Getting rid of thistles

Contents

Controlling Pasture Weeds

If you notice bare patches, you can get ahead of weeds by sowing seed in late February and early March, or later in very cold climes. “Cast cool-season grasses by hand–such as orchardgrass and Kentucky bluegrass,” Waite says. “The freezing and thawing process during that time will help the seed work its way into the cold soil.”

Knock existing weeds out before they get a chance to spread. Witt says one or two plants might not seem like a big deal but, if left to go to seed, some plants can spread hundreds and thousands of seeds throughout the pasture. “If you don’t mow early enough,” he said, “and you let the weeds make seed, when you mow you’ll scatter the seeds in all directions. Pigweed may have a few thousand seeds per plant. The common ragweed has hundreds if not thousands on a big plant.”

Healthy vegetation can help keep weeds at bay, especially if you have hardy forage such as fescue, which has a particularly deep root system. “Fescue competes with weeds better than bluegrass or orchardgrass,” Witt says. “Nimblewill doesn’t grow as well in tall fescue. If you have riding horses and you don’t breed (as endophyte-infected tall fescue can be toxic in pregnant mares), growing the old Kentucky-31 fescue can be a good option. Nontoxic endophyte-friendly tall fescue isn’t widely used because it’s so expensive, but there’s a new fescue (also nontoxic and endophyte-free) that has the potential to be excellent. Fescue breeders have done grazing trials with beef animals with it, and this spring we will do grazing trials with pregnant mares.”

Removing Weeds

Weeds can’t seed if you eliminate them in the first place. There are three ways to do this: Pull them out by hand, mow them, or spray them with herbicide. The method you choose depends upon the weed, how much space it’s occupying, and its maturity.

As a sidenote Witt says some weeds can be eaten when small and are nutritious, but once they grow and get woody they -become unpalatable to horses, as with buckhorn plantain, so it’s still important to manage these intermediate weeds.

Plant identification can be tricky for the layman, but Witt says your local county agriculture extension agent can help you identify weeds and evaluate your pasture. “We have a pasture evaluation program for horses here in Kentucky,” he says. “We go out to the (owner’s farm) and take counts across the pasture. (The owner) gets back information that details the percentage of weeds, forage, and bare ground. It also identifies the weeds.”

Owners might decide to hand-weed plants that are poisonous (e.g., poison hemlock) or low in number, but for the most part this removal method is not practical.

Mowing is not as useful for weed management in pastures as you might think, says Witt. “You have to kill low-growing weeds with low-mowing, which will kill good forage, too–you shouldn’t mow a pasture below four inches. Mowing might stop some weeds from going to seed, but you won’t get rid of the existing plants.”

If many invasive weeds have taken hold, you might need to spray with an herbicide and kill off all vegetation. “Killing the entire pasture and starting over is something that no one wants to hear,” says Waite. “But sometimes this is the only answer. Once a pasture is killed off and reseeded, you won’t be able to let horses back on the pasture until the plants are established, which means one growing season.”

Witt advises keeping horses away from cut or sprayed weeds, and to remove cut weeds from the pasture. “Do not let horses have access to a decaying plant,” he says. “Once the plant is cut or sprayed, your horses might eat it when they wouldn’t otherwise. (In the case of poisonous weeds or those sprayed using herbicides) the toxin is still in those plants while it’s decaying.”

Noxious and Nuisance Weeds

Our sources have selected nine weeds prevalent across North American to describe in further detail:

1. Thistle (Asteraceae family) This is the common name for a group of nontoxic plants (including Canada thistle, musk thistle, and plumeless thistle) that have spiny leaves. Thistles are very difficult to control once they’ve invaded a pasture, and mowing is ineffective. Their roots can be deep, and if you break one another plant will grow in its place. The only effective way to remove thistles is using herbicides.

Witt says that because thistles have spines, horses avoid grazing any good plants growing near or in a patch of thistles. Left alone, thistles can spread quickly and occupy 20% or more of your pasture.

2. Hoary alyssum (Berteroa incana) Hoary alyssum is widespread in the northern states and in Canada. It grows well in dry areas and flourishes during droughts. It is toxic to horses, causing depression and lower limb swelling. It might also cause founder and colic. “Hoary alyssum was a big problem for us in Michigan last summer because it’s been so dry,” says Waite. “Ordinarily horses will eat around hoary alyssum, but last year they didn’t. For the most part horses can tolerate a little of the plant, but some horses might be more affected by it than others.”

Hoary alyssum is toxic fresh or dried. It resembles alfalfa when dried and can be difficult to identify in hay. University of Minnesota Extension veterinarians recommend not feeding hay that’s more than 30% hoary alyssum.

3. Buckhorn plantain (Plantago lanceolata) Widespread throughout the United States, buckhorn plantain thrives in agricultural and developed land. It’s also drought-tolerant. Generally horses nibble on the plant, but they won’t graze it fully unless they have no other plant choice. Buckthorn plantain falls into the nuisance weed category. Dig the weed up as you would a dandelion, removing the taproot. You can also use herbicides to eradicate this plant.

4. Hemp dogbane (Apocynum cannabinum) Hemp dogbane contains a white milky sap similar to the common pasture dweller milkweed, but while milkweed has leathery leaves and large seedpods, hemp dogbane has smaller leaves, a different branching habit, and reddish stems. It grows throughout the United States, especially in the mid-Atlantic region, and it has toxic properties. The plant’s underground roots grow in plant communities similar to the thistle. Mowing and herbicides suppress hemp dogbane.

5. Poison hemlock (Conium maculatum) Poison hemlock is a tall, multistemmed biennial that grows in shaded wet areas across the United States. Poison hemlock resembles Queen Anne’s lace, but Witt says the easy way to tell the two apart is to look at the stem. Poison hemlock, even in its immature state, has reddish or purplish blotches on the stem. Poison hemlock is very toxic to horses; it contains neurotoxins that cause tremors, incoordination, and respiratory failure. Hand-pull plants prior to flowering to remove them from the pasture. Multiple close mowings can kill the plant completely.

6. Nimblewill (Muhlenbergia schreberi) Nimblewill is a native perennial grass that forms a thick mat over the ground. It has short, narrow leaves and stolons (stems that form roots and buds, also called runners). Nimblewill resembles Bermuda grass, but it’s not nearly as useful; in fact, horses refuse to touch it. Its stems and stolons die off in the fall, resulting in brown patches of pasture that used to be green.

“We have not found a way to control nimblewill,” says Witt. “It’s not poisonous, but nothing will eat it. It produces thousands of little tiny seeds, and anywhere there is a bare space it will germinate. It also runs along the soil. You can start over, and you’ll keep it at bay for a few years, but it will grow back. In Kentucky it’s our No. 1 problem in pastures.”

7. Spiny amaranth/Spiny pigweed (Amaranthus spinosus) This summer annual is widespread in the eastern half of the United States and can take over an entire pasture. It thrives in compacted soil, particularly around watering or feeding stations, or along fence margins where horses walk, and each plant produces thousands of seeds. It’s fairly easy to control with herbicides, but you must spray it early in the year. Horses avoid grazing near it because of the sharp spines at the base of the leaves.

“It’s hard to see before it’s mature,” says Witt. “You can mow it but it’s one that you have to mow close to the ground to kill. We don’t do a lot of spot mowing in pastures, but this would be where we could do that and be fairly efficient. Mow when it’s a foot tall.”

8. Common ragweed (Ambrosia artemisifolia) Common ragweed tolerates dry conditions, thrives in bare areas, and competes with desirable forage. Although toxicologists say it might cause horses to accumulate harmful nitrites in their bodies, this is uncommon. Mowing is ineffective, as branches below the mowing line produce seed heads.

9. Black walnut trees (Juglans nigra) don’t immediately spring to mind when considering pasture weeds, but Waite says not to discount them. “At MSU we often get calls about black walnut trees in pastures,” she says. “Most people know the issue with black walnut shavings in bedding, but there have been cases of founder from horses standing on the roots. Black walnut trees are fascinating because the toxic principal, juglone, is really toxic. It kills everything growing around the tree.”

Waite says that although wilted black walnut leaves can be an issue, the toxin of concern lies with the wood. “I always discourage people from planting black walnut trees on horse farms,” she says. “Even if they aren’t in the pastures, a storm can cause the trees to fall down or branches to blow into the pastures.”

Take-Home Message

Our sources agree that weed-free pastures start with managing the good forage you have. “Soil test, reseed, and fertilize as appropriate, and don’t let weeds go to seed,” says Waite. “Mow before that happens. Impose rotational grazing strategies, and don’t let pastures get grazed below three inches. All of these strategies will help keep weeds under control.”

Buckhorn Plantain

Common name: Buckhorn Plantain
Scientific name: Plantago lanceolata L.
Life Cycle: Perennial
Poisonous: No

Buckhorn plantain is widespread across North America and is a commonly occurring plant in all types of pastures and rough turf. It readily survives overgrazing and compacted horse pastures, especially when rainfall is limited. Buckhorn plantain is relatively easy to control with several herbicides; however, mowing in pastures is generally ineffective. | Photo: Wikimedia Commons

Bush Honeysuckle

Common name: Bush (Amur) honeysuckle
Scientific name: Lonicera maackii (Rupr.) Herder
Life Cycle: Perennial
Poisonous: None reported

Bush honeysuckle describes several species of woody honeysuckles found in the eastern United States. All grow rapidly and produce multiple stems and can reach heights of about 30 feet. Large bush honeysuckle plants are difficult to remove by hand due to an extensive root system. Herbicidal control is effective. | Photo: Wikimedia Commons

Buttercup

Common name: Buttercups
Scientific name: Ranunculus species
Life Cycle: Perennial
Poisonous: Yes

Buttercup is the common name for several Ranunculus species distributed across much of the United States. Buttercups can be poisonous to horses, but the plants are not palatable and usually not eaten by animals. Mowing is usually ineffective for controlling buttercups, however; they are easily controlled with several herbicides. | Photo: Wikimedia Commons

Chicory

Common name: Chicory
Scientific name: Cichorium intybus L.
Life Cycle: Perennial
Poisonous: None reported

Chicory is a commonly occurring plant in all types of pastures and rough turfs across North America. It is relatively easy to control with several herbicides. Mowing in pastures might reduce flower formation but is generally ineffective in killing the plant. Hoeing or digging the taproot is successful and should be done before the seed heads form. | Photo: Wikimedia Commons

Common Milkweed

Common name: Common Milkweed
Scientific name: Asclepias syriaca L.
Life Cycle: Perennial
Poisonous: Yes

Common milkweed grows throughout North America except in the extreme southern, southwestern, and far western states. It produces cardiac-glycosides that are toxic to horses and might cause depression, irregular heartbeat, diarrhea, weakness, labored breathing, and even death. Plant control is difficult. Hand weeding and removing the deep taproot might help. Mowing is generally ineffective. | Photo: Wikimedia Commons

Common Ragweed

Common name: Common ragweed
Scientific name: Ambrosia artemisiifolia L.
Life cycle: Warm season annual
Poisonous: No

Common ragweed is distributed widely across the United States and occurs in pastures and cultivated crops. Infestations in pastures are usually more of a problem during periods of drought or when overgrazing occurs. Common ragweed control is relatively easy: apply herbicides to plants less than 12 inches tall that have not been mowed. | Photo: Wikimedia Commons

Hemp Dogbane

Common name: Hemp dogbane

Scientific name: Apocynum cannabinum L.
Life cycle: Perennial
Poisonous: Yes

Hemp dogbane grows throughout North America. It’s poisonous to horses, with the leaves toxic at all times, including dried in hay. The toxic substance is a glycoside that can cause digestive disturbances, diarrhea, and weakness. Controlling it in pastures is difficult. Mowing is often ineffective and herbicidal treatment requires multiple applications. | Photo: Wikimedia Commons

Nimblewill

Common name: Nimblewill
Scientific name: Muhlenbergia schreberi J. F. Gmel.
Life Cycle: Perennial
Poisonous: No

Nimblewill is a warm season perennial grass that is widespread across the eastern United States. A commonly occurring plant in many pastures and turf types, it is found especially in Kentucky bluegrass fields. Mowing is ineffective to control it. Currently no herbicide is available that will control the nimblewill and not cause severe damage to desirable pasture grasses. | Photo: Wikimedia Commons

Poison Hemlock

Common name: Poison Hemlock
Scientific name: Conium maculatum L.
Life Cycle: Biennial
Poisonous: Extremely

Poison hemlock is distributed across the United States and grows most frequently along fence borders in shady and moist areas. This plant is extremely poisonous to horses and humans. All plant parts contain the poisonous alkaloids; however, the fruits contain the greatest concentration of the alkaloids. Poison hemlock control is relatively easy with herbicides. | Photo: Wikimedia Commons

Spiny Pigweed/Amaranth

Common name: Spiny pigweed, Spiny amaranth
Scientific name: Amaranthus spinosus L.
Life Cycle: Warm season annual
Poisonous: No

Spiny pigweed is distributed widely across the United States and grows most frequently along fence borders, feeding/watering areas, and other compacted areas. Control is relatively easy with herbicides applied to plants less than 12 inches tall. Mowing and hand weeding are effective if done before flower production to prevent seed formation. | Photo: Wikimedia Commons

Star-of-Bethlehem

Common name: Star-of-Bethlehem
Scientific name: Ornithogalum umbellatum L.
Life Cycle: Perennial
Poisonous: Yes, all parts, especially bulbs and flowers

Star-of-Bethlehem grows in the eastern half of the United States and the Pacific Northwest. It grows in pastures, landscape beds, gardens, fields, and roadsides. The plant contains cardiotoxins and glycosides that are toxic to horses; the bulbs and flowers contain the highest amounts. Few pasture herbicides are effective on mature plants. Remove small patches by hand, digging the bulbs. | Photo: Wikimedia Commons

Tall Ironweed

Common name: Tall ironweed
Scientific name: Vernonia gigantea (Walt.) Trel.
Life Cycle: Warm season perennial
Poisonous: No

Tall ironweed is distributed widely across the eastern half of the United States and is found most frequently in low damp areas of pastures and roadsides. Tall ironweed control is possible when herbicides are applied to plants less than 12-15 inches tall that have not been mowed. Mowing is an effective treatment to prevent seed production. | Photo: Wikimedia Commons

White Snakeroot

Common name: White Snakeroot
Scientific name: Eupatorium rugosum Houtt.
Life Cycle: Perennial
Poisonous: Yes

White snakeroot is a warm-season perennial frequently found in shaded areas of pastures near streams or woods. These plants are toxic to horses; both fresh and dried leaves contain the toxin. Cumulative intake between 1 and 10% of body weight is toxic and can be lethal. Removing these plants from the pasture by hand is often the best course of action. | Photo: Wikimedia Commons

Wild Carrot

Common name: Wild Carrot (Queen Anne’s Lace)
Scientific name: Daucus carota L.
Life Cycle: Biennial
Poisonous: Slightly

Wild carrot, also known as Queen Anne’s lace, is an erect biennial that can grow to about 4 feet in height. It is found in pastures, native areas, fields, and roadsides. Mild neurotoxicity to horses was reported in Europe but is not considered a serious threat in North America. Controlling wild carrot in pastures is easy using timely mowing before flowering and herbicidal treatment. | Photo: Wikimedia Commons

How to Get Rid of Thistle Permanently

It’s tough, it’s spiny, it’s hairy, and it’s taking over your yard. Thistle—whether it’s of the musk, tall or Canada variety—is a very difficult weed to eliminate from your lawn or garden once it has taken hold; a single musk thistle can generate 120,000 seeds from one flower, and can grow to six feet tall. Eliminating thistle for good my take several years, because thistles are either biennial or—even worse—perennial. If you have thistle one year, you will have even more the next, unless you take action. Biennials, such as musk thistle and tall thistle, germinate in summer and fall, spend the winter as rosettes, then produce many flower heads the next spring. The fluffy purplish flowers of thistles are the only visually pleasing part of the plant, but don’t be fooled; they carry the seeds that guarantee another year of thistle invasion. Canada thistle, a perennial, reproduces in the same way, but it has the added advantage of spreading by way of its roots; this makes control even more challenging.
If you want to banish thistle, you have to go to war against it. Fortunately, you do have some weapons at your disposal.

North Dakota State University

Canada Thistle Biology and Identification


Canada thistle flower and buds
(P. Gregoire, NDSU)

Canada thistle (Cirsium arvense (L.) Scop.) is a long-lived perennial weed that spreads rapidly due to the generation of new shoots from creeping, extensive roots. This species was introduced to North America probably as a contaminant in seed or forage.

Oblong, deeply lobed dark green leaves with spiny margins clasp the stems in an alternate arrangement. The leaf surfaces are smooth to slightly hairy. Unlike other exotic thistles in North Dakota, Canada thistle does not have a highly spiny stem.


Canada thistle leaf upper surface; note that is it smooth
(P. Gregoire, NDSU)


Underside of Canada thistle leaf
(P. Gregoire, NDSU)

Small (¾ inch diameter), composite flowers are borne at the tops of stems. Flowers vary from purple to pink, and are less commonly white. Seeds are light brown and have a featherlike pappus, which aids in wind dispersal. Seeds can be dispersed long distances by the wind, but most seeds tend to fall near the mother plant.


Seed head of Canada thistle displaying seeds with feathery pappus
(E. Burns, NDSU)

Canada thistle rosettes emerge in early spring once the soil temperature reaches 35 to 45 F. Flowering occurs from June through September.

North Dakota’s Noxious Weed Law

Canada thistle is on the North Dakota noxious weed list. What does this mean? A North Dakota law (NDCC § 4.1-47-02) requires every person to “do all things necessary and proper to control the spread of noxious weeds.”

The North Dakota Department of Agriculture coordinates the efforts of county and city weed boards and state and federal land managers to implement integrated weed management programs. To access city and county lists and find out more about North Dakota’s noxious weed law, visit the North Dakota Department of Agriculture website.

The North Dakota Century Code states that noxious weeds must be controlled and that control is defined as “preventing a noxious weed from spreading by: A) suppressing its seeds or propagating parts or B) destroying either the entire plant or its propagating parts.” The method by which control is achieved is not specified nor is it limited to herbicides.

Organic farmers face a serious challenge when controlling Canada thistle and other noxious weeds, but insisting that noxious weeds must be controlled by synthetic herbicides is incorrect. Effective nonchemical methods are valid options for noxious weed control in North Dakota.

Problems Caused by Canada Thistle

Why is Canada thistle such a concern in North Dakota? This aggressively spreading weed is the most common noxious weed in the state and infests millions of acres. The ability of this species to reproduce via vegetative shoots leads to dense infestations that compete with and crowd out desirable native plants or crops.

If you see a dense patch of thistle in a field or pasture, chances are good that this is Canada thistle. And even though the species is poorly dispersed via seed, just one seed can produce a new patch. New patches can spread quickly if control measures are not taken.

Canada thistle can displace endangered or threatened plant species. In pastures, Canada thistle is a deterrent to grazing because cattle avoid eating the spiny leaves. Grazing Canada thistle with goats is a control option because goats tolerate and often prefer spiny, tough fodder that other livestock reject.

Canada thistle infestations in lawns are undesirable and may reduce property values. In rangeland, Canada thistle may cause changes in fire frequency and intensity, leading to ecological disruption.

Unlike many noxious weeds, Canada thistle also is a serious problem in cropland, especially in organic production systems. If an organic farmer is unable to prevent the spread of this noxious pest, he or she may be forced to choose between organic management and weed control. Canada thistle infestations can cause substantial yield losses in most annual field crops.

OMRI-approved Products for Use in Organic Systems

Synthetic chemical herbicides often are used to control Canada thistle, but these products are not allowed for use on certified organic land or land transitioning to certified organic production. However, Organic Materials Review Institute (OMRI)-approved weed control products are commercially available and can be used to suppress Canada thistle.

One commercial preparation contains the active ingredients citric acid and clove oil. The active compound in clove oil is eugenol, which disrupts cell membranes, leading to tissue death. Acetic acid (vinegar), caprylic acid and other organic acids are contact-type herbicides that burn plant tissues. These organic acids are the most common ingredients in many organic weed killers.

Research conducted to assess the effectiveness of these natural products has yielded mixed results. In general, these products can kill small annual weeds, but the effectiveness varies widely among weed species. Generally, broadleaves are better controlled than are grasses, but the key is spraying small (less than 2 inch) weeds. Results from one study showed that acetic acid effectively burned and destroyed Canada thistle shoots, but the plants readily regrew from roots.

Repeated applications eventually may control small Canada thistle patches, but multiple years of treatment are usually necessary. Organic acid herbicides usually require very high carrier volumes (30 to 100 gallons/acre) and high application rates to be effective.

These use requirements, combined with a high per-gallon cost, make OMRI-approved herbicides cost prohibitive for broadcast application in most agronomic crops grown on large acreages. However, spot treatment can be more practical and cost-effective.

Another issue with these OMRI-approved herbicides is that they are nonselective, meaning they can damage crop and weed species, so in-crop application is usually not possible.

Biological Control Agents for Canada Thistle

Various insects, as well as bacterial and fungal pathogens, attack Canada thistle. Two non-native insects that feed on Canada thistle have been studied and widely released in North Dakota as biological control agents.

The first is Hadroplontus litura, a stem-mining weevil. This insect emerges as an adult in the spring, feeding on thistle leaves and depositing eggs. Larvae migrate into the stems, where they mine the pith, eventually exiting into the soil, where they pupate. Although the larvae may feed on the roots briefly, the damage is mostly confined to the shoots, so the plants easily recover.


Hadroplontus litura stem mining weevil feeding on Canada thistle leaves
(E. Burns, NDSU)

The second released insect is Urophora cardui, a fly that lays eggs on the plant. The presence of larvae causes the plant to form a woody gall on the stem, which directs nutrients away from the growth of the plant. However, like the weevil, the gall fly inflicts little lasting damage and has not reduced Canada thistle infestations substantially when used alone.


Canada thistle gall
(G. Gramig, NDSU)

Two pathogens (Puccinia punctiformis and Pseudomonas syringae pv. tagetis) infect Canada thistle, but attempts to develop effective commercial products from these have failed. Canada thistle infected with fungal pathogens exhibit white or yellow upper leaves.

When combined with other control methods, biological control agents may help reduce Canada thistle infestations to a minor degree. For example, insect feeding can give pathogens an entry point. Some growers report that mowing a Pseudomonas-infected thistle patch immediately before a rain can help spread the fungus and increase infection.

Overall, biological control agents have limited ability to control Canada thistle infestations when not used in combination with other control tactics.

Mechanical Control of Canada Thistle

The primary reason Canada thistle is difficult to manage is its extensive root system that stores carbohydrate reserves and allows the plant to regrow after shoots have been destroyed. Because of this system, long-term control of this weed requires methods that destroy the roots or exhaust them of their carbohydrate reserves.

Tillage and mowing are two mechanical tactics that can exhaust these tenacious root systems. Timing and frequency of mowing are important to achieving acceptable levels of suppression. Mowing should start at early bud stage to prevent seed production. The plants should be cut as short as possible, with repeated mowing as plants regrow.

Persistence is key. Several years may be required to deplete the root stores.

Adding alfalfa to an annual crop rotation and mowing it for hay can suppress Canada thistle. Repeated tillage also will weaken the roots through time.

You must use an implement that will sever the shoots completely from the roots. Because thistles regrow, tillage should be repeated frequently, or at least every three weeks. Otherwise, tillage will make the problem worse because Canada thistle can regenerate from even small pieces of roots if conditions are favorable.

The roots are susceptible to frost damage, so repeated tilling late in the fall can help kill roots by bringing them to the surface.

Whether mowing or tilling, these methods rely on severing the shoots from the roots to weaken the roots.

Cultural Control of Canada Thistle

Prevention and sanitation are important for limiting Canada thistle spread. Plant clean seed, especially forages, and do not allow patches to produce seed. After tilling thistle-infested areas, clean equipment to prevent it from carrying root fragments into clean fields. Even a tiny root fragment can produce a new plant.

Scout clean fields for new patches. You often can remove a small patch completely if you dig it up carefully and remove the roots.

Canada thistle does not tolerate shading, so establishing vigorous, healthy crop stands is key. Maintaining good soil fertility is another tactic to help foster the growth of desired plants instead of Canada thistle. The best crops for outcompeting Canada thistle are winter annual cereal grain crops and perennial crops such as alfalfa or forage grasses.

However, competition alone seldom will provide long-term control. Combining competitive crops with mowing, tillage or grazing may increase success. Grazing with goats also can provide suppression of Canada thistle.

Target grazing to the bud stage, when flower buds are present but not yet open, because this will ensure the goats take the maximum amount of energy reserves possible from the thistle and prevent seed set. A dense patch of thistle may need to be grazed more than once during the summer to prevent seed set.


Canada thistle flower bud
(E. Burns, NDSU)

The key to using crop rotations to manage Canada thistle is growing crops with different life cycles than the weed. For instance, alfalfa works well because it emerges and starts growing in the spring about three weeks earlier than Canada thistle. After cutting, alfalfa recovers more quickly and continues to grow tall in the fall, when Canada thistle remains in a rosette.

Narrow row spacing or undersowing a cash crop with a cover crop such as red clover also can increase competition. Seeding a highly competitive cover crop such as winter cereal rye after a season of frequent tillage and cutting in the spring also is an option for suppressing Canada thistle.

Crop rotations should balance crops that compete well, such as alfalfa and winter wheat, with crops that compete poorly, such as soybeans or corn. Avoid growing a simple corn-soybean rotation. Using a high-residue cultivator in wide-row (30 inches or greater spacing) corn or soybeans can provide in-season Canada thistle suppression.

Wildlands, Pasture, Rangeland and Roadsides

Early detection also is important in perennial systems such as wildlands, pasture and range. A small patch of Canada thistle can be controlled more easily than a large infestation.

Canada thistle thrives in disturbed soil, so preventing disturbances in wild areas, pasture and rangeland can limit its spread. Overgrazing pastures or rangeland also encourages the spread of this weed.

Canada thistle does not do well when shaded, so maintaining a cover of desirable plant species will help suppress Canada thistle and prevent establishment. Revegetate disturbed areas immediately. Strategic grazing and mowing before flowering can prevent further spread.

Prescribed fire often is used to encourage native plants, but fire has not been shown to control this weed effectively. Biological control agents may provide some suppression in pasture and range settings.

Despite its many problematic attributes, Canada thistle can provide nectar to native pollinators, a needed service in many landscapes dominated by grasses.


Pollinator feeding on a Canada thistle flower
(E. Burns, NDSU)

Summary of Best Practices to Control Canada Thistle

Remember: Canada thistle is an aggressive and hard-to-control weed because it easily regenerates shoots from its extensive creeping root system. Without the use of synthetic herbicides that kill these roots, you must rely on methods that deprive the roots of additional energy that is produced by the leaves.

Organic management of Canada thistle relies on 1) prevention and 2) depletion of root energy reserves. Organic farmers should plan to use a combination of biological, mechanical and cultural tactics to manage Canada thistle.

Canada thistle
 (Cirsium arvense (Sunflower Family: Asteraceae)
)

Click on images to enlarge

Canada thistle, a perennial broadleaf plant, is found in most of the western states except for southern Sierra Nevada, the Sonoran and Mojave deserts, and the Channel Islands. It is scattered throughout California to about 5900 feet (1800 m). Canada thistle inhabits agricultural land and other disturbed locations. According to some taxonomists, four varieties or biotypes exist that differ in growth habit, leaf characteristics, seed germination, and development.

Canada thistle is a state-listed noxious weed in California and many other states. It is important to control plants before they regenerate food reserves in their roots or produce seed. Do not let this weed move to new areas and eliminate it from noncrop locations.

Habitat

Stream banks, forest openings, rangeland, hillsides, moist depressions, gardens, crop fields, roadsides, and other open, disturbed sites.

Seedling

Cotyledons (seed leaves) are oval to oblong, usually from 1/5 to 1/2 of an inch (5–14 mm) long, thick, fused at the base, and smooth or slightly glandular. The midvein on their lower surface is shiny. The first leaves are alternate to one another on the stem and their edges are somewhat wavy to unevenly toothed. The leaf surface is covered with stiff hairs, but lower surfaces are often covered sparsely with soft, webby, hairs. Seedlings have poorly developed rosettes.

Mature plant

Canada thistle stands erect to 3-1/3 feet (1 m) tall and grows in clumps or patches. Its stems are slender and hairless or nearly hairless. Leaves are oblong to lance shaped, mostly 2 to 8 inches long (5–20 cm), prickly, and are alternate to one another along the stem. Sometimes leaf bases extend down the stem joints as prickly wings that are 1/2 of an inch (1 cm) long. Leaf edges range from nearly smooth to shallow lobed and toothed. The upper leaf surface is hairless to nearly hairless and the lower surface is sometimes sparsely woolly. Rosette leaves are either few or lacking altogether.

The extensive root system consists of a network of vertical and creeping horizontal roots. Although most roots occur in the top 1-1/2 feet (45 cm) of the soil, vertical roots from 6-1/2 to almost 10 feet (2–3 m) deep are common.

Flowers

Flowers bloom from June through October. The flower head is cylindrical or narrowly egg to bell shaped, about 1/5 to 4/5 of an inch (0.5–2 cm) in diameter, and consists of several overlapping rows of scalelike structures (bracts) tipped with spines. Narrow, white, purple, or pink flowers (disk flowers) cluster above.

Fruits

Fruits are single seeded (achene), egg to football shaped, tan, about 1/12 to 1/6 of an inch (2–4 mm) long, and end in a long tuft of feathery bristles that are about 1/2 to 4/5 of an inch (12–20 mm) long.

Reproduction

Reproduces by seed and from cut roots. Seeds are wind dispersed and can be viable for many years.

More information

  • Broadleaf ID illustration
  • Calflora’s distribution map
  • For agriculture: UC IPM Pest Management Guidelines

Milk Thistle – May 2018 Weed of the Month

For more information on control options, see the following pages:

Be sure to identify milk thistle carefully before removing it. Close-up, milk thistle’s white marbling is a give-away, but at a distance it might be confused with a number of other thistle species. Two similar invasive thistles are bull thistle (Cirsium vulgare) and Canada thistle (Cirsium arvense), both non-regulated Class C noxious weeds.

Like milk thistle, bull thistle reaches 2–6 feet tall with one flower at each stem end, but its lobed, hairy leaves lack white marbling. Its flowers also have smaller spines around their bases, as opposed to the larger, fleshier bracts of milk thistle.

Bull thistle rosette with deeply lobed, hairy leaves. Bull thistle in bloom with one magenta flower head per stem.

Canada thistle grows to 5 feet tall with narrow leaves and flower heads in clusters at stem ends. Unlike the other two invasive thistles—but like the two native thistles below—this species has spineless stems. It spreads via both seeds and creeping rhizomes, making it extra difficult to control.

Canada thistle rosettes with narrow leaves. Canada thistle in bloom showing clusters of flowers at stem ends.

Two native thistles also resemble milk thistle. Edible thistle (Cirsium edule) is a 6-foot-tall perennial with short spines on alternate leaves and single pinkish-purple flower heads at stem ends. Tops and bottoms of leaves are lightly hairy.

Mature edible thistle, showing spiny, alternate leaves and pinkish-purple flower heads at stem ends. Photo by Yan S / CC BY 2.0.

Short-styled thistle (Cirsium brevistylum) looks similar to edible thistle, but with less-lobed leaves and clustered flower heads. Only bottoms of leaves are slightly fuzzy.

Mature short-styled thistle, showing less-lobed leaves than edible thistle. Photo by NatureShutterbug / CC BY 2.0.

Both native thistles lack spines on their stems, which helps to distinguish them from milk thistle and bull thistle. Be careful, though, because Canada thistle’s stems are spineless, too. Unlike all three invasive thistles, both of these native species have noticeable cobweb-like hairs wrapped around the bracts at the bases of their flower heads.

For more information on native and invasive thistle species, check out Native Thistles: A Conservation Practitioner’s Guide by the Xerces Society for Invertebrate Conservation.

Also, it’s important to note that milk thistle is widely known as a medicinal plant, especially for liver problems. The Washington State Noxious Weed Control Board’s categorization of milk thistle as a Class A noxious weed is based entirely on the plant’s invasive behavior in Washington, and is not related to any medicinal qualities. Local medicinal gardens can get permission to grow this plant from King County if they are careful not to let it spread.

Milk thistle growing in an educational medicinal herb garden.

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Categories: Weed Control, Weed Identification, Weed of the Month

Washington State

Onopordum acanthium

Family: Asteraceae

Other Common Names: cotton thistle, woolly thistle
Weed class: B
Year Listed: 1988
Native to: Europe and Asia
Is this Weed Toxic?:

not known to be

Legal listings:

WAC 16-752; WSDA Quarantine list (prohibited plant list)

Why Is It a Noxious Weed?

Scotch thistle is a problem in rangelands and other open areas. Infestations of Scotch thistle reduce forage production and virtually prohibit land utilization for livestock and block access for people and wildlife. Dense stands of the large, spiny plants exclude animals from grazing and access to water.

How would I identify it?

General Description

Scotch thistle is a branched, biennial or annual with a broadly winged stem that can grow up to 8 feet or more in height and 6 feet in width.

Flower Description

Plants flower in mid-summer. The globe-shaped flowerheads solitary or in groups of 2-7 on branch tips. Flowerheads are up to 1 to 3 inches in diameter, rounded, with long, stiff, needle-like bracts at the base. Flowerheads contain many disk flowers that range from dark pink to lavender in color, though occasionally white.

Leaf description

Leaves are up to 2 feet long and 1 foot wide, are covered with sharp yellow spines and have a gray-green appearance from being covered with a thick mat of cotton-like or woolly hairs.

Stem description

Stems have vertical rows of prominent, spiny, ribbon-like leaf material or wings that extend to the base of the flower heads.

Fruit Seed Description

Seeds are smooth, slender, and plumed.

Where does it grow?

Scotch thistle will grow in wet meadows and pastures as well as dry pastures and rangelands. It may also be found alongside streams and rivers. Please click here to see a county level distribution map of Scotch thistle in Washington.

How Does it Reproduce?

Scotch thistle reproduces by seed. Each plant can produce 8,400 to 40,000 seeds.

How Do I Control It?

General Control Strategy

Establishing a dense well-maintained pasture is effective in preventing and competing with a Scotch thistle infestation.

Manual/Mechanical Control

Small infestations can be dug out. Dig out rosettes by severing plant’s taproot with a shovel below the soil’s surface. Mowing has limited effectiveness for controlling Scotch thistle, usually only prevents seed production. Mow too early and plants can recover and flower. Flowerheads that are cut when already in bloom may still release seed if left on the ground – so bag up and trash cut flowerheads. Repeated mowing may prevent flowering. Make sure to mow before flowering to prevent seeds development. Repeated mowing may be needed on moist sites.

Biological/cultural Control

Goats will graze Scotch thistle plants, eating flowerheads, and sheep may feed on small rosettes. There are no insects currently approved for the biological control of Scotch thistle.

Herbicide Control

There are a number of herbicide options for Scotch thistle. Please refer to the PNW Weed Management Handbook, or contact your county noxious weed coordinator.

For More Information

Scotch thistle weed report from the book Weed Control in Natural Areas in the Western United States

See our Written Findings for more information about Scotch thistle (Onopordum acanthium).

Thurston County NWCB Fact Sheet on Scotch thistle

Franklin County NWCB Fact Sheet on Scotch thistle

Stevens County NWCB Fact Sheet on Scotch thistle

Whatcom County NWCB Fact Sheet on Scotch thistle

Pierce County NWCB Fact Sheet on Scotch thistle

Lincoln County NWCB Brochure on Scotch thistle

Getting Rid of Thistles with Vinegar: Green Gardening Tips

Getting rid of the thistles in my gardens, lawn, and rock border always proves arduous for me during the spring and summer months. I live across from an open field, so the thistle seeds just keep floating onto my property, sprouting more and more prickly weeds. My husband and I tried pulling the thistles out by hand, but the task was tedious at best and painful at worst. Even with thick gardening gloves, the prickles on the weeds poked into our skin. Because I am allergic to most plants, I always broke out with itchy, red welts whenever I got poked by a thistle thorn. I needed a better solution to my thistle problem.

One day while perusing my Pinterest feed, I happened across a pin about using household vinegar to get rid of weeds in a stone path. According to the writer at A Garden for the House, spraying regular old vinegar on the weeds kills the plants without the use of harmful chemicals. Vinegar is a herbicide that is truly biodegradable. As a food, I knew that I could use vinegar around my house without worrying about the dangers to my daughter and pets. I was excited to have possibly found a solution to my weed problem that was truly safe for my family. Vinegar is also quite a bit cheaper than chemical herbicides.

After buying a few huge bottles of vinegar and a spray bottle, I proceeded to spray the weeds and grass taking over the rock border and path in the front of my house with straight vinegar. I sprayed every plant that I saw: grass, dandelions, violets, wild flowers, thistles. The next day when I went outside to check on the progress, I was thrilled to discover that many of the weeds were yellowed and wilting. Even more to my surprise, most of the thistles looked completely dead. I sprayed everything again that second day for good measure.

On the third day, I went outside and discovered that most of the small thistles that I had sprayed with vinegar had died and completely disappeared. Excited about my discovery, I started spraying all the thistles in my grass and taking root around my trees. After just one application, the majority of the thistles in my yard disappeared. I had killed the evil thistles taking over my property with a little kitchen vinegar and without the need to pull the prickly little weeds up!

The only caveat to using vinegar as an herbicide is that vinegar is not selective. Any plants sprayed with the vinegar will yellow and possibly die. As I quickly discovered, not only did the thistles in my yard die, but the grass around the thistles yellowed considerably. Fortunately, grass is pretty hardy. Unlike the thistles, the grass is still in my yard, just more yellow in color than usual. I am keeping an eye on the progress, but I expect the grass to make a come back eventually. However, in the battle against thistles, I am more than happy to sacrifice a little of my grass.

As for other weeds, the vinegar herbicide works okay. The other plants in my rocks are slowly dying. With multiple applications, I suspect that the weeds in my rocks will disappear completely too. However, for thistles, vinegar is also an immediate death sentence. Even if none of the other weeds die, I am still happy with the results of the vinegar on the thistles. If you too have a thistle problem in your garden or yard, try a little white vinegar. You will not be unhappy with the results!

Thistles in Rocks

Weeds in Rocks

Thistles

Thistles in Grass

To recap, vinegar is an environmentally-friendly method for killing thistles and other weeds in gardens, grass, and rocks. Simply pour some undiluted distilled white kitchen vinegar into a spray bottle. Spray the plants that you want to kill with the vinegar. You may need to spray the plants a few times over a few days depending on the hardiness of the plants. Be careful about spraying other plants as vinegar is not selective and will harm any plants sprayed. After spraying the weeds, rinse the bottle out with water to avoid corrosion. Vinegar is a safe, simple, and effective herbicide, especially against thistles.

What other safe methods have you discovered that work to kill weeds around your house?

Image Credits

Getting Rid of Thistles with Vinegar: Green Gardening Tips © 2013 Heather Johnson
Thistle in Rocks © 2013 Heather Johnson
Spraying Thistle with Vinegar © 2013 Heather Johnson
Thistle Dying from Vinegar © 2013 Heather Johnson
Thistle Destroyed by Vinegar © 2013 Heather Johnson
Weeds in Rocks © 2013 Heather Johnson
Spraying Weeds with Vinegar © 2013 Heather Johnson
Weeds Yellowed from Vinegar © 2013 Heather Johnson
Weeds Dying from Vinegar © 2013 Heather Johnson
Big Thistle Dying from Vinegar © 2013 Heather Johnson
Thistle Wilting to Death from Vinegar © 2013 Heather Johnson
Thistles in Grass Dying from Vinegar © 2013 Heather Johnson
Dead Thistle in Grass from Vinegar © 2013 Heather Johnson
Yellow Patched in Grass from Vinegar © 2013 Heather Johnson

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How to Kill Thistles Organically – Without Chemicals

You’re heard the saying about a weed being any plant growing where it isn’t wanted? I can tolerate a variety of “weeds” in a lawn, but thistles are not one of them. Thistles are frustrating if you’re trying to have a kid friendly lawn – what kid wants to walk barefoot on a prickly thistle? Not mine. So, what to do. I don’t want prickly weeds for my kids to step on but also don’t want to use potentially dangerous chemicals on the lawn.

Here is one approach I’ve been using over the last year with good success. It’s a little unorthodox, but if you’re stuck between a thistle and a chemical, it might just be worth a try for you.

Step 1: Purchase a Syringe

Yes, it’s a little strange to use a syringe for organic weed control, but stay with me here. There are a variety of places online that sell syringes…here’s a 10 ml syringe that will work well available from a website I used.

Step 2: Purchase Adios Organic Weed Killer

Adios Organic Weed Killer is a newer organic weed killer that works better than others on thistles. There are a lot of organic weed and grass killers out there (like Burnout Organic Grass & Weed Killer) that work great at killing the above ground portion of plants but don’t work so well at killing the roots.

Burnout is perfect for spraying on weeds in your mulch beds and sidewalk cracks etc. but isn’t ideal for spraying in the lawn. Adios has some degree of an ability to travel into the plants root system and be absorbed giving it a much better chance to kill the entire plant, rather than just the above ground portion.

I’ve used the Adios Concentrate in my trials to date, but you might be able to get the same results with the ready to use Adios – I’ll try that next.

Step 3: Mow the Lawn

Mowing should cut the top off the thistle and expose the stem. If it doesn’t to this, you can break off the top with your hand (heavy glove recommended). Just try to weed it without pulling out the entire root.

Step 4: Apply Adios to Kill your Thistles

After you’ve cut the grass, get out your Adios and your syringe and fill the syringe with Adios by sucking it either directly out of the container or by pouring the adios into a bowl and sucking out of that.

Insert the syringe needle into the stem of the thistle as far as you can and slowly push in the plunger to release the Adios into the stem. If you pull the syringe out slightly as you press in the plunger, it will allow the adios into the stem more easily. I usually do this twice on each plant.

Step 5: Soak the thistle with Adios from a Sprayer as well.

Go ahead and spray the leaves and into the stem with a ready to use Adios mixture to make sure you’re getting as much adios into the plant as possible. You can mix a batch of concentrate in a pump sprayer or just use a ready to spray bottle like the one pictured below.

Step 6: Watch the Thistle Disappear Slowly.

This is the fun part. It will take some time. If it doesn’t seem affected after a week, try a reapplication. This isn’t a magic bullet, but it can be really effective and is a much better tool to get rid of thistles organically than we’ve ever had.

More Information about How to Kill Thistles Without Chemicals

Thistles have a very invasive root system which is one reason why they are so difficult to get rid of. The ideal time of the year to do these injections is in the late fall when the thistles are pulling all their energy into their roots – they’ll pull the Adios into their root system most effectively when this is happening in the fall. The thistles I injected this way last fall have yet to return and it’s already July, so the results look very promising. I have done some this summer that have not returned yet, but I won’t be surprised if they do and I need to do a reapplication this fall.

So, if you’re trying to get rid of thistles, the best organic way I know of to do it is to inject Adios directly into the stem.

Good Luck!

Yes, REALLY Tough Weeds Like Thistle and “Running Bamboo” CAN Be Beaten!

Question. Mike: A neighbor planted ‘running’ bamboo as a “natural fence” around his property, and not surprisingly, it’s out of control. The culms are about 12-16 feet high; both plant and root system are invading my property. It also blocks out the sun and sucks up all the available water so that I can’t seem to get anything else to grow in my backyard. I’ve read that bamboo is technically classified as a “perennial grass” which means that growing it violates a township ordinance requiring grass and weeds not to be higher than 10 inches. If the township orders it to be removed, how do we get rid of it safely without damaging the environment? Thank you.—Curtis in Cherry Hill, NJ.

Hello, Mike McGrath: What can you recommend to get rid of thistles in my garden and lawn? I have tried extreme weeding and professional chemical treatment, but they’re back!!!!!!! Thank you.

    —Valerie, Rockville, MD

style=”font-weight: bold;”>Answer. These are two of the toughest weeds you can face, especially the bamboo, which ‘runs’ at an astonishing rate and will happily take over acres I fallowed to— although your terminology is somewhat off, Curtis. Those aren’t plants AND roots invading your domain—its one big plant with one giant honkin’ root system that expands to cover the earth like horticultural concrete a few inches under the surface. Some thistles form similarly impenetrable root systems. (Val— never let thistles flower; if you do, then you’ve got roots and seeds to worry about!)

These are not easy plants to beat, and as Val discovered (Bad girl!), toxic chemical herbicides won’t do the trick. Those poisons are good at killing off single plants, but they don’t affect huge underground root-systems; so don’t waste your time, money and life fooling around with them.

There are three basic ways to do the job well and safely, all of which involve you first cutting the above ground growth to the ground repeatedly. Cut it all down, allow it to grow again; cut it all down again, let it grow again, etc. Two, three, four times; the more the better to deny the roots their solar energy collectors. In fact, if you just do this continual cutting for several years, the plants and roots will eventually die.

For more immediate satisfaction, cut and then do one of the following:

    1. To kill a patch on your property alone, mulch, mulch, mulch the entire area with something THICK and HEAVY (sheet-metal, old carpeting …) weighted down with a few inches of soil or woodchips on top. May be soak the area with a high-strength vinegar (see #3) first. Make sure the mulch extends a good couple of feet past where the plants were growing. Regardless, the root system will likely send plants out on a scouting mission and try to creep up around the edges. Be vigilant, and mulch these pioneers and/or spray them with high-strength vinegar. Leave the mulch in place for at LEAST a year. Or better still, leave it there forever, and make a nice raised bed filled with ‘wanted’ plants over top. READ COMPLETE ANSWER

  1. 2. If the bamboo is spreading to your property from a neighbor’s place and you just want to keep it on their side, cut yours down and drive barriers deep into the ground at the property line to keep more from spreading over. Then use high-strength vinegar and/or mulch to kill the roots on your property.

Here’s some barrier info from one of our favorite sites, www.Americanbamboo.org (this is also a great place to learn more about bamboo—not all types are bad; there are many well-behaved varieties that grow in tidy clumps): “To prevent running bamboo from spreading, a “rhizome barrier” two or three feet deep is essential. It should be slanted outward at the top so that when the rhizomes hit the barrier they will bend upwards. A barrier does not stop a running rhizome; it only deflects it. The barrier should project an inch or two above ground level. Check the barrier once a year, and cut off rhizomes that arch over the top.

“Barriers can be concrete, metal, or plastic. The usual recommendation is high-density polypropylene (40mil or heavier), glued, taped, or clamped with stainless-steel at junctions. This material comes in rolls or as hinged sections, and is available from some landscape suppliers and bamboo nurseries, frequently termed root barrier. More elaborate barriers with corner posts that hold the material at the proper angle are also available.” McG: We don’t need more plastic in the world, so I strongly suggest metal instead of the poly.

Essentially what you’re doing here is building an underground fence, and before you can build that fence you’ll have to dig a trench to hold it. (Unless you have John Henry illusions, rent a machine to do the job.) And, if you make it wide enough; say a foot across; that trench alone will make an excellent bamboo barrier. Even better, turn your problem into a water feature! Dig a deep trench in between you and the bamboo/thistle/other super weed and then fill it with water and make it a kind of canal running along your property. Put in some fish and some a quatic plants and you’re happening! (Like vampires, these weeds can’t cross over running water.)

  • 3. And then there’s the favorite high-intensity weed-killing solution of YBYG listeners, high strength vinegars.
  • Cut everything down, wait till the soil is bone dry and no rain is predicted for at least a few days and then, in the heat of the day, soak the earth containing those unwanted roots with one of the products below. (Be careful; you must wear protective gear, especially safety glasses). The high acidity of the vinegar will lower the soil pH down to something like 3—the surface of the moon. All plant and soil life will die, earthworms and larger creatures will quickly run or squirm away and that region will become a dead zone. Leave it like that for at least a month—longer if you can. (And if you fear that ‘your’ plant has more lives than Christopher Lee in an old Dracula movie, do it again a week or two later.) When you’re sure it’s really most sincerely dead, raise the pH back up with wood ashes or lime to between 6 and 7 (use teststrips or a meter) and soil life will return and the ground will be fertile again— but the roots will stay dead.

    “Burn out weed killer” is St. Gabriel’s Labs mixture of vinegar and lemon juice; it now also contains clove oil and is called “Burn Out II”(the sequel!)— but this is for normal weeds; it’s not strong enough for things like thistle or bamboo. They recently introduced a double-strength version called “Poison IvyDefoliant” that should do the job. St. Gabe’s products are available at retail outlets or direct from them at 800-801-0061; www.milkyspore.com.

    “Green sense 20% acidity vinegar” is white vinegar that’s four times more potent than the household variety. (“You watch the weeds die.”) Its available in some retail stores in the Southeast, and Rohde’s in Garland, Texas (near Dallas) will ship you a gallon for $11.95 plus $8.50 shipping; call 972-864-1934, or visitwww.beorganic.com and enter “white vinegar” in the search function. (There’s a photo at www.greensense.net) Although, this stuff is incredibly powerful, it is all-natural—and not all high-strength vinegars are. The folks at Rhode’s stress that their Greensense product is a grain based vinegar, not a petroleum-based product like the Acetic Acid used in photography. Once it’s done its work, a grain-based vinegar will return its nutrients to the earth, and allow life to colonize the soil once again. Chemically produced acetic acid will leave toxic resides that will destroy soil life for perhaps years to come; don’t use them.style=”font-weight: bold;”>SAFETY NOTE: All of these products require extreme caution on the users part, especially the 20% vinegar! This is not harmless stuff! Vinegar with such enormous acidity is really caustic! You have to be careful not to get any on your skin or eyes–gloves and goggles are a must!!!!

    And that, dear readers and listeners, is why many of you asked Rhode’s to come up with something a bit gentler, and they did. (And yes, they tell us, it was entirely at the request of YBYG listeners, from whom most of their mail-order sales come! Thank you, Rhode’s!) The new creation is “Greensense 10% Acidity vinegar”; $7.95 plus $8.50 mail order. You’ll have to use it more than once (the 20% is like dropping a ‘natural nuke’ on those roots), but it is much safer to handle. “You pays your money and you takes your choice.”

    “Greenergy Blackberry & Brush Block”, 8% vinegar (apple cider or wine) and 5% citric acid, is a West Coast product (apparently, wild blackberry vines attack people’s cars and children out there). You can get a gallon from Professional Turf Center in Portland for $36 (includes shipping); call 1-800-894-7333(Regular household 5% white vinegar will only kill ‘easy’ annual weeds.)

    You Bet Your Garden ©2004 Mike McGrath

    Getting rid of thistle in 3 steps

    Q uestion: How can I get rid of thistles in my ground cover? I do not want to kill off my ground cover. It is on the hillside in front of my house. The area is very sunny. Full sun almost the whole day. The ground cover is very thick. It has small, needle-like leaves and is low to the ground. I can’t remember the name of it but it’s very old and took a long time to fill in. Please help. I don’t want to rip it out and start over, but it is beginning to look like my only option.

    Answer: Getting rid of any weeds in ground cover is problematic, but thistles present an extra challenge. Not only do thistles spread via seed, their thick, white roots also spread underground, causing new plants to pop up on a regular basis. Weeds that spread in this fashion should never be tilled or the problem will become worse as each root piece the tilling process leaves behind will develop into a new plant.

    A few months ago, I wrote a column on how to manage thistles in an empty garden bed, but managing them in a ground cover is a whole different ballgame.

    No matter what weed you’re dealing with, when it grows throughout a bed of ground cover, I recommend following these steps.

    1. Never let the weed set seed. Whether it’s grass, clover, bindweed, or thistles, it’s important to not let the weed’s flower turn into seed. Doing so will only make the problem far worse. Even if it means you chop down the weeds with a string trimmer or loppers every week or two, it will prevent seeds from being produced. It also weakens the weeds over time, making it easier to control.

    2. The second step in controlling weeds in ground covers is to hand pull them religiously. While this task isn’t easy, it’s crucial to controlling the weed population. Even if you don’t get every bit of a weed’s root out, you’ll be weakening the plant, and with repeated top-killing like this, the weed is eventually starved of carbohydrates and dies. It may take a season or two, but going out and weeding the ground cover bed on a weekly basis makes a huge difference.

    3. You can also target-apply organic herbicides to the weeds. The brands I most often recommend are Avenger and BurnOut (both of which are available from various online sources, including Amazon and planetnatural.com). The trick with using these products in ground covers is always to apply it just to the weeds and not to the ground cover itself. These products do not discriminate and will kill any plants they come in contact with. To target the application, put on a chemical-resistant glove and then soak a sponge with the herbicide. Hold the sponge with your gloved hand, and wipe it up the weed, from its base to its top, squeezing the herbicide onto the plant’s leaves and stems. This keeps the herbicide off the ground cover. You may have to repeat this process a few times in order to eliminate all the weeds, but it is very effective.

    As with all herbicides, it’s important to follow label instructions carefully. Organic herbicides are often based on acetic acid which can be very caustic to skin, so heed all the precautions on the label.

    A combination of all three of these steps, done in a consistent fashion, will clear your ground cover of its weedy visitors.

    Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden,” “Good Bug, Bad Bug,” and her newest title, “Container Gardening Complete.” Her website is jessicawalliser.com. Send your gardening or landscaping questions to [email protected] or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.

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      Controlling Canada Thistle – Canada Thistle Identification And Control

      Perhaps one of the most noxious weeds in the home garden, Canada thistle (Cirsium arvense) has a reputation for being impossible to get rid of. We won’t lie to you, Canada thistle control is difficult and requires a significant amount of effort to be successful, but the effort you put into controlling Canada thistle will pay off when you have a garden that is free from this annoying weed. Let’s look at how to identify Canada thistle and how to get rid of Canada thistle.

      Canada Thistle Identification

      Canada thistle is a perennial weed that has soft green, deeply lobed, spear-like leaves and these leaves have sharp barbs on them. If allowed to go to flower, the flower is a purple pom-pom shape that will be produced in clusters at the top of the plant. If the flower is allowed to go to seed, the flower will become white and fluffy, much like a dandelion seed head.

      How to Get Rid of Canada Thistle

      When starting a Canada thistle control program, it is best to first understand what makes Canada thistle such a difficult weed to control. Canada thistle grows on an extensive root system that can go quite deep into the ground, and the plant can grow back from even a small piece of root. Because of this, there is no one and done method of Canada thistle eradication. Whether you are controlling Canada thistle with chemicals or organically, you will need to do so repeatedly.

      The first step towards getting rid of Canada thistle is making your yard and garden less friendly to it. While Canada thistle will grow anywhere, it grows best in soil with low fertility and open areas. Improving your soil’s fertility will weaken the Canada thistle and help desired plants grow better and, therefore, make them better able to compete with the Canada thistle. We recommend having your soil tested at your local extension service.

      Chemical Canada Thistle Control

      Canada thistle can be killed with weed killers. The best time to apply these is on sunny days when the temperatures are between 65 and 85 degrees F. (18-29 C.).

      Because many weed killers are non-selective, they will kill anything they touch, so it is best not to use these on windy days. If you need to treat Canada thistle where it is close to wanted plants, you might be better off using a paintbrush to paint the weed killer on the Canada thistle.

      Check back weekly and reapply the weed killer as soon as you see the Canada thistle reappear.

      Organic Canada Thistle Control

      Controlling Canada thistle organically is done with a sharp eye and an even sharper pair of scissors. Find the base of the Canada thistle plant and simply snip it off at the base. Do not pull Canada thistle out, as this can split the root, which causes two Canada thistles to grow back.

      Check the location weekly and snip off any new growth that you may see. The idea is to force the weed to use up its energy reserves by regrowing but removing the new leaves before the Canada thistle has a chance to build its energy reserves back up.

      Note: Chemical control should only be used as a last resort, as organic approaches are more environmentally friendly.

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