It’s a glorious spring day, and I’m walking through our Indiana woods enjoying the amazing display of wildflowers. Large-flowered trillium, red trillium, phlox, geranium, putty root, trout lily, spring beauty and many others all shouting “look at me” with their flashy swaths of color. Not to be undone by the wildflowers, many shrub species including serviceberry, spicebush, redbud and flowering dogwood are also presenting eye-grabbing flowers of their own. Half way through a large-flowered trillium patch, I spot a white flowering plant that looks out of place. Fear takes hold of me. “It can’t be,” I mutter to myself. Moments later, I’ve confirmed my worst fear - I have garlic mustard.
According to most accounts, garlic mustard was introduced into North America in the mid-1800s and is now one of our worst invasive species of woodlands. Once it gets a foothold in a forest community, it will dominate the herbaceous layer and greatly reduce the diversity of native species. One plant can produce a ridiculous amount of seeds. Studies vary, but cited seed production ranges from 600 up to 8,000 seeds per plant and sometimes more in optimum conditions. The Ontario Invasive Plant Council cites a study that reported 105,000 seeds per square meter! Seed viability, or how long it can live in the soil before germinating, is thought to be in the 6- to 8-year range.
Without control, garlic mustard will consume your woodland. For those of us who enjoy spring wildflowers and appreciate nature’s biodiversity, we are faced with a difficult choice. Either go-all-in and expend considerable resources to control the demon, or steer clear of the woodlands until fall to avoid being reminded of what has been lost.
Garlic mustard is a biennial, two-year life cycle. Seedlings emerge in spring, overwinter as rosettes, and bolt, flower and die the following year. I’ve discovered that effective control involves a variety of techniques targeting all stages of its growth cycle. What follows is a summary of landscape-scale control methods I’ve used regularly during my 30-year battle with garlic mustard.
In the seedling stage, fire can destroy an entire generation of garlic mustard. The key is to time the fire in the early spring to just after the mustard seedlings emerge but before any fire-sensitive native plants emerge. While a well-timed fire will do wonders for native spring flowers, conversely, burning after these flowers emerge can greatly reduce their vigor. In practice, early spring fires are a balancing act between burning when the most mustard seedlings have emerged and the fewest fire-sensitive native flowers have emerged. Because fire removes the litter layer, allowing more sunlight to any seeds that haven’t germinated before the fire, be prepared for more second-year garlic mustard plants the following year.
Don’t burn in the fall if you have a moderate or more severe invasion of garlic mustard. There will be trillions of seedlings the following spring. Fire won’t destroy most mustard plants in the rosette stage unless they are weakly rooted. Fire top-killed rosettes will re-sprout later in the growing season complicating follow-on control. The delayed re-sprouting will occur when most other plants are growing making the mustard plants harder to find. Also, blooming will be out of sequence (much later) compared to plants untouched by fire.
Garlic mustard rosettes stay green most of the year, so dormant season spot spraying is highly effective. Early spring, late fall and sometimes winter are great times to spray mustard rosettes with little chance of collateral damage because most native plants have no living tissue above the ground. There are many effective herbicides to use against garlic mustard. Just be sure to read the herbicide label in order to use them properly. My preference is to use a broadleaf herbicide to avoid damaging native cool-season grasses and sedges. I call this dormant season “spot spraying” or “first-pass spraying.” While it will knock down large quantities of mustard, it won’t get them all. Skips, rosettes hidden under leaves, behind logs, etc. will get through to produce plenty of seed to keep the invasion well supplied. A second-pass spraying is required for decent control.
Second-pass spraying is much more difficult without collateral damage. Many native species are now up and mixed in with the garlic mustard rosettes. I often work dawn to dusk to get through second-pass spraying before garlic mustard reaches the flowering stage. To spray effectively, plant Identification skills are essential. One must be able to recognize native plant species at very early growth stages to avoid killing what we are working so hard to protect. Some common native plant look-alikes to the garlic mustard rosette stage include hyssops, wood violets, wood mints, kidneyleaf buttercup, figwort, tall bellflower.
Use a lot of dye in your herbicide. This will make it much easier to see your spray pattern. Keep the spray pressure low. Practice spot spraying on a white sheet of paper. This will show you how easy it is to overspray your target. I will often pull a mustard plant if I feel it is too close to a desirable native plant. Other times I accept some collateral damage if the native plant species is present in good numbers. Again, identifying plant species at early growth stages is key to good mustard control and rebuilding native plant diversity.
In order to find all the mustard plants, one needs to learn to “grid” the forest floor carefully. I tend to divide the woodlot into sections and use geographic features, down trees, and other landmarks to help guide me to cover all areas. Go slow and look carefully – including behind you. I have not found large herbicide crews effective, as they tend to leave uncovered gaps between adjacent workers when the terrain gets tough. I recommend chaps to armor yourself against the countless brambles and thorny shrub thickets where mustard loves to hide.
Unfortunately, we often run out of time. For large patches of mustard mixed in with desirable native species, mow it down while in the early flower stage. Weed whackers work well for this, as you can cut plants low to the ground and avoid plants pushed over by tractor tires that can still set seed. Some follow-up mowing may be necessary should plants re-sprout. Better to mow too early and recut than to mow too late. Yes, mowing will also set back the native plants, but you won’t kill them as you would with herbicide spraying.
What about neighboring lands overrun with garlic mustard? This is a major issue, as seed rain from neighboring lands will reinvade your property. Ideally, if you obtain permission, spraying or mowing a buffer zone of 10 – 15 feet along the property boundary will greatly reduce the amount of seed rain that reaches your property.
Woefully, the basic control steps that I’ve summarized above will be needed annually into perpetuity. Garlic mustard is all over the landscape, hardly anyone controls it, and each year it will find a way onto your property even if you are successful in removing every plant the year before. I remember all the talk about biocontrol in the early 90’s, or how mustard runs its course and disappears. Well, my reality has been that garlic mustard is still here, more formidable than ever, and likely only to get worse.
While spring garlic mustard control can be stressful in the race against time, I’ve learned to enjoy many aspects of it. I still get to see amazing displays of native flowering plants, participate in nature’s renewal processes, witness spring animal migrations, and experience nature re-healing itself with my helping hand. Because I spot spray all invasive species during my spring rounds, including woody species’ seedlings such as Japanese barberry, honeysuckle, buckthorn and multiflora rose, I don’t have to perform periodic back-breaking woody invasive species removal anymore.
Each spring, during heated moments in my battle against garlic mustard, I ask myself why? I think about that first mustard plant I found 30 years ago and how its species has controlled my life since. Will society awaken and see what I see? Or will I be the last to witness such great diversity in the woodlands I manage?
My tenuous answer is always the same – I care, and hopefully that’s enough.