Good Oak News

Saturday, April 6, 2013

Eradicating Garlic Mustard Requires A Holistic Approach

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 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 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.

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.

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 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. 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.

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:

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!?