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| Garlic Mustard: flowering second-year plants above, young first year plants below. |
Doing some research online for herbicide rates recently, I ran across a lot of incorrect and misleading information on controlling garlic 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 thousands of seeds, allowing it to reproduce and spread rapidly. Despite this I believe that, technically speaking, garlic mustard is among the easiest invasive species to control in our region. As a biennial, it only lives 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. 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 through an area that you suspect might have 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.
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| Garlic mustard seeds are black, cigar shaped and just over 1/8th inch long. Photo courtesy www.http://foragingpictures.com |
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| Dead garlic mustard stalks can drop seeds that get caught in your clothes. Photo by http://www.kwfn.ca. |
Prescribed Burns: 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.
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| 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.
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| 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 have more than a few spring ephemerals, you will need to get the burn completed before these wonderful woodland wildflowers sprout. Most native perennials will survive fire, but repeated burns will weaken them and interrupt reproduction.
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. The seedlings are quite vulnerable and a flame weeding will kill them off. 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.
This is a fun way to kill garlic mustard. It's a little faster than an herbicide sprayer, hundreds of them will die very quickly 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 effectiveness may go down slightly, but it's probably worth doing until mid-May.
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| 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. 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 high wind when using the propane torch, and only use it when the humidity is above 50% and the leaves are NOT dry and crinkly. 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 as Phydura or 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. You can't quite move as fast spraying as you can torching and let's face it, its not nearly as much fun. But Smokey The Bear would prefer you use this method.
Is Treating Seedlings Worth It?: There are two problems with fire as a seedling control method. First, is a little thing called "intraspecific competition", which basically means that the seedlings all compete with each other for survival, and only a small fraction, perhaps 1 in 10 (or less) make it to adulthood. So any plants that you miss will have an easier go of it with plenty of growing space since they aren't fighting 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.
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 anyway, 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 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. Just be sure 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. 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. 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.
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 though, 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.
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| 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. 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 seen 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.
You can start 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. 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. At this point, you need to bag them out.
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| 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. |
If you wait too long, 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.
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 garlic mustard, 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.
The technique involves complete obliteration of the plant. You want 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 to resprout. With a string trimmer, we often start at the top of the plant and mow down the stalk to the ground. Or, we might sweep through a patch of garlic mustard with the weed whip held so that the strings are spinning perpendicular to the ground, mincing up the seed heads, and then coming back with a couple horizontal swipes to cut the plant stalk up all the way to the ground. 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 lawn mower, you'll 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.
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.
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| 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, 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. In a healthy woodland, garlic mustard won't have a chance against our hardy native perennials.
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| 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 in this woodland. How could a 4" garlic mustard rosette ever survive under this dense layer of native wildflowers!? |
