Images of thistle plants

Bull thistle identification and control

Bull thistle is a widespread biennial thistle originally from Europe and Asia, but now introduced throughout North America. Although it is intimidating in appearance and can sometimes form large infestations, this thistle is not as challenging to control as many others and is mainly a problem in hay fields and pastures. Bull thistle is also commonly found along trails, roads and vacant fields.

Legal status in King County, Washington

Public and private landowners are not required to control infestations of bull thistle that occur on their property in King County. Bull thistle is a Class C Noxious Weed in Washington, first listed in 1988. Because control is not required in the county, it is on the list of Non-Regulated Noxious Weeds for King County. For more information, see Noxious Weed Lists and Laws or visit the website of the Washington State Noxious Weed Control Board.

Bull thistle might be confused with native thistles as well as more serious noxious weeds such as milk thistle, a Class A noxious weed in Washington, and Canada thistle, a Class C noxious weed that is much more difficult to control.

Identification (see below for more photos)

  • Branching, erect biennial, 2 to 6 feet tall
  • Rosettes form in first year, flowering stems the second
  • Long, sharp spines on the leaves at the midrib and the tips of the lobes
  • Leaves are deeply lobed and hairy – there are coarse hairs on leaf tops, making leaf feel rough to the touch, and woolly hairs on the underside
  • Leaf bases extend down onto stems and form spiny wings along the stems
  • Pink-magenta flower heads top each stem
  • Flower heads are “gumdrop” shaped and spines extend all around the base of the flower heads
  • Flowering June to September

Habitat and impact

Bull thistle prefers sunny, open areas and can tolerate a wide range of conditions, from moist to dry soils, and is typically found in disturbed areas such as roadsides, trails, logged areas, vacant land, pastures and cultivated land. Overgrazed pastures are susceptible to bull thistle encroachment, and it can sometimes form dense stands that reduce productivity and stocking levels. Bull thistle may also dominate forest clear cuts and reduce growth of tree seedlings.

Growth and reproduction

Bull thistle has a two year life cycle, flowering and setting seed in the second year. Seeds are short-lived on the soil surface but can persist for many years when they are buried, such as form cultivation activities. Seed germination generally occurs in the fall and spring. Basal rosettes form and continue to grow until winter and can grow quite large, up to 3 feet in diameter. Rosettes that are not large enough by spring may not flower until the following year. Flowering usually starts in mid-June and continues into early fall. Plants can be self-pollinated or insect-pollinated. Bull thistle does not reproduce vegetatively and does not have rhizomes.

Control

Prevention: Bull thistle only reproduces by seed so prevention of seeding and takign care not to spread seeds are key to preventing new infestations. Contaminated hay is a primary means of spread of this species so be careful to purchase weed free hay or watch closely for new plants in the areas hay is kept or spread. Do not leave cut stems of flowering bull thistle on the ground because they are likely to form viable seed after they are cut.

Manual control: Bull thistle can be dug up with a shovel. Usually removing the top couple of inches of root is sufficient to kill the plant, especially after it has bolted (produced stems). A shovel or other tool can used to chop off leaves from one side of the plant to gain easier access to the roots, which can then be dug up. Flowering stems should be collected and destroyed to keep them from forming viable seed.

Mechanical control: Mowed thistles will produce new branches from basal buds but close cutting or cutting twice per season will usually prevent seed production and reduce the population over time. More more effective control, cut plants with a sharp shovel at 1-2 inches below the soil surface prior to flowering. If only one cutting a year is possible, cut when plants are in bud for best results. Cultivation and tilling can also be effective to control bull thistle.

Chemical control: For information on herbicides and recommendations for control, please see the Pacific Northwest Weed Management Handbook. There are several herbicides that are effective on bull thistle, but it is important to carefully match the product with your site, local conditions and regulations, and other weed and land management issues, and always follow the label directions for the product you are using. If your site has animals grazing, please read instructions carefully on grazing restrictions. For grassy areas, it is best to use a selective broadleaf herbicide to keep the competitive grasses intact. After spraying, wait two weeks or more to give the herbicide time to work.

Biological control: The bull thistle seed head gall fly (Urophora stylata) lays eggs on closed flower buds in June and July. After hatching, the larvae burrow into the seed-prodcuing tissues to feed, forming galls and reducing seed production. If the bull thistle population is large enough to support a good sized population of this insect, it can be an effective way to reduce seed production of the bull thistle. This insect will not get rid of the bull thistle, however, just reduce its impact. See the biological control page for more information.

Cultural/grazing: Bull thistle is not as competitive in a well-managed pasture and regular cutting combined with good pasture management can reduce bull thistle to a manageable level. Also, goats and sheep have both been used for grazing management of bull thistle. Even horses will help by picking out many of the nectar-rish flowers and eating them before they go to seed.

Additional information on bull thistle

What to do if you find this plant in King County, Washington

Because bull thistle is so widespread, property owners in King County are not required to control it and we are not generally tracking infestations. We can provide advice on how to control bull thistle, but there is generally no legal requirement to do so.

Knock Thistle Out Of Your Garden For Good Without Herbicides

I’ve become an unwilling expert on the weed known as thistle. Living where I do in Pennsylvania, we’re besieged by the plant in spring, and when not controlled, thistle causes problems for years. Fortunately for you, I have a lot of experience successfully ridding thistle from my own and other gardens and am happy to report that it’s not an impossible task if you’re willing to do a little work when it first appears. Okay, I just lied right there – sometimes it requires a LOT of work. But no matter how good you are at exterminating this devil of a plant, you’ll never be completely free of thistle, so get used to it and learn its habits.

Thistle – specifically Canada Thistle – appears over a wide area of the U.S. and obviously, Canada (Bull Thistle and Creeping Thistle are closely related). This cool season perennial (Zones 2-8), known for its difficulty to kill and its desire to take over your garden completely and totally, is a scourge for gardeners. But once we get over our thistle anxiety and learn how to keep it in check, it becomes a routine weeding task.

Flowers of Canada Thistle are very beautiful. But don’t be tempted to keep them or you’ll be overrun.

Canada Thistle spreads by seeds and by creeping roots called rhizomes, which makes it especially invasive when left unchecked. Give it an inch or two of bare ground near your Joe Pye Weed, and it will fill it in eagerly, and also kill your Joe Pye Weed in the process.

Thistle appears in spring, shows its flower buds in late May or early June, and by the end of June is in full flower. Shortly after this, and definitely by mid-July, thistle flowers have produced seeds, easily and widely dispersed by wind, ignorant landscapers (don’t get me started…), and wildlife. In September, after the dog days of summer have passed, new shoots emerge from the roots, which survive until cold weather sets in, usually in November. This new growth replenishes underground store reserves to give the plant lots of power to overtake more of your garden the following spring.

Chemical herbicides aren’t always successful against thistle

Many gardeners, frustrated at the seemingly perpetual battle with this garden tyrant, reach for chemical herbicides out of sheer desperation. Unfortunately, chemicals only kill the leaves and flowers. But the uninitiated gardener is at first satisfied since the scourge of her garden appears to be deceased.

What the gardener doesn’t realize is that the herbicide has done nothing to the thistle’s massive and deep root system, which will produce new shoots and foliage shortly after, confounding the chemical-wielding gardener in all manner. Of course, one could perpetually spray, but they would also wipe out all of the surrounding plants. Herbicides kill indiscriminately, not to mention their effect on wildlife, soil, and your health. Organic control of thistle is more effective than chemical control, although it requires some diligence and considerable elbow grease.

How to control thistle organically

You must keep the flower heads of thistle, and hence the seeds in check. When flowers appear, yank them off immediately and toss them in the compost bin (unless they’re mature – a few weeks old and well-developed – then toss them in a trash bin). By keeping the plant from flowering and seeding, you’ll save yourself a lot of work next year. Much to every gardener’s chagrin, thistle seeds have been known to remain viable in the soil for up to twenty years, so get those flowers when they first appear, if not the whole plant.

Keep thistle in check and exhaust its energy by cutting off the foliage whenever it appears. Foliage collects energy from the sun, which is used to produce more foliage, roots, flowers, and shoots. Interrupt this flow and the plant will eventually give up. By eventually, I mean the end of that year’s season, so keep cutting the foliage whenever you see it (you’re in this for the long haul, right?). The roots will continue to send up new shoots each time you hack off its foliage, so keep checking the surrounding area – it’s a persistent plant, to be sure. Also, attempt to dig the plant out wherever you can – use hand tools that extend below the surface and have a serrated blade to cut through the roots.

Young Canada Thistle. Weed this out as soon as you see it.

Shade is by far the best way to control thistle. When it pops up in your shade garden, it has little energy to pop up again if you nip it in the bud. When it appears early in the season near your tomatoes, don’t fret too much. By mid-season, the tomatoes will shade it out and the thistle will give up, assuming you’ve been knocking it back while young.

But give thistle a wide-open, sunny expense of clay soil, and all hell breaks loose, as it will fill up that space almost overnight. So the idea is to fill in every possible space in your garden with plants you love, to shade-out and out-compete the thistle seed and young shoots. If you have any open space, pop an annual or a ground cover in there, or use a nice vegetative mulch to fill it in and smother the thistle. If you don’t allow thistle the opportunity to start, you’ll rarely see it.

Best hand tool for weeding thistle: CobraHead Original Weeder & Cultivator Garden Hand Tool

But let’s say you have a massive thistle problem, like if you’ve neglected an open space. The best way to control thistle, in this case, is to mow it down very early in the season and then smother it. Use old blankets, carpeting, sheet metal, untreated lumber, etc., and then cover that material with 3 inches of mulch. Make sure you extend the material about a foot in each direction beyond where you see the thistle, as the plant will attempt to send new shoots up from the fringes of its root system. A heavy mulch alone may work, but you’ll probably need 6 inches or more to keep the thistle from finding its way out.

If you’re especially concerned with its return, after mowing, soak the thistle and the soil with a white vinegar solution before covering it up. The vinegar should knock out the remaining foliage, and once deprived of light by the material on top, it will expire. But only soak the area with vinegar when dry weather is forecast for at least 3 days. The vinegar solution takes some time to fully work, and wet weather will dilute the vinegar, even when covered. Leave the area covered for an entire year before planting anything in that space. I do not recommend that you use a vinegar solution to control thistle in an actively growing garden, as it will make the soil surrounding the thistle very acidic, which may kill delicate plants and even shrubs.

Thistle is what’s called a “pioneer” plant, like dandelion. Give them bare earth, and they will fill it in promptly. This “pioneering” actually provides a great service in the wild, by protecting the soil from erosion until larger plants can take over, and by feeding pollinators like butterflies where little else may exist. And by virtue of thistle’s deep tap roots and extensive root systems, many air pockets and water pathways are created in the hard soil, encouraging the soil ecosystem along. So don’t hate on thistle too much – it’s only doing its job.

Read more on controlling thistle from Cornell University.

Fewleaf thistle (Cirsium remotifoilum), a native thistle

Thistles are one of the most common weed problems that property owners deal with in Clackamas County. Many of the thistles we encounter are invasive and can grow to dominate a property. But did you know there are at least four thistles that are not only non-invasive, but native to western Oregon? These native thistles are not only well-behaved, but are beneficial to the health of our working lands and natural areas.

Native Thistles are Important!

Native thistle provide important habitat and food sources for native fauna. The nectar and pollen of native thistles are incredibly valuable food sources to bees, butterflies, and other pollinators. Many insects feed on the leaves, stems, flowers and seeds, while some songbirds also feed on thistle seeds. These nectar sources help support pollinators year-round, and can help to increase yields for many valuable crops.

The presence of native thistles also makes it harder for the aggressive non-native thistles to invade an area. Native thistles help to support healthy populations of beneficial insects that will also consume non-native thistles. Our native thistles also remain in balance with other native plants and do not aggressively displace other plants. Despite their benefits, native thistles are either knowingly or unknowingly killed because it is assumed they will become a big problem, or simply because they have spines, or are considered “weeds”. Because of this, many native thistles are in danger of being completely eradicated from our area.

The Problem with Non-Native Thistles

Milk thistle (Silybum marianum) infestation in Clackamas County

On the other hand, non-native thistles can overrun an area and displace native plants, reduce agricultural yield, and create problems for grazing animals when they infest a field or pasture. They also cost a lot of money and time to control. One of the struggles with controlling thistles is that it can be difficult to distinguish between them. We are here to help!

General Thistle Characteristics

Thistles are members of the sunflower family (Asteraceae) and can be annual, biennal, or perennial. True thistles have spines along the leaf margins and their flower heads are generally pink-purple and surrounded by bracts that are typically spiny. Below are some questions to help you determine whether your thistle is native or not:

  • Is the thistle spiny along the entire length of the stem? (a YES answer indicates Italian thistle, slenderflower thistle, Scot’s thistle, or bull thistle)
  • Are the bracts triangular, firm, and spine-tipped? (a YES answer indicates Italian thistle or slenderflower thistle)
  • Are the bracts thick, and leathery, and jagged? (a YES answer indicates milk thistle)
  • Are the roots rhizomatous? (a YES answer indicates Canada thistle)

If the answer to all of these questions is “NO”, it is likely you have a native thistle!

Native Thistles

Clustered thistle (Cirsium brevistylum) Edible thistle (Cirsium edule) Fewleaf thistle (Cirsium remotifoilum) Wavyleaf thistle (Cirsium undulatum) Clustered thistle (Cirsium brevistylum) Edible thistle (Cirsium edule) Fewleaf thistle (Cirsium remotifolium) Wavyleaf thistle (Cirsium undulatum)

  • Clustered thistle/Indian thistle/Short-styled thistle (Cirsium Brevistylum):
    • Annual, biennial or short-lived perennial
    • Stems are fuzzy and mostly unbranched (branched/clustered and more fuzzy near the top)
    • Flower heads are “cobwebby”
    • Flower heads are red- purple and have small white stalks (the “styles”) extending out individual flower tubes less than 1 mm
    • Bracts around the flower heads are long and tapered
    • Leaves point upward, with the top leaves extending above the flower head
    • Leaves are shallowly lobed, have somewhat weak spines, and are woolly on the underside (sometimes slightly woolly on the top)
    • Plant grows up to 7 feet tall
  • Edible thistle (Cirsium edule):
    • Biennial or short-lived perennial
    • Looks similar to the clustered thistle, but stems are more branched, and leaves are more strongly spiny
    • Flower head is very cobwebby and hangs on young plants
    • Flower tubes have styles that extend out 3-8 mm
    • Leaves are somewhat woolly on the underside, especially along the midrib
    • Leaves at the base can be 15 inches long
    • Plant grows up to 9 feet tall
  • Few-leaved thistle (Cirsium remotifolium)
    • Biennial or short-lived perennial with a lot of variation between plants
    • Stems can be either woolly or smooth
    • The plant has weak spines, and is sparingly branched on the upper half
    • Flower heads can be creamy-white to purple and are only moderately cobwebby
    • Bracts are spiny with papery margins that might by slightly fringed
    • Leaves are woolly on the underside, deeply lobed, and spiny
    • Leaves may have short stalks, or they may be clasping the stem with no stalk
    • Plant grows up to 6 feet tall
  • Wavy-leaved thistle (Cirsium undulatum)
    • Perennial
    • Stems are branched on the upper half and are matted with white hairs
    • Flower heads are typically solitary, pale lavender to white, and up to 2 inches wide
    • Bracts have a sticky midrib and are typically tipped with spines that stick out
    • Leaves are hairy on top and bottom
    • Leaf margins have shallow wavy lobes with a yellow spine at the tip
    • Plant can grow up to 4 feet tall

Non-native Invasive Thistles

Bull thistle (Cirsium vulgare) flower head Canada thistle (Cirsium arvense) flowers Blessed milkthistle (Silybum marianum) flower head Italian plumeless thistle (Carduus pycnocephalus) flowers Scot’s thistle (Onopordum acanthium) flowers Bull thistle (Cirsium vulgare) plant Canada thistle (Cirsium arvense) plants Blessed milkthistle (Silybum marianum) rosette Italian plumeless thistle (Carduus pycnocephalus) Scot’s thistle (Onopordum acanthium) plant

  • Bull thistle (Cirsium vulgare):
    • Common in Clackamas County, moderately invasive, see our weed profile
    • Biennial
    • Stems are leafy, stout, branched, and somewhat woolly
    • Flower heads are large, rose to purple with many sharp spine-tipped bracts
    • Leaves are irregularly lobed, with the end lobe being longer than the side lobes, and all tipped with yellow prickles
    • Leaves are woolly on the underside
    • Leaves have noticeable “wings,” (flat projections extending down the stem)
    • Plant can grow up to 6.5 feet tall
  • Canada thistle (Cirsium arvense)
    • Common in Clackamas County, highly invasive, see our weed profile
    • Perennial plant that spreads by underground stems
    • Stems are slender, ridged, and hairy
    • Flower heads are purple, small (less than 2 cm), and clustered
    • Bracts are broadly triangular, and typically not very spiny
    • Leaves are small, lobed, and the margins have very sharp yellowish prickles
    • Plant can grow up to 6.5 feet tall
  • Blessed milk thistle (Silybum marianum)
    • Uncommon but present in Clackamas County, highly invasive, see our weed profile or Best Management Practices
    • Biennial (sometimes annual)
    • Stems are ridged and branched in the upper half, with sparse hairs
    • Flower heads are large (up to 2 inches) and purple
    • Bracts are stout, jagged, very sharp, 1-2 inches long
    • Leaves are dark green with distinct white marbling
    • Leaves at the base can be up to 20 inches long
    • Plant can grow up to 8 feet tall
  • Italian plumeless thistle (Carduus pycnocephalus) and slenderflower thistle (Carduus tenuiflorus)
    • Not known to exist in Clackamas County
    • Two annual, similar-looking species
    • Stems are woolly and very spiny-winged, with branches that point upward
    • Flower heads are pink – purple, small (0.5 inches wide), and grow in spiny clusters of 2 to 5 (on Italian) and 5-20 (on slenderflower)
    • Bracts are hairy and spiny, the hairs on Italian thistle bracts are stiff and point forward
    • Leaves are deeply lobed and woolly on the underside, lobes are tipped with spines
    • Plant can grow up to 6 feet tall
  • Scot’s thistle (Onopordum acanthium)
    • Not known to exist in Clackamas County
    • Biennial (sometimes annual or short-lived perennial)
    • The whole plant is very branched, strongly spiny, with a bluish gray appearance
    • The stem is woody and very spiny-winged (flat leaf projections extend down stem)
    • Flower heads are purple and 1-3 inches wide
    • Bracts are tipped with flat, pale, orange spines
    • Leaves have triangular lobes, are spiny, covered in silvery hairs, and can be up to 1 foot long
    • Plant can grow up to 8 feet tall

Want to dig deeper?

There are several other species that we did not discuss here, either because they aren’t known to be in western Oregon, or because they aren’t true thistles… and this post is already long enough! Follow the links below for more information about these “thistle-like” counterparts .

  • Purple starthistle, Iberian starthistle, and yellow starthistles are not true thistles, but are actually knapweeds. These are a serious problem! Please report them if you find them.
  • Woolly distaff thistle, smooth distaff thistle, and Russian thistle (aka. “tumbleweed”) are also not true thistles. They are not known to occur in Clackamas County, so please report them.
  • Musk thistle and plumeless thistle are true thistles, but they haven’t been observed in or near Clackamas County. Please report them if you think you have them.
  • Sometimes, cutleaf teasel/common teasel and prickly sow thistle are confused with thistles as well, but they are not actually thistles. These weeds are quite common so they don’t need to be reported, though we encourage active management by landowners when they occur on their property.

Purple starthistle Woolly distaff thistle Musk thistle Common teasel Prickly sow thistle

Slender Thistles

Common and scientific names

Slender thistle, shore thistle

Carduus pycnocephalus L. (slender thistle) Carduus tenuiflorus Curt. (winged slender thistle) Family Asteraceae (daisy family)

Origin and distribution

Figure 1. Slender thistle, Carduus tenuiflorus.Figure 2. Slender thistle, C. tenuiflorus rosette.

Slender thistles are native to Europe and North Africa. The range of C. pycnocephalus extends to Asia Minor and Pakistan while that of C. tenuiflorus extends northwards to Britain and Scandinavia. They are a problem in many areas of the world. Both species were present in Victoria during the 1880s and now occur throughout much of the state. Slender thistles are troublesome weeds in pastures and wastelands, favouring areas of winter rainfall and soils of moderate to high fertility. The two species oftenoccur together in mixed populations.

Description

Erect annual herbs, commonly 60 to 100 cm high but up to 2m, reproducing by seed. Seed germinates in the 6 weeks following the autumn break. Seedlings develop into rosettes and remain in the rosette stage over winter. Flowering stems are produced in early spring and flowering continues from September to December. Plants die in early summer after flowering, but dead stems can remain standing for months.

Stems – flowering stems are single or multiple from the base, branched, strongly ribbed and slightly woolly. Spiny wings occur along most of the length of flowering stems.

Leaves – rosette leaves 15 to 25 cm long, stalked and deeply lobed with numerous spines on each lobe. Stem leaves shorter, deeply lobed with spines along the margin and with the base forming wings along the stem.

Flowers – pink or purple, in slender heads 2 cm long or less, lacking stalks; surrounded by numerous spiny bracts (modified leaves at the base of flower). Heads in groups at the ends of the branches or in the leaf axils.

Seeds – two types in each head: inner seeds and outer seeds. Inner seeds, comprising about 85% of seeds, occur in the central part of the head and have longitudinal striations. They are 4 mm long and grey brown in C. tenuiflorus and 5 mm long and yellowish fawn in C. pycnocephalus. Outer seeds are the same size but are grey in colour. All seeds have a group of plumes (the pappus) about three times as long as the seed for wind dispersal.

Roots – branched, slender or stout tap root.

The problem

Figure 3. Slender thistle, C. pycnocephalus flowering heads.

Slender thistles are found mainly in areas with an annual rainfall exceeding 500 mm. Seedlings establish preferentially on bare and disturbed sites such as stock yards, sheep camps, rabbit burrows and heavily grazed annual pastures. They are competitive weeds in improved pastures, outperforming subterranean clover and ryegrass, and significantly reduce pasture production, but tend to occur irregularly from year to year. Slender thistles are avoided by stock because of their spines and this encourages their spread in heavily grazed pastures. The spines can cause mechanical injury to stock and dogs. Slender thistles are potentially toxic to stock due to high levels of nitrate. Their spines and dead leaves contribute to vegetable fault in wool. The pollen produced in late springis a useful bee forage.

Table 1. Features used to distinguish the two species of slender thistle.

Carduus pycnocephalus Carduus tenuiflorus
Rosette Stage
Stalks of rosette leaves always green Stalks of rosette leaves often purplish or with red/purple flecks
Leaves shiny, green, with lighter coloured areas at base of leaf spines (usually) Leaves bluish-green. No lighter coloured areas at base of spines on leaves (usually)
Flowering Plants
Wings discontinuous on the stems Wings continuous on the stems
Stems usually without wings beneath the flower heads Stems with wings to the base of the flower heads
Flower heads in clusters of 3 or 4 Flower heads in clusters of 3 to 8
Inner spiny bracts on flower heads are shorter than the adjacent florets Inner spiny bracts on flower heads are as long or longer than the adjacent florets

Dispersal

Slender thistles are dispersed solely by seed which can be carried for long distances in the wind. Seeds have a parachute of barbed hairs (the pappus) which aids wind dispersal and attaches to clothing and animal coats, particularly the fleece of sheep. Goldfinches and other granivorous birds eat the seed but they remove the husk before consumption so do not disperse viable seed. Seed is spread in contaminated hay, silage and grain and on farm machinery, and can be dispersed in water.

Management

Figure 4. Slender thistle, C. pycnocephalus.Figure 5. Autumn-spring spores of slender thistle rust are purplish on the leaf undersides and reddish-brown on the uppersides.

Prescribed measures for the control of noxious weeds

  • Application of a registered herbicide
  • Physical removal

Important information about prescribed measures for the control of noxious weeds.

Other management techniques

Changes in land use practices and spread prevention may also support slender thistle management after implementing the prescribed measures above.

Biological Control

Biological control is a long term program which is best used on large, chronic infestations with a low priority for control due to inaccessibility, remoteness or low threat of spread. The single agent released to control slender thistle in Victoria, the rust fungus, Puccinia carduipycnocephali, will assist in integrated management of slender thistles.

The slender thistle rust has occurred in Australia since early this century. It infects both species of slender thistle and is common, but this particular strain of rust has had little effect on slender thistle infestations. Two new strains selected in Europe by the CSIRO Division of Plant Industry, have been released at numerous sites in Victoria since 1995. Strain IT2 from Italy and strain FR3 from France, are extremely virulent on Australian types of C. pycnocephalus and C. tenuiflorus respectively, although each strain will attack the other species. Thorough host specificity studies have demonstrated that the rust presentsno danger to other species of plants.

Rust infection starts on new seedlings in autumn and builds up to epidemic proportions by spring, covering most of the leaves and flowering stems. Infection reduces the vigour and subsequent seed set of plants and makes them more susceptible to competition from desirable pasture species. Rust infection reduces plant height, plant mass and the production of viable seed. The rust has spread considerable distances and control appears to be most effective in wetter summers.

The thistle receptacle weevil, Rhinocyllus conicus, released in Victoria to suppress spear and variegated thistles, and the thistle rosette weevil, Trichosirocalus horridus, released in Victoria for control of spear thistle, attack slender thistles in New Zealand. Specific strains of these insects suitable for control of slender thistles have not been released in Australia.

Further advice

  • contact your local landcare or friends group for further assistance and advice.
  • Call the DEPI Customer Service Centre on 136 186.
  • Visit the Weeds Australia website at: www.weeds.org.au

Slender thistle suppression with the slender thistle rust fungus. Landcare Notes: Biological Control LC0146, Frankston, Keith Turnbull Research Institute.

Code, G. (1998) Weed Control in Pastures. Rutherglen, Department of Natural Resources and Environment.

Parsons, W.T. and Cuthbertson, E.G. (1992) Noxious Weeds of Australia. Melbourne, Inkata Press.

What’s that Weed? Creeping Thistle

June 18, 2018

Type: Creeping Thistle is an herbaceous perennial weed with sharp, prickly leaves (sometimes with lilac-pink flowers) that emerges in early June and can infest landscapes throughout the U.S. through late October. A member of the Asteraceae family of thistles, this weed can spread rapidly via its highly aggressive spreading root system, or its ability to scatter thousands of seeds in a short time.

Where It Grows: Native to Europe and Asia, this weed has been widely introduced to most of the globe. In the eastern U.S., it dominates north of the Ohio River and the Mason-Dixson line (with exception to West Virginia, where it is has also become well-established). West of the Mississippi River, Creeping Thistle has spread across most of the United States with exception to the southern Great Plains.

Creeping Thistle is the most common thistle species in the U.S. It prefers recently disturbed/cultivated grounds (such as vegetable gardens), road side and field edges, and other patches of relatively barren soil. Barren patches of turf provide an ideal germinating for wind-dispersed thistle seeds. The best defense against a creeping thistle invasion is a dense, well-established turf.

How to Identify: Creeping Thistle consists of tiny pink/lilac florets which comprise a larger flower head. Each complete head sits atop a spiny bract (a circular arrangement of leaf-like tissues at the apex of the stem). Its slender green stem ranges anywhere from 30 to 150cm tall. Its spiny leaves alternate around the stem, topping out around 20cm in length. Regularly mowed turf is unlikely to see this weed in the flowering stage, however Creeping Thistle is notorious for its spiny leaves which can leave a painful prick.

Growing Season: Creeping Thistle tends to emerge from the soil in early June, with its bright purple flowers dominating roadside landscapes by mid-July. When large stands of this weed begin to seed, the flower heads take on a dense cotton-like appearance. A burst of wind can send thousands of seeds into the air in a matter of seconds. Creeping Thistle continues growing and flowering through October.

How To Manage: There are two ways to remove Creeping Thistle, hand removal or herbicide. You can hand remove Creeping Thistle from gardens or lawns using thick gloves to avoid their prickly leaves. However, since Creeping Thistle produces an extensive root network so you need to assure all roots have been successfully removed to avoid re-establishment (even traces of roots can lead to new thistle shoots). Each individual plant can send up multiple shoots, giving the appearance that there are many more individual plants than in reality. In addition, when removing by hand you want to avoid spreading the flower’s light fluffy seeds. In order to prevent germination of thistle seeds, make sure any large thistle stands are removed before flowering. Creeping Thistle seeds can quickly be blown to other areas of your landscape and rapidly take over. Once removed, throw the weed away and do not try to compost them because traces of roots or seeds will quickly establish new plants.

If herbicides are the preferred method of removing Creeping Thistle, apply when the weed is actively growing in the spring or early fall. You can use a broad-spectrum herbicide that will not damage trees or other ornamentals in a shared root zone (such as 2,4-D MCPA, Triclopyr or Dicamba) or a non-selective herbicide (glyphosate) when you can safely target the weed directly. In most cases, apply the herbicide in a heavier than normal concentration, as Creeping Thistle has a root network often larger than its above ground shoot mass and multiple treatments may be necessary. Once applied, DO NOT mow or cut back the thistle in any way; allow the herbicide to absorb into the shoot tissue so that it may be transported down to the roots.

Prevention: Creeping Thistle typically thrives in low fertility soils so amending the soil and improving the environment for your preferred plants and turf can reduce its appearance. You can also avoid this weed by applying a pre-emergent herbicide/fertilizer application in the early spring that will prevent it, or any thistle species, from taking hold of your lawn throughout the season.

Check out TurfCare’s catalog of professional turf products to view the pre emergent or post emergent herbicides and soil amendments available.

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Ten Weeds to Pull Now

Ask any of our horticulturists to name the most important job in their gardens right now and they’ll all answer the same way:

“Weeding.”

In this month’s Smart Gardener, we take on the topic that gardeners love to hate, focusing on the top ten weeds most commonly found in gardens in the Chicago area.

What is a weed? Essentially, it’s a plant that’s growing where you don’t want it to be. Weeds are opportunistic, springing up where there’s a void in the landscape, where the soil has been disturbed, or where birds and mammals have eliminated the seeds of the fruits they’ve eaten.

Weed ruthlessly in spring and early summer, rather than waiting for weeds to grow—they’ll be easier to remove, less likely to have spread, and won’t use up the precious nutrients and water from the soil that you want your other garden plants to have. Cultivate a love of weeding—smart gardeners know it’s worth it.

#1: Bindweed
Convolvulus arvensis

Every gardener knows the horror of bindweed: the perennial, twisting, vine-like weed that climbs up the stems of other plants, defying attempts to unwind or pull its counterclockwise cling. If neglected, bindweed forms a thick mat and an extensive root system that overwhelms any garden bed—or even farm field.

Get familiar with bindweed’s arrow-shaped leaf and search for the first tendrils at ground level while weeding. If the weed is already established, pull and clip the plant repeatedly to exhaust its roots. Our plant healthcare manager recommends this trick if you’ve spotted an established vine: set up wooden stakes for it to cling to (rather than other plants); then remove the plant stake.

#2: Buckthorn
Rhamnus cathartica

North Shore residents know buckthorn well. It is the shrubby tree that pops up in a hedge or wooded area, then chokes out every other plant, cutting off sunlight as it spreads. As its name warns, buckthorn has thorns, adding injury to insult for those who forget to wear gloves and goggles while removing it.

Like all weeds, buckthorn is best removed when small. Dig the plant up entirely. Not sure if that sapling is a buckthorn? Identify it in fall, as its leaves stay green on the branches much longer than most other trees’ branches.

#3: Canada thistle
Cirsium arvense

Canada thistle looks like a weed, with tufty seedheads and spiny, pointed leaves that stick out like a sore thumb (wear gloves while pulling it) in your garden bed.

One reason that Canada thistle is so common: its root system spreads by runners, allowing it to produce many new plants and return year after year. To eradicate it, pull all running roots.

#4: Crabgrass
Digitaria sanguinalis

Some weeds are indicators of soil health and condition; crabgrass indicates compacted soil, which is one reason you’ll find it in lawns and at the edges of sidewalks.

Unlike lawn grass, crabgrass grows in a rosette of leaves, spreading by both seeds and creeping stems. Its seedheads name it, as they resemble crab claws.

Because crabgrass is an annual, the efficient way to prevent it from spreading is to prevent it from flowering. Cut the rosette off with a knife so it won’t self-sow. (Mowing doesn’t work, as the low-growing plant is below blade height.)

#5: Creeping Charlie
Glechoma hederacea

The name says it all: insidiously creeping stems allow this weed to spread like a ground cover. A perennial, creeping Charlie can thread its way through, above, below, and around other plants, making it difficult to remove.

Identify it by its scalloped leaves and those square, creeping stems. To remove: trace it back to its root nodes, then dig those up repeatedly throughout the season.

#6: Dandelion
Taraxacum officinale

It’s the poster child of weeds, yet dandelion has undeniable value, too. Its young leaves are vitamin-rich edible greens, and its bright yellow flowers are one of the earliest and best sources of nectar for emerging insects, especially bees. And who among us hasn’t delighted in picking a posy of dandelions or blowing clouds of fluffy seedheads?

But it’s the dandelion’s taproot that’s the real issue for gardeners. To remove it, wait until after a rain, then dig the taproot out completely, or that same dandelion will return next year. An old-fashioned dandelion fork is a great tool for the job.

And if you don’t quite get all the roots this year, let the survivors bloom for the bees next year before you tackle that taproot again.

#7: Garlic mustard
Alliaria petiolata

So invasive is garlic mustard that just one plant can spread by seed quickly, forming colonies that choke out native plants in woodlands and shady gardens.

Garlic mustard is a biennial that grows into a clumping rosette of toothy leaves in its first year, then flowers and sets seeds in its second year. The key is to pull it in its first year—a task that our ecologists and volunteers repeat each spring here in the McDonald Woods. As its name declares, the plant smells strongly of garlic when crushed.

#8: Nightshade
Solanum nigrum

Nightshade can be crafty in a garden bed, vining and climbing and camouflaging its wavy-edged leaves through other plants. Most gardeners don’t notice it until it has already flowered and set its distinctive clusters of mostly black berries, which are both numerous and potentially poisonous.

Nightshade grows where the ground has been disturbed; keep your garden beds healthy and uncompacted to prevent this weed. Pull or dig it out completely at the root.

#9: Poison ivy
Toxicodendron radicans

Even children learn the phrase “Leaves of three, let them be,” the apt descriptor of poison ivy.

The perennial plant can be a trickster, growing low and shrubby or high and vining. But it’s the allergic reaction that some humans have that’s the real risk—touching the shiny leaves or burning the plant can release the chemicals that cause rash and uncomfortable itching.

Always wear rubber gloves to pull or dig the plant, and dispose of immediately.

#10: Yellow nutsedge

Indicative of wet conditions, and often found in lawns, yellow nutsedge can look like regular lawn grass until it grows taller, powering up to a potential 8- to 36-inch height. Check the leaves for identification—they feel stiffer than lawn grass, and the stem is triangular (it’s a sedge, not a grass).

Pull this weed as soon as you spot it—if left unchecked, it develops underground tubers and nutlets that make it difficult to control later. Feathery, golden flowers can produce seeds too.

Karen Zaworski is a garden writer and photographer who lives and gardens in Oak Park, Illinois.

Washington State

Cirsium vulgare

Family: Asteraceae

Weed class: C
Year Listed: 1988
Native to: Europe, Asia and Northern Africa
Is this Weed Toxic?:

not known to be

Why Is It a Noxious Weed?

Bull thistle may outcompete native plants and desirable wildlife and livestock forage plants. It can invade most any disturbed habitat and grow in dense thickets. Hay price may decline with the presence of bull thistle.

How would I identify it?

General Description

Bull thistle is a biennial herbaceous plant growing between 3 to 7 feet tall with one upright branched stem. It grows a rosette (cluster of radiating leaves at plant base) in its first year and blooms in its second year.

Flower Description

Flowerheads many, 1.5 to 2 inches in diameter. Bracts at base of flowerheads are spine-tipped. Flowers are purple or rarely white, blooming July through September.

Leaf description

Leaves alternate and coarsely lobed. Each lobe has a spined tip. Leaf bases extend downward from the leaves along prominent ridges of the stem. Upper leaf surface is rough with bristle-like spines while the undersides are covered with white woolly hairs.

Stem description

Stems are spiny-winged from leaf bases extending downward producing a winged ridge effect.

Fruit Seed Description

Seeds are less than 0.16 inches (4 mm) long.

Where does it grow?

Bull thistle colonizes primarily in disturbed areas such as pastures, roadsides, and ditch banks as well as in hayfields, disturbed prairies and logged mountain areas. Please click here to see a county level distribution map of bull thistle in Washington.

How Does it Reproduce?

Bull thistle reproduces by seed and not by vegetative means.

How Do I Control It?

Mechanical Control

Hand-pull and dispose of flowering plants in trash to prevent seed spread. Mowing can be effective, but make sure the plants do not flower. If cut too early before flowering, plants may re-sprout and flower again that season. Remove stems from site if plants are cut or pulled with flowers.

Bull thistle seed production is impacted by the seedhead gall fly, Urophora stylata. This gall fly’s larvae induce and feed on gall tissue in the developing bull thistle seedhead, reducing seed production up to 60%. Urophora stylata may significantly reduce seed production if bull thistle populations are sustained for many years. For more information about the biological control of bull thistle, please visit WSU Extension Integrated Weed Control Project

Herbicide Control

Please refer to the PNW Weed Management Handbook, or contact your county noxious weed coordinator.

For More Information

See our Written Findings for more information about bull thistle (Cirsium vulgare).

Report on bull thistle from the book “Weed Control in Natural Areas in the Western United States”

Cowlitz County NWCB Fact Sheet on bull thistle

Whatcom County NWCB Fact Sheet on bull thistle

Lincoln County NWCB Fact Sheet on bull thistle

Clark County NWCB Fact Sheet on bull thistle

San Juan County NWCB Fact Sheet on bull thistle

King County NWCB Fact Sheet on bull thistle

King County NWCB Control Options for bull thistle

Legend has it that the thorny thistle once saved Scotland from a marauding Norse army, a feat that earned this tenacious plant its status as a Scottish national symbol. But these days it’s staging an invasion of its own, causing people all over the British Isles, and elsewhere, to declare war on this invasive weed.

More’s the pity in my opinion, because I rather like the prickly beauty of thistles. Luckily there are some garden-friendly varieties, that with proper care, won’t run amok. Here are five of our favorites:

(Invasive) Scots Thistles

Above: Photographer Francisco Gonzalez captured this quintessential image of Scotland: Scottish thistle in the Highlands. But throughout the world, invasive Scots or cotton thistles and their cousins threaten native species. Prairies and grasslands in America and Canada are being overrun, and in Australia the problem was so bad that Parliament imposed heavy penalties on those who did not control thistles on their land.

Globe Thistles

Above: Misty globe thistles (Echinops ritro) by Isidre Blanc via Wikimedia.

The gentler cousin to the Scottish thistle, non-invasive globe thistles (Echinops ritro) are ideal for the garden. With deep blue or violet orbs perched on silvery stems that are from 2 to 4 feet tall, globe thistles provide a striking architectural and textural element to the garden. Drought resistant, these hardy perennials are also easy to care for. Bees, butterflies, and lady bugs love them too. Hardy in growing zones 3-9. Globe thistles are readily available at most garden nurseries. A 5-Inch Pot Of Blue Glow Globe Thistle is $8.99 at High Country Gardens.

Giant Cotton Thistles

Above: A giant cotton thistle in Prague. Photograph by Karelj via Wikimedia.

Giant cotton thistle (Onopordum acanthium) was prized during the Middle Ages for its herbal properties, and by the poor who used its fluffy “down” to stuff mattresses. At from 10 to 15 feet high with a 5-foot spread, this dramatic biennial is not for small gardens. It benefits from staking and from stony soil (which provides better support for its roots). But note: Giant cotton thistle is invasive. To prevent the seeds from scattering to the wind, it is important to cut the heads off after flowering. You can also see that, with all those spikes, it should only be handled by gloved hands.

Plume Thistles

Above: Cirsium rivulare ‘Atropurpureum’ by Jean Jones.

Magenta heads atop long, leafless, and spike-less stems make ornamental plume thistle (Cirsium rivulare ‘Atropurpureum’) an excellent garden choice. Rivulare literally means “growing by a stream,” and this perennial prefers moist, yet well-drained ground; fertile, slightly acidic soil, and full sun. Cut back after early summer flowers to promote another flowering, and then to the ground after the last bloom in the fall. Though popular in Europe, this Cirsium is pretty rare stateside, so you’ll get points for originality with this one. A 1-Gallon Pot Of Cirsium Rivulare ‘Atropurpureum’ is available at Far Reaches Farm seasonally for $16.

Globe Artichokes

Above: A globe artichoke in flower in South Africa. Photograph by Marie Viljoen. For more, see My Mother’s Garden in Constantia.

Last year, Michelle discovered that artichokes are members of the thistle family, Asteroideae, as well. (See A Thistle That Won’t Misbehave.) Though its wild counterpart artichoke thistle is invasive, stately globe artichokes (Cynara cardunculus) and their cardoon cousins make a dramatic statement in the garden, and, of course, can be eaten as well. (With cardoons, you eat the stem, which is said to taste like a celery artichoke medley.) Artichokes and cardoons prefer lots of sun and rich, well-drained soil. To build up a plant’s strength, during the first year cut off the heads as soon as they appear. Replace the entire crop every four years. For more information, refer to this extremely thorough article from The Daily Mail.

Sea Holly

Above: Photograph by Erin Boyle.

Not really a thistle at all (actually a distance relative of the carrot family), sea holly (Eryngium), is often considered a worthy stand-in for thistles. I love it for my seaside garden, where it endures not only drought and poor soil, but also salty air. About the only care it requires is deadheading to encourage additional blooms.

N.B. Thistles and sea holly also dry well for use in winter arrangements and crafts. One of my favorites is Erin’s Black Thistle Bouquet.

Finally, get more ideas on how to successfully plant, grow, and care for thistle with our Thistle: A Field Guide.

Finally, get more ideas on how to plant, grow, and care for various perennial plants with our Perennials: A Field Guide.

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