- Killing Garlic Mustard: Learn About Garlic Mustard Management
- Why Garlic Mustard Management is Important
- How to Control Garlic Mustard Plants with Small Infestations
- Garlic Mustard Weed Control with Large Infestations
- Don’t Spread the Seeds!:
- Prescribed Burns:
- Flame Weeding of Seedlings:
- Treating Seedlings with Organic Herbicides:
- Is Treating Seedlings Worth It?:
- Dormant Season Herbicide Treatment of Rosettes:
- Pulling During the Flowering Stage:
- Mowing, a Last Resort:
- Spraying Garlic Mustard Over the Summer?:
- While you’re out there, control these other invasive plants too!
- Healthy Woodlands Resist Garlic Mustard:
- Management summary
- Detailed management options — Small infestations
- Detailed management options — Medium to large infestations
- Detailed management options — Regional infestations
- Biological control of garlic mustard
- Garlic mustard management strategy in Michigan
- Garlic mustard identification and control
- History and impacts
- Legal status in King County, Washington
- Identification (see below for additional photos)
- Biology and morphology
- Additional information on garlic mustard
- What to do if you find this plant in King County, Washington
- Project Overview
- Garlic Mustard
- Additional Images
Killing Garlic Mustard: Learn About Garlic Mustard Management
Garlic mustard (Alliaria petiolata) is a cool-season biennial herb that may reach up to 4 feet in height at maturity. Both the stems and leaves have a strong onion and garlic odor when crushed. It is this odor, particularly noticeable in the spring and summer, that helps to distinguish mustard weed from other mustard plants commonly found in woodlands. Occasionally garlic mustard can become weedy; therefore, it is important to become familiar with garlic mustard weed management.
Why Garlic Mustard Management is Important
Garlic mustard was first introduced to Europe and used both medicinally and for cooking. But garlic mustard plants are also known as garlic mustard weeds because they produce hundreds of seeds per plant. These seeds travel on the
fur of large animals, such as horses and deer, and also in flowing water and by human activity.
Because of this, garlic mustard spreads in woodlands and quickly takes over native woodland wildflowers. When this happens, it’s a good idea to know how to control garlic mustard plants.
How to Control Garlic Mustard Plants with Small Infestations
When infestations are small, hand-pulling plants is the best way of killing garlic mustard. Pull plants early in the season before they flower. Also, pull plants, being sure to get as much root as possible, while the garlic mustard weeds are small and the soil is wet.
Tamping the ground after removal will help keep the plants from re-sprouting. If it is too difficult to pull plants, you can cut them as close to the ground as possible before they form seeds as part of your garlic mustard weed control.
Garlic Mustard Weed Control with Large Infestations
Garlic mustard weed control needs to be aggressive when infestations are large. Burning large patches of garlic mustard in the fall or spring is sometimes effective. However, three years of burning may be required to fully eliminate the weed.
More severe infestations may be controlled chemically by using a glyphosate solution in late fall or early spring. However, caution should be used when working with glyphosate for killing garlic mustard, as it will also kill other vegetation in its path.
Garlic Mustard: flowering second-year plants above, young first year plants below.
UPDATE (3/3/17): I’m continually updating this post over the years as new information comes to light, we develop new techniques, and as I find new typos! You can consider this an up-to-date, thourough guide to garlic mustard control.
Doing some research online for herbicide rates recently, I ran across a lot of incorrect and misleading information on controllinggarlic mustard (Alliaria petiolata). So I was inspired to write this post. A lot of people don’t realized that fall and early spring are actually the best times to manage garlic mustard, so if you think you have more garlic mustard than you can handle, get out there and get ahead of it!
Each garlic mustard plant can produce hundreds, even thousands of seeds. This allows there population to increase rapidly and establish a lot of seeds in the seed bank. The seeds don’t generally move far from the parent plant, most fall or are flung just 2′-5′ away. Often you can see small patches of garlic mustard in rough circles around 4′ in diameter where an adult plant was able to go to seed the previous year. Only a few seeds will travel further, carried in flowing water, on animal fir, or in mud on animals’ feet (including your shoes!), and vehicle tire treads.
From a technical standpoint, garlic mustard is among the easiest invasive species to control in our region. Despite the large number of seeds they produce, garlic mustard is a biennial, they only live for 2 years. The first year they remain a low rosette and gather energy. The second year they bolt, flower, produce seed and die. So all you have to do is prevent them from developing seeds before their time is up. Garlic mustard is susceptible to a variety of herbicides, it’s shallow-rooted, and has a low fire tolerance. It stays green at times of year when most native plants are dormant. This gives us a lot of management tools that we can wield against this otherwise rapidly reproducing pest.
The two most important factors in control garlic mustard are: 1) Determination: to completely exterminate every plant each year so there is no seed production, and 2) Patience: since the seeds can live for up to 7 years in the soil, you have to figure that this battle is going to take a decade to win. Below is a ‘battle plan’ for the year-round management of this invasive pest.
Don’t Spread the Seeds!:
You can’t do much to kill off the garlic mustard seeds in the soil. You have to wait for them to sprout so you can do something about them. But you can prevent their spread! If you have been walking or driving through an area that has garlic mustard, thoroughly clean your boots’ treads to avoid moving these seeds to other sites. This goes for vehicle tires and the feet of animal companions too. If you’re moving through an area where there are upright dead stalks from last year’s plants, be sure to clean out your socks and pant cuffs too.
Garlic mustard seeds are black, cigar shaped and just over 1/8th inch long. Photo courtesy www.http://foragingpictures.com
The garlic mustard seedlings look a lot like the sprouts you buy from the health food store, but often carpet the woodland floor almost as thick as moss. I have seen them sprouting as early as mid-February. They are delightfully easy to kill. Prescribed fire can wipe them out en-mass.
Freshly sprouted garlic mustard seedlings. These can be hard to differentiate from the seedlings of other plants visually, but they have the characteristic garlic mustard smell when crushed. Photo courtesy of The Buckthorn Blaster.
Prescribed burns will also negatively impact the second-year rosettes to a greater or lesser extent depending on the intensity of the fire and the health of the individual plants. I have seen hot burns kill over 50% of the mature garlic mustard plants in a woodland, whereas other times it seems like it’s 0%.
We feel that prescribed burns are a critical tool for controlling garlic mustard, particularly the seedlings. Considering the many other benefits of prescribed burns in woodlands, repeated spring burns during the early years of your garlic mustard control work will improve the health of your woodland greatly.
A prescribed fire like this one can be the most efficient tool in your garlic mustard control arsenal.
Time the burn for early-to-mid spring (typically this is in late-March or early-April) after a lot of them have come up, but be aware of any early sprouting native plants, particularly spring ephemeral wildflowers that might be in the area. If you spring ephemerals in your woods, you will need to get the burn completed before these wonderful woodland wildflowers sprout. These spring ephemerals are perennials, and will survive a fire, but repeated burns will weaken them and delay or prevent flowering for the year.
Flame Weeding of Seedlings:
Not able to do a full-out prescribed burn? A propane torch like those sold by the Red Dragon company works very well at killing off seedlings. The fire will only top-kill any mature plants (the roots will survive to resprout). So it won’t kill second year garlic-mustard rosettes. On the other hand any native perennials that might literally get in the line of fire will also readily resprout from the roots as well.
Flame weeding is a little slower than an herbicide spray application, but has a very low risk for collateral damage and no residual impact that some herbicides may have. It is an extremely satisfying control method as you watch hundreds of them wilt and die immediately. It makes me laugh like a mad scientist every time. With a flame-thrower in my hand and a propane tank on my back, I’m sure I look like one too.
Torching garlic mustard seeds should work very well from the early seedling stage when they just have a pair of their cotyledons (baby leaves) up until their first set of real leaves are about the size of a quarter. After that the kill rate may go down slightly, but it’s probably worth doing until mid-May.
These more mature seedlings have developed their first true leaves, but are still vulnerable to prescribed fires, flame weeding and organic herbicides.
Most people try a little too hard with the flame weeder. It only takes a little heat and you will see a slight color change to the plant, they may wilt slightly. That’s all you need. When you come back in a few minutes you will see that they have wilted and are goners. Usually I sweep through an area torching them lightly and when I’m done, go back over it for any spots that didn’t quite get hot enough.
An important note here is that you want to be careful not to accidentally start a little wildfire. Avoid days of low humidity and high wind. Its safest if the humidity is above 50% and the leaves are NOT dry and crinkly. If there is little or no leaf litter or dead plant material in there area, you’re probably ok. Test it out in a small area first to see if the burning leaves fizzle-out or if the fire spreads on its own. Remember that conditions can and will change as the day goes-on, so even though a fire might not carry in leaf litter in the morning, that doesn’t mean it won’t flare up and take off in the afternoon. Always keep a hose or portable water sprayer on hand just in case. Also, have a phone in your pocket ready to call the fire department. Better safe than sorry.
Treating Seedlings with Organic Herbicides:
If running around your property with a flame thrower seems a little extreme to you, another option for killing seedlings is to use an organic herbicide such asPhyduraor Nature’s Avenger. Both of these herbicides are mild, naturally derived acids that dissolve the waxy cuticle layer on a plant’s leaf causing them to dry out and die. Like fire, these organic herbicides only top-kill plants, but that’s enough to wipe out little garlic mustard seedlings that haven’t developed much of a root yet. Its not nearly as much fun as flame weeding, but Smokey The Bear would prefer you use this method.
Is Treating Seedlings Worth It?:
There is some debate about weather its worth the effort to conduct control work on the first-year seedlings, there are a several key talking points:
First, is a little thing called “intraspecific competition”, which basically means that the seedlings of the same species all compete with each other for survival, and only a small fraction, perhaps 1 in 10, 1 in 100, or less, make it to adulthood. So any seedlings that you fail to treat or kill will have an easier go of it, with plenty of growing space since you’ve taken out the competition, and they no longer to compete with their siblings for resources.
Because of this, some people will argue that treating seedlings is a waste of resources. If you have the time to do it, curtailing the raw numbers of plants make later management much easier. It also eliminates the effect that a “carpet” of garlic mustard can have on the native woodland flora, giving our wildflowers some growing space back so they can reclaim some of the woodland floor that had previously been lost.
UPDATE (4/11/16): I don’t talk a lot about the ecological impacts of garlic mustard in this blog post, its long enough just covering the topic of control! However, I think its worth pointing out that garlic mustard has an allelopathic impact on soil fungi, that is to say, it releases chemicals into the soil that kill our native soil fungi. Many native plants are dependent on, or strongly benefit from a symbiotic relationship with these soil fungi. The plants provide carbohydrates from photosyntasis to the fungi. In return, the fungi absorb various nutrients from the soil through their fine mycelium, which they pass on to the plant. As a result, the garlic mustard can have a very dramatic impact on the ground layer flora, strongly reducing the species diversity and abundance of native plants, more from this chemical warefare than from direct competition. And it appears that the impacts to the soil biotic community are significant and potentially permanent. It stands to reason the loss of these soil fungi will negatively impact tree health as well. Considering these impacts, I’d recommend trying to control garlic mustard seedlings if you have the time and resources to do so. Working to preventing them from getting established in the first place can reduce or prevent harm to soil micro and macro-organisms, wildflowers and potentially trees that live in your woodland.
The second issue to consider when tackling garlic mustard in the seedling stage is that by killing off this first wave, you’ve effectively recruited their reinforcements. New seedlings will often immediately germinate to take their place. This may seem discouraging, but I believe that it’s better to flush as many of these seeds out of the seed bank, so we can start killing them too. Bring them on! The second wave of seedlings is rarely as dense as the first, so either way you’re better off.
Dormant Season Herbicide Treatment of Rosettes:
The real key to controlling garlic mustard is targeted herbicide applications when most of our native plants are dormant. This works both in late fall and in early spring. In spring, you can begin shortly after the snow has melted away. In order for this method to be effective the air temperature needs to be above 35 degrees (often we’ll wait until it’s above 40 just to be sure). At these times of year the garlic mustard is easy to find since they are just about the only thing that is green. Try to get it done before the native woodland wildflowers come out, or else you will have to be very careful to spray around them.
The herbicides we use to spray garlic mustard includes glyphosate-based herbicides (same active ingredient as Round-Up®) since glyphosate breaks down fairly quickly in the environment and has no residual activity as it binds strongly to clay particles in the soil upon hitting the ground and is broken down by bacteria within a few weeks. Other glyphosate-based herbicides include Accord®, Razor® Pro, Makaze® and Glypho-Star. If you’re working near wetlands of any kind, use an aquatic-approved formulation such as Rodeo®, Aquaneat®, or AquaMaster®.
A Note On Recent Glyphosate Research:
There have been some studies recently that correlated the use of glyphosate-based herbicides by farmers to an increased risk of certain forms of cancer. Bear in mind that this group of people is at a very high exposure level to the chemical, often using handling concentrated pesticides and applying it with no personal protective equipment, day-in, day-out for decades. They also have a very high exposure to many other agro-chemicals. Also remember that correlation does not imply causation. More research needs to be done to determine if there really is a causative link here here, and what the mechanism of pathogenesis might be. For the time being, many, many other studies indicate that glyphosate is very safe for animals (less toxic than salt-water!) and that it breaks down fairly quickly in the environment so there’s no risk of long-term contamination. We will update our practices and recommendations if future research suggests a greater risk.
If there is a risk of harming native grasses or sedges nearby, many of which also stay green pretty late into the fall and green up early in the spring, then we will use a triclopyr-based herbicide such as Garlon® 3A or Vastlan®. Triclopyr takes longer to break down, roughly 3 months, and has some residual activity, so it may kill any broadleaf seedlings that try to sprout in the treated area during this time. On the other hand, triclopyr is “practically non-toxic” to animals (though you always have to wonder about the unspecified “inactive” ingredients in the herbicide solution as they constitute 55% of the mix!)
Tryclopyr herbicides seem to be harder to find, but you should be able to find Garlon 3A at Ben Meadows, Forestry Suppliers or your local farm co-op should be able to order it for you. We like to work with Rick Schulte of Crop Production Services in DeForest as he is highly knowledgeable about the safe use of pesticides to control invasive plants.
Which herbicide to use and at what application rates are a bit of a contentious subject among land managers. One mistake we often see is people mixing their herbicide with tap water, which is often alkaline (a mild base) in our region. This can partially de-active some herbicides, such as glyphosate, which are mild acids. Its best to use rain water or use a water condition such as Choice or Watersoft to neutralize your water before mixing in the herbicide.
We also add a dye to our herbicide mix, such as Hi-Lite Blue to help us keep track of what’s been sprayed and what hasn’t. I like to use 1 oz of this dye per gallon of water to allow us to see which plants were sprayed and which weren’t even a week or more later.
Before I recommend herbicide application rates, I need to say that my recommendations are not a substitute for the pesticide label. The label is the law; read it and follow the instructions before applying any pesticide.
I also want to add that you should always wear the personal protective equipment (PPE) as recommended on the label. I would go above-and beyond to recommend anyone applying herbicides should be wearing long pants, long sleeves, waterproof boots, “rubber” gloves (we like disposable nitrile gloves) eye protection and a hat or other head covering, to reduce the risk of direct exposure. If there is a breeze, always spray down-wind of yourself and use extra caution when mixing or otherwise handling concentrated herbicides.
For early-spring or fall application when there is a relatively low leaf area compared to the root mass we like to have a higher concentration of active ingredient in the mix. For glyphosate-based, terrestrial herbicides, we recommend using 6 oz of concentrate per gallon of water. The aquatic-approved formulations of glyphosate have a higher concentration of active ingredient, so use those at around 5 oz/gallon. For triclypyr-based herbicides like Garlon 3A, we recommend using between 4 and 5 oz/ga for this dormant-season application.
As the plants mature and grow they have more leaf surface area to absorb herbicide, so you can dial down the concentration of active ingredient a bit: 3-4 oz/ga of terrestrial glyphosate, 2.5-3 oz/ga of aquatic-approved glyphosate or 2.5-3 oz/ga of triclopyr.
Though its best to apply herbicide as early as possible in the spring before native plants come up (or even in the fall), provided that you take care not to get any overspray on native plants, you can continue to apply herbicide to garlic mustard until it starts to flower and still expect the plant to die before it sets seed.
As for application technique, we use backpack sprayers like this one or for smaller jobs, hand sprayers like this or this. We cruse through the woods in teams in a systematic grid pattern to be sure we traverse the entire area. If time is limited, I recommend focusing on areas with the least garlic mustard so you can cover the most area per time allotted. Then work your way into the more dense areas as time allows. Hopefully this will allow you to push the garlic mustard back further and further to the core of the population each year.
Dormant season spraying does a very good job at killing most of the garlic mustard. Still, we find that roughly 5-10% survive this treatment. Either the herbicide wasn’t entirely effective on a plant or we simply miss them among the hundreds of others as we walk through. As a result, pulling is a necessary step to follow-up after dormant season spraying. The early season spraying reduces the hand-pulling workload considerably, making managing large sites or dense patches of garlic mustard manageable.
Pulling During the Flowering Stage:
This is when most people start treating garlic mustard, when really, it should be the final stage in the management process. If you wait until late-April or May to start working on garlic mustard, you’ve waited too long. If you’re dealing with a small area and just a few dozen or hundred plants, then pulling them can be effective. But hand pulling is labor-intensive and time-consuming, so it’s critical to start early when dealing with sites larger than an acre in size, or dense patches of garlic mustard in smaller areas.
Pulling garlic mustard can be a big job!
The technique here is fairly simple. Grab the plant firmly at the base and pull steadily. If the plant breaks off leaving the root in the ground, try again to get the root out. A small weeding tool, or even a dinner folk can be helpful here. If you don’t get the root, you don’t kill the plant!
Once the plant is out of the soil and in your hands, I recommend shaking the dirt off the roots and snapping the stem below the flower head and/or above the root. This prevent the plant from moving nutrients and water up from its roots to the flower heads. You don’t actually need to break them into multiple pieces, just bend the stem back on itself until you feel it snap. This is especially important for plants that are well into the flowering stage, and we have often see pulled garlic mustard plants that are thrown back on the ground turn their flower head up and continue flowering! It only takes a split-second to snap the plant and it makes for good insurance, just in case. Now repeat this process until you have removed every last garlic mustard in the area. If even a single plant is allowed to successfully reproduce, the result could be hundreds or thousands of seeds, most of which will develop into plants that you will need to pull in years to come.
I like to start pulling-season just before the garlic mustard starts to bloom. At this point the roots have gotten weak and the plants pull out easier. If you get to this task early enough you can simply snap the stems and drop the plants where you pull them and move on or even put them in the compost bin. If you wait until well into the flowering season and you’ll have to bag them out and get them in the trash, which is a lot more work. How do you know when it’s safe to leave the pulled plants just laying on the ground? Our general rule of thumb is that once petals start to fall off the flowers, that means those flowers have been pollinated and are starting the process of producing seeds. Bag ’em.
Notice how the petals have fallen off the lower flowers on this stalk and they are beginning to elongate into seed pods. At this point, you definitely need to be bagging the plants out of the site and disposing of them in the landfill.
For late-season pulling, you will have to collect the garlic mustard in garbage bags and send it to the landfill so that it does not develop seeds. They should not be composted or put on the curb with other lawn waste. As long as you bag out the dead plants, you can continue this work into mid-summer up until the seed pods start to open up and release seed.
In fact, if the plant is showing clear signs of dying back, you can just snip the seed heads off with pruners and only pack that out in your trash bag. This greatly reduces the weight and bulk of what you need to carry around, and prevents soil disturbance and potential erosion caused by pulling. Time is up when the seed pods are dry and shatter easily, meaning you are spreading as many seeds as you are removing.
Mowing, a Last Resort:
Often times we get a call from a new client, asking us to control the garlic mustard in their woods. Unfortunately, they wait until May to call us and when we get there, their woods look something like this:
There’s more garlic mustard here than can be pulled in a reasonable amount of time. It’s too late to herbicide. Though it will kill the plants, they will be able to develop seeds as it dies anyway. So what to do?
Mowing or weed whipping garlic mustard isn’t a perfect solution, but it can cut the reproduction rate down dramatically. We know it doesn’t prevent seed production entirely, but we suspect it cuts it by about 90%. And we always say that 90% control is better than 0% control. Timing is critical though. You need to wait until there are just a few flowers left blooming per plant. At this stage most of the seeds will have started to develop, but will not be mature enough to be viable, and the plant is near-enough to the end of its life that it won’t have the resources to put much of an effort into resprouting from the roots once cut.
We call the technique “obliteration mowing” because it involves complete obliteration of the plant. It uses a string trimmer, aka weed whip, to mince up those seed heads so that the little bits of plant can no longer provide nutrients to the developing seed. You also want to be sure that you cut it all the way to the ground so that the roots have as little reserves left as possible from which to resprout.
For individual plants or small clusters, we often start at the top of the plant and mow down the stalk to the ground. This technique is more targeted and reduced damage to neighboring ‘good plants’.
For larger patches, we sweep the string trimmer head with the machine held so that the cutting strings are spinning perpendicular to the ground through the patch of garlic mustard at the height of the flowers/seed-heads. This first step minces up the seed heads preventing further development. The second step is to sweep back through with a couple horizontal swipes to cut the plant stalk as low to the ground as possible.
Be warned that you will get little bits of garlic mustard all over you. It’s a bit like an outdoor food processor if you’re doing it right.
With a mower, you may need to go over each area two or three times to be sure the plants are minced up enough. If you’re just cutting and knocking them to the ground with the mower deck, then it’s not going to do much good. A heavy-duty field and brush mower is good for this task, our you can take your standard push mower and rear it up on hind wheels for the first pass.
If you get any resprouts you will need to go back and mow it all again, or pull the few straggling survivors. Fortunately its a lot less work the second time around.
Spraying Garlic Mustard Over the Summer?:
We usually allow first-year garlic mustard plants to grow and fester over the summer since we don’t want to do any harm to the native plants that live next to them. But if you have some really bad areas that are nothing but garlic mustard, it wouldn’t hurt to spray these patches over the summer.
While you’re out there, control these other invasive plants too!
Garlic mustard isn’t the only invasive plant in the woods that overwinters green and sprouts early. Take a look at the linked Weed Identification and Control Sheets and get to know them, its critical to control these right along side of the garlic mustard, or they may take over your woods as well:
- Dames Rocket
- Japanese Hedge Parsley
Healthy Woodlands Resist Garlic Mustard:
Just like a healthy person is less likely to get pneumonia, a healthy woodland is going to be much more resistant to garlic mustard invasion. The reason garlic mustard is such a prolific pest is because virtually all of the woodlands in our region are in really, really poor health.
Derek here is standing in a sick woodland. In this case we have a “sugar maple deadzone” blocking out all light to the ground, where there should be a proliferation of woodland wildflowers. Its easy for garlic mustard to thrive if there is no ground-layer competition!
Garlic mustard is a bit of a cheater. It greens up early in the spring, and stays green late into the fall. It can even be photosynthesizing during snow-free periods in the middle of the winter! This gives it a big advantage over our native flora in capturing energy from the sun.
Historically, regular ground fires in our woodlands kept tree seedlings and shrubs in check, creating an environment where a fair amount of sun can hit the ground under an open tree canopy. Our native woodland plants are therefore accustomed to more sunlight. They are also adapted to going dormant in the fall, because naturally wildfires occurred in our region in late fall and early spring. If they had stayed green all winter they would have often lost tissue, and the energy needed to create it, to the regular fires.
Garlic mustard on the other hand evolved in an environment free of fire, and with a dense forest canopy. So it developed adaptations to allow it to gather more light in the off-season.
Decades of abuse in the form of overgrazing by cattle, irresponsible logging practices such as high-grading, and more recently grazing by overpopulated deer, has decimated the plant communities in most woodlands. We have introduced exotic shrub species such as buckthorn and honeysuckle which have spread rapidly in these disturbed woodlands and crowded out native flora. Even trees native to the region that were once rarely or never found in our woodlands due to their intolerance of fire (often including box elder, among others), now crowd the woodland canopy, creating dense shade. Once our woodlands contained hundreds of species of wildflowers and grasses that grew vigorously and bloomed all through the growing season. Today most woodlands have only a handful of ground-layer plants in them, most of which bloom early in the spring and disappear by mid summer, leaving a lot of growing space available for garlic mustard.
Our Midwestern woodlands are now too shaded for our native plants, but just right for garlic mustard. So in order to control garlic mustard in the long term, we must return our woodlands to health. Remove exotic brush. Thin out weedy, fire-intolerant “canopy invading” trees to let more light reach the ground. Re-establish native woodland wildflowers and grasses by seeding and planting. In a healthy woodland, garlic mustard won’t have a chance against our hardy native perennials.
Vestal Grove at Somme Prairie Grove is a rare example of a healthy oak woodland (photographed in August, during the height of the 2012 drought!). Notice how much sunlight reaches the ground layer plants in this woodland. How could a four-inch tall garlic mustard rosette ever survive with this dense competition from native wildflowers!?
Detailed management options
– Small infestations
– Medium to large infestations
– Regional infestations
Biological control of garlic mustard
Garlic mustard management strategy in Michigan
A comprehensive plan for managing garlic mustard via conventional means includes the following elements adapted from Nuzzo (1991). Because garlic mustard is a disturbance-adapted plant, all management efforts should strive to reduce soil and vegetation disturbance to prevent giving further advantage to garlic mustard.
- Where garlic mustard is not well established, efforts should focus on detecting and eradicating new satellite infestations before a seed bank develops (i.e. dormant seeds in the soil). Monitoring should focus on areas where garlic mustard seeds are likely to be dispersed and find disturbed areas suitable for germination. Trails, parking areas, transportation corridors and recreation sites in suitable habitats are known sites of early infestation. (See small infestations)
- Once garlic mustard has established an invasion front (several years of flowering plants), the goal is to prevent further seed set until the seed bank is exhausted; a period of up to 10 years. Depending on the site characteristics and infestation level, pulling, cutting, applying herbicide or repeated fire will be required.
- In small infestations or sensitive plant communities, hand clip or pull seedlings, rosettes or flowering plants prior to seed set. (See small infestations)
- For core infestations (long established, dense populations) in fire-intolerant communities, apply herbicide in spring (if few or no non-target species will be damaged) or dormant season if non-targets are present. In fire-adapted communities, use a mid-intensity burn as late in the spring as possible to avoid undue harm to non-targets, followed by herbicide, cutting or pulling of surviving individuals. (See medium to large infestations)
Pattern of spread and management strategy
Garlic mustard spreads from established (core) infestations along an invasion front. Satellite infestations occur when seeds are transported to new areas. This often occurs along trails, roads or forest edges. Top priority should be given to annual removal of all satellite infestations to prevent further spread.
Detailed management options — Small infestations
Where garlic mustard is not established, efforts should focus on detecting and eradicating infestations before a seed bank develops. Pulling individual garlic mustard plants by hand is the simplest and most effective approach to managing small or isolated infestations. When pulling plants, it is important to remove the upper portion of the roots as well as the stem, since buds in the root crown can produce additional stems. All pulled plants should be removed from the site as seed ripening continues even after plants are pulled. Repeatedly hand pulling of garlic mustard is reported to be effective for control in small areas but has limitations. Because seeds remain viable in the soil for up to 10 years, it is important to pull all garlic mustard plants in an area every year until the seed bank is exhausted and seedlings no longer appear. This will require multiple efforts each year as rosettes can continue to bolt and produce flowers over an extended period (April-June). In fire-adapted communities, regular prescribed burns may deter garlic mustard from entering by stimulating native communities and killing early invaders.
Detailed management options — Medium to large infestations
In medium to large sized infestations, a combination of hand pulling, cutting or localized herbicide treatment is appropriate for managing garlic mustard. First, an aggressive effort at hand pulling all newly established or “satellite” infestations is recommended to limit garlic mustard’s spread. Next, consider managing well-establish “core” populations with a combination of pulling, cutting and or herbicides.
Cutting: Cutting flowering plants at ground level by hand or with a string trimmer or lawn mower will kill a high percentage of garlic mustard plants. The lower the cut, the more effectively the plant will be killed. If the plants are flowering, it is important to remove all cut stems since seed development will continue even if the stem is severed from the root. Because mechanical cutting does not remove the root crown, it may be necessary to cut multiple times in a season to prevent seeds from developing on secondary stems that sprout from the rootstock. It will also be necessary to cut for many consecutive years until the seed bank is exhausted. Unselective cutting with a string trimmer or lawn mower damages desirable vegetation and may slow re-establishment of desirable vegetation or even allow garlic mustard populations to expand. The goal is to selectively remove garlic mustard, leaving the desired plant community intact.
Herbicides: Several herbicides have a role in garlic mustard management. Herbicides should only be applied according to their label directions and using the protective equipment specified. Use of application equipment that can direct the herbicide to the target plant and reduce overspray or drift onto non-target plants is required. This is referred to as spot treatment.
It is very important to limit damage to non-target vegetation. If other plants are killed, garlic mustard will likely replace them. Indiscriminate herbicide applications can thus increase garlic mustard populations! As with cutting, the goal is to selectively remove garlic mustard leaving the desired plant community.
As a cool season herb, garlic mustard continues to grow on snow-free days when temperatures exceed freezing. This provides an opportunity for selective treatment of garlic mustard if applications are made when other plants have not yet appeared (spring) or have died for the year (late fall).
Application of 1-2% glyphosate (Roundup) provides effective control of garlic mustard seedlings and rosettes. Note: glyphosate is a non-selective herbicide meaning that it will kill or damage most plants it comes into contact with (including woody plants). However, to be effective, this herbicide must be absorbed by growing leaf tissue or bark, i.e. the plant must be actively growing. Applications in very early spring (March-April) can often be timed for periods when few if any other plants beside garlic mustard are actively growing. Similarly in late fall, applications can be made with reduced risk to many non-target species. However, glyphosate will damage sedges and other species that are actively growing at this time and therefore susceptible to herbicide uptake. Always take precautions to avoid contacting desirable plants with the herbicide. This may include the hard to see stems of small woody shrubs and trees. Bentazon (Basagran) applied at 8 ounces (by weight) per acre may be an acceptable substitute, less effective on garlic mustard but with reduced risk to some non-targets particularly annual and perennial grasses.
Prescribed fire: Professional land managers trained in the use of prescribed fire may also consider burning for control of medium to large infestations of garlic mustard. The effectiveness of fire differs based on site characteristics and management goals. Most research shows that dormant season fires (March) are ineffective in garlic mustard control and that growing season fires (May) suppress garlic mustard but also adversely affect native understory forbs. Carefully timed spring fires (after garlic mustard emergences but prior to emergence of desirable plants) may be effective. While a single dormant season fire (fall) might actually increase the abundance of garlic mustard the following spring, repeated burns (fall, spring, spring; or spring, spring, spring), have been used to maintain garlic mustard in a reduced condition and stimulate herbaceous species richness and cover. This fire regime did not reduce the number of woody shoots but did decrease their height in the fire-adapted oak woodland where it was tested. Fire accelerated loss of woody seedlings on upland but not lowland sites.
Detailed management options — Regional infestations
Extensive work on conventional controls has failed to yield practical methods for control of generally infested areas on county, state or regional scales. Natural area managers report that the above measures are generally too labor intensive for all but the most critical of sites and overall they are losing the battle with this invasive species. Biological control appears to be a promising option for long-term control of extensive infestations.
For more information
Nuzzo, V.A. 1991. Experimental Control of Garlic mustard in Northern Illinois Using Fire, Herbicide and Cutting. Natural Areas Journal. 11: 158-167.
Nuzzo, V.A., 1996. Impact of dormant season herbicide treatment on the alien herb Garlic mustard (Alliaria petiolata (M.Bieb.) Cavara and Grande) and groundlayer vegetation. Transactions of the Illinois State Academy of Science 89: 25-36.
Biological control of garlic mustard
Research is currently being conducted to explore the potential for biological control of garlic mustard. Classical biological control (also known as importation biological control) is a technique for controlling exotic species by introducing natural enemies of the specific target species from the native range. Ideally, the biological control agents will become naturalized in the new range and maintain populations that swell and recede with the population of the target pest.
In its native range, garlic mustard is a component of forest ecosystems but does not form the same extensive and damaging infestations that it does in North America. In Europe, garlic mustard is found in similar habitats as in North America; however, populations are typically scattered and smaller in size and are consistently attacked by a community of plant feeding insects (herbivores) resulting in reduced shoot number, shoot height and seed output as well as direct mortality. This constant attack by herbivores in the native range appears to reduce the competitive advantage that garlic mustard exhibits in North American habitats. The long-term management goal for garlic mustard is to identify insects from the native range that feed and breed exclusively on garlic mustard and to release them in North America if they are proven safe and effective.
In 1998, work was initiated to investigate the potential for classical biological control of garlic mustard in North America. In this process, potential biological control agents are tested for their effectiveness at controlling garlic mustard and for their host specificity. An ideal control agent will cause extensive damage to garlic mustard, but will not feed on other non-target species, even if the supply of garlic mustard is exhausted. Additionally, the larvae of the control agent must only develop on garlic mustard. If these conditions are met, once a garlic mustard population is reduced, the population of control agents will diminish as well and will not damage other non-target species.
Importation biological control of weeds is highly regulated in the United States. All prospective importation projects fall under the supervision of the USDA Technical Advisory Group (TAG) on Weed Biological Control. This committee reviews potential importations considering specific potential risks and benefits. Extensive testing is required by the TAG to evaluate the host specificity of a natural enemy. Once approved for importation by TAG a natural enemy must undergo further testing under quarantine conditions to determine that it is the desired species and is free of diseases or other contaminates.
From an initial literature survey, 70 insects and 7 fungi attacking garlic mustard in the home range were identified. Of these, 5 species of weevils (Curculionidae) and one leaf-feeding beetle (Chrysomelidae) were determined to have the greatest potential impact and were selected for further testing. The leaf beetle, Phyllotreta ochripes (Curtis), has since been rejected because it feeds too broadly. Testing is currently focusing on five species of Ceutorhynchus weevils. Adult C. alliariae and C. roberti feed on leaves while the larvae mine in stems and leaf petioles. These two species exhibit very high attack rates in the field (generally greater than 80 percent of garlic mustard infested). Larval C. scrobicollis are root miners while C. constrictus and C. theonae are seed feeders. Host-specificity testing on the Ceutorhynchus weevils is promising. The USDA Technical Advisory Group (TAG) on weed biological control may permit initial introductions of C. scrobicollis in 2004. Other natural enemies will likely follow as indicated by host specificity tests and USDA TAG review.
The effort to explore garlic mustard biological control is being conducted by a consortium involving many state and federal partners. A similar consortium was highly successful in implementing biological control of purple loosestrife in North America. Michigan was a part of the purple loosestrife consortium contributing research and education products for national use and benefiting from the receipt, release and establishment of the effective Galerucella natural enemies.
Garlic mustard management strategy in Michigan
The Michigan Invasive Plant Council (MIPC) convened a meeting in December 2002 to outline a broad based strategy for managing garlic mustard in Michigan. It was generally agreed that an effective strategy would seek to: 1) exclude garlic mustard from uninfested parts of the state, 2) eradicate localized populations in areas where garlic mustard is not widely established and 3) manage existing infestations to reduce their impact and limit spread. To these ends, the following activities were considered high priorities:
* Document the current distribution of garlic mustard in Michigan.
* Determine the impact of garlic mustard on Michigan ecosystems.
* Educate the public about garlic mustard biology, impacts and management.
* Develop a list of non-target plants in Michigan that could potentially be attacked by garlic mustard natural enemies and to assure their safety in advance of biological control by pre-release host specificity testing in Europe.
* Establish baseline information on garlic mustard in Michigan to determine the potential for biological control.
Information prepared by: Doug Landis, Jeff Evans, Department of Entomology, Michigan State University. Funding support: Michigan Department of Agriculture and Rural Development – Pesticide and Plant Pest Management Division, AgBioResearch, and MSU Extension.
Garlic mustard is a very invasive, fast-spreading weed, and Multnomah County has the worst infestation of it in Oregon. The roots produce a chemical that is toxic to other plants, and it can grow in most soil types. It can also grow in full sun or full shade, making it a threat to a wide variety of our native plants and habitats. You can help get rid of it, though – read on for some important tips about pulling up and getting rid of garlic mustard.
Many other plants are often mistaken for garlic mustard, especially before the flowers come up. Control is easiest when garlic mustard plants are in bloom (usually beginning in April), unless you can easily identify the rosettes (leaves) of the plant. Hand removal can be a successful technique in small patches that can be visited often and re-pulled frequently. Learn how to pull up garlic mustard and see more photos after the break!
Keys to Successful Hand Removal:
Garlic mustard is extremely invasive – once it spreads, it will blanket natural areas and crowd out native plants
Garlic mustard is already taking over this forest floor
Many other plants look similar to garlic mustard in the “rosette” (leaf) stage before flowering, making it difficult to identify!
- It’s best initially to pull during flowering, before the plants produce seed.
- Pull at the base of the plant and try to remove the entire root.
- Pulled garlic mustard material will still complete flowering and set seed – do not leave it on the ground! Be sure to bag and dispose of pulled plants as garbage.
- Mowing garlic mustard is not an effective control because plants will still bolt (send up flowers) and seed. To prevent spreading, do not mow garlic mustard when seed pods are present (May-September).
- Revisit pulled sites as often as possible to re-pull plants that sprout from left behind root fragments. This is especially important later in the spring as seeds develop.
Larger infestations of garlic mustard may require other approaches. Check out our Garlic mustard page for more information on garlic mustard and control methods – certain residents in our District (our District is all of Multnomah County east of the Willamette River) may be eligible for free control!
Garlic mustard identification and control
Garlic mustard, a Class A noxious weed, is a biennial or winter annual herb that generally grows 2-3 (up to 6) feet tall. Lower leaves are kidney-shaped with scalloped edges. Leaves feel hairless, and the root has an “S” or “L” shape just below the stem base. In spring, roots and new leaves smell like garlic, and small, four-petal white flowers appear clustered at stem ends, followed by long, skinny seedpods. This weed spreads by seed and can self-pollinate, helping it rapidly displace native plants along trails, in forests, and on riverbanks, among other areas.
History and impacts
Introduced from Europe originally as a food plant, this species is now a serious concern in forests across North America.
Garlic mustard is an invasive non-native biennial herb that spreads by seed. Although edible for people, it is not eaten by local wildlife or insects. It is difficult to control once it has reached a site; it can cross-pollinate or self-pollinate, it has a high seed production rate, it out competes native vegetation and it can establish in a relatively stable forest understory. It can grow in dense shade or sunny sites. The fact that it is self fertile means that one plant can occupy a site and produce a seed bank. Plant stands can produce more than 62,000 seeds per square meter to quickly out compete local flora, changing the structure of plant communities on the forest floor. Garlic mustard is also allelopathic, producing chemicals that inhibit the growth of other plants and mychorrizal fungi needed for healthy tree growth and tree seedling survival.
The majority of the known infestations in King County are on City of Seattle Parks properties and nearby private properties, but garlic mustard has also moved into Bellevue, properties along the Cedar River, North Bend, Tukwila, Shoreline and other parts of the county. Early detection, containment and eradication of new sites is of the highest priority. If you think you see this plant, please contact our program as soon as possible.
Garlic mustard is also a growing problem in other parts of Washington as well as Oregon and Alaska. Regional collaboration to share information and build partnerships to combat garlic mustard will be the key to stopping this plant in the Pacific Northwest. See the PNW Garlic Mustard Working Group Poster sharing highlights from the October 2014 meeting. If you see garlic mustard in our region outside of King County, please notify the local or state weed board or conservation district office.
Legal status in King County, Washington
Garlic mustard is a Class A noxious weed with a limited distribution in Washington, and eradication is required state-wide. This species is also on the Washington quarantine list (known as the prohibited plants list) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts, seeds in packets, blends or “wildflower mixes” of this species, into or within the state of Washington. For more information, see Noxious weed lists and laws.
First identified in Seattle in 1999 and listed as a Class A noxious weed in 2000, the King County Noxious Weed Program is working closely with landowners to prevent new infestations and eradicate existing infestations. Without cooperation and vigilance we will lose the battle to keep garlic mustard out of woodland areas throughout western Washington.
Identification (see below for additional photos)
- Biennial or winter annual herb that typically grows to about 3 feet tall, but can be anywhere from a few inches to over 6 feet tall depending on conditions
- Plants are usually single-stemmed, but may have more stems if they were cut
- Small, white 4-petaled flowers appear in early spring and are in clusters at the top of the stem
- First year plants are low-growing rosettes with rounded, kidney-shaped leaves, scalloped on the edges
- Leaves are not noticeably fuzzy or hairy (unlike most look-alike species)
- Upper leaves on mature plants are more triangular, becoming smaller toward the top of the plant, coarsely toothed
- Plants often smell like garlic, especially when leaves are crushed
- Each plant usually produces one flowering stem. If a plant is cut or stepped on, many stems will form
- Roots typically have a characteristic s-shaped bend
Biology and morphology
Garlic mustard seeds typically germinate in fall or early spring and the plant first forms a low, mound of leaves called a rosette that grows from mid-summer through the following spring. Plants typically bolt and form upright, flowering stems in March and April. Flower buds can be seen on the tops of stems as the plants begin to bolt and then flowers open soon after stem elongates, usually late April through May. Seed production soon follows.
Each plant usually produces one flowering stem. However, if a plant is cut or stepped on, many stems will form. Seeds can form on plants that are cut and left on the ground. Roots crowns will grow new stems if they are not removed completely or if garlic mustard is cut.
Garlic mustard is competitive in a wide range of soils, sun, shade and moisture. It grows in wet soil near creeks and on dry, steep slopes. Garlic mustard’s curved root helps the plant hold on to the soil even on steep slopes with loose soil. It can grow under the shade of other plants like nettles or in bright sunny spots. Flowering plants can range in size from sover six feet tall to tiny plants with just a few seed pods. Seeds can last in the soil for at least 10 years.
Seeds are small and easily spread on animals, people, vehicles and also by water, birds and other vectors. Deer tracks and dog trails through infested forests are often lined with garlic mustard.
Because this plant is so difficult to eradicate once it is established, familiarize yourself with the flower, the plant and the habitat where it grows to find infestations early. Monitor sites regularly to remove plants prior to seed set.
Hand-pulling individual plants is effective if the entire root is removed. Flowering or seeding plants must be put in a bag and discarded in the garbage. Carefully and thoroughly clean off boots, clothes and tools before leaving the area to avoid carrying the tiny seeds to new sites.
Herbicide may be needed for large, dense infestations and should be applied in the spring or fall on seedlings and rosettes, with care taken to avoid native and other desirable plants. Follow the product label and all laws and regulations regarding herbicide use on the site. Contact your local noxious weed program or county extension office for recommendations on herbicides.
After pulling or spraying dense infestations of garlic mustard, it can help to cover the bare areas with wood chip mulch to reduce seed germination. Infested sites should be carefully monitored every year for new plants, and checked for at least three or four years after no more plants have been found to ensure the population has been eradicated.
Additional information on garlic mustard
- Washington State Noxious Weed Control Board (external link)
- Garlic mustard weed alert (1.69 Mb) Acrobat file).
- For in-depth information on impacts, biology, identification and control of garlic mustard in King County, please read the garlic mustard best management practices (294 Kb Acrobat file)
- Wisconsin garlic mustard video (external link)
What to do if you find this plant in King County, Washington
Please notify us if you see garlic mustard growing in King County. Our program staff can provide the property owner or appropriate public agency with site-specific advice on how best to remove it. We map all known locations of regulated noxious weeds such as garlic mustard in order to help us and others locate new infestations in time to control them.
Garlic mustard infestation in the USCeutorhynchus constrictusCeutorhynchus alliariae on Alliaria petiolataCeutorhynchus alliariae on Alliaria petiolata Ceutorhynchus scrobicollis larvae in Alliaria root-crown Ceutorhynchus roberti Alliaria Alliaria flower Open field tests Oviposition and development tests with C. constrictus Previous
So, what’s the problem
Garlic mustard (Alliaria petiolata) is not related to garlic but is a biennial cruciferous (brassica) plant. Native to Eurasia, it was accidentally introduced to North America in the 19th century. It is one of the few non-indigenous herbaceous species that can invade and dominate the understory of North American forests. It is considered one of the most serious invaders in the northeastern and midwestern USA and southeastern Canada. Several methods have been used to control its proliferation in natural areas; hand removal can be effective in small infestations, while fire, cutting and herbicide treatments have been used to reduce densities in large infestations. However, these treatments are costly, need to be repeated over several years and may face regulatory restrictions.
What is this project doing?
This project to investigate the potential for biological control of the weed was initiated in 1998 by Prof. Bernd Blossey (Cornell University, USA). A team from CABI’s centre in Switzerland has been surveying for natural enemies and assessing host specificity of selected insects.
Biological control is based on the concept that a plant may become invasive because of the absence of natural enemies that keep it in check in its area of origin. The project aims to identify and introduce host-specific natural enemies as biological control agents. A guiding principle is that an agent should not impact plants other than the target. Risk of non-target damage is assessed by testing whether a potential agent feeds or develops on other plant species.
By reviewing the literature, we found records of 69 herbivorous insect species and seven fungi associated with garlic mustard in Europe. 30 species were collected in subsequent field surveys in Switzerland, Germany, Austria and Daghestan (Russia) in 1998–2000. Six insects were prioritized as potential biological control agents: the root-feeding flea beetle Phyllotreta ochripes, the two shoot-mining weevils Ceutorhynchus alliariae and C. roberti, the two seed-feeding weevils C. constrictus and C. theonae and the root-crown weevil C. scrobicollis.
Garlic Mustard Flower
Lisa Johnson, Commercial Horticulture Agent, UW-Extension Milwaukee/Waukesha Counties
Item number: XHT1081
What is garlic mustard? Garlic mustard (Alliaria petiolata) is a European woodland plant introduced to North America by early settlers for its culinary and alleged medicinal qualities. In North America, European insects and diseases that control the plant’s population are not present. Garlic mustard starts growing earlier in the season than our native plants, and outcompetes them. It also produces large quantities of seed. For these reasons, garlic mustard spreads rapidly in wooded areas, forming tall, dense stands that smother native wildflowers, and native tree and shrub seedlings. It can overrun a forest floor in a few years, destroying a previously healthy ecosystem by eliminating many plant species. In addition, animals, birds and insects that depended on a diversity of plant species for food and shelter can then no longer live in the infested area.
What does garlic mustard look like? Garlic mustard is a biennial plant with a two-year life cycle. The first year, it forms a rosette of round, scalloped-margined leaves that stay semi-evergreen through winter. The second year, it sends up a flower stem with triangular toothed leaves that bears tiny white flowers with four petals. The plant dies after producing long narrow seedpods. At maturity, garlic mustard plants may be 3 to 4 ft. tall and bear up to 500 seeds per plant.
How can I control garlic mustard? Repeat any control method for several years since garlic mustard seeds can survive in the soil for up to 7 years. Hand-pull small infestations, but do not compost the plants because most compost piles do not get hot enough to kill the seeds. Dispose of pulled plants by burying deeply in an area that will not be disturbed, or landfilling. Call the Bureau of Endangered Resources at 608-266-7012 if you need permission to landfill garlic mustard. To burn collected plants, burn them while still moist, because dried garlic mustard seedpods can burst open and spread the seed. If you use an herbicide, spray early in spring or late in fall, because our native plants are dormant at these times, but garlic mustard is still green and vulnerable to sprays. A 1-2% solution of a glyphosate-containing herbicide is very effective. Glyphosate is a nonselective herbicide, so avoid spraying nontarget plants. Read and follow all label directions on the herbicide product. Encourage your community to scout for garlic mustard in your area and remove it, if found.
Links & Downloads
- Garlic Mustard (PDF)
- Herbicide Effectiveness on Invasive Plants in Wisconsin