Invasive species alert: Garlic mustard threatens native plants

Spring wildflowers arrived fashionably early this year. It's always a treat to see native spring wildflowers, whether early or late, but there's a downside this year: nonnative plants blossomed early, too.

One such plant in particular, garlic mustard, has a jump on things this year.

Garlic mustard is an aggressive plant that threatens the health of our native woodlands. It defies conventional weed control methods. We can sever the plants' flowers, pull the plants up by the roots, stomp on them, compost them and herbicide them. But we're outnumbered. Literally millions of vibrant green garlic mustard plants advance across the forest floor year after year, and this invasion is as inexorable as the incoming tide on the ocean shore.

A biennial plant, garlic mustard takes two years to mature. The first year seedlings put out small green leaves close to the ground. In the second year a flowering stem grows 2 to 3½ feet tall, sporting triangular or heart-shaped leaves with scalloped edges. At the top of the stem numerous small white flowers grow, each with four delicate white petals. Each flower develops a long slender pod containing 12 to 16 tiny black seeds. The entire plant has the pungent odor of garlic when crushed.

Garlic mustard originated in Europe and was introduced to North America in the 1860s as a culinary and medicinal herb. After a century of behaving itself in gardens (in other words, staying put), the European herb began to turn up in woodlands and along shaded waysides. There was no cause for alarm when its numbers were sparse. But after a century of low-profile living, garlic mustard eventually found an ecological niche in our woodlands, and it took off. In fact, it exploded.

Garlic mustard now pops up in wooded areas everywhere, from shaded gardens to natural areas, roadsides and parks. It has made its way across 30 eastern states. In Illinois, it grows in 41 counties and is most abundant in the northern part of the state. In Kane County, it has made inroads in every one of our 16 townships.

Although the plant is edible, it seems to have fallen out of favor in our cuisine. Unlike the morel mushroom, no one's hankering to harvest garlic mustard from the wild. Wildlife doesn't want it either. Even the most voracious deer will selectively munch on wildflowers all around a garlic mustard plant, and leave the garlic mustard untouched.

Garlic mustard's success is largely due to its phenomenal reproductive strategy. A single plant may produce anywhere from two to 100 or more pods. Each pod has about 16 seeds. Doing the numbers, you can see that a well-endowed garlic mustard plant is capable of producing 1,600 seeds. Multiply that by the number of garlic mustard plants in a woodland and … yikes! Researchers at The Ohio State University estimated that “in dense patches of garlic mustard, over 20,000 seeds per square foot can be produced annually.”

Garlic mustard pods are green now and will ripen in mid May. They will shed seeds continuously all summer. The tiny black seeds are dispersed by animals — including us — traveling through the woods. They hitch a ride on muddy hoofs or the soles of our shoes, in fur or in the folds of our clothing. After the seeds come off the unwitting chauffeur, they lie dormant in the soil. Most garlic mustard seeds remain dormant until the second spring after their production. Those that don't germinate the second spring remain viable for up to five or six years.

New crops of garlic mustard seeds accumulate in the soil year after year. Collectively, the seeds in the soil are called a seed bank. The seed bank grows richer with millions of garlic mustard seeds each season, and these long-lived seeds are poised for germination the instant conditions are right.

With legions of seeds lying in wait, the indomitable advance of garlic mustard has a tremendous ecological impact. Garlic mustard displaces native species in a short period of time. Within a decade of its first appearance it can turn a botanically diverse woodland into a monoculture bereft of native plants.

What can be done to stem the tide? There are several options, but to the consternation of ecologists and gardeners alike, garlic doesn't cry uncle easily. The first approach to the problem is mechanical control. Hand pulling each plant has a drawback, however. Weeding disturbs the soil, which only brings new recruits to the fore. To offset this, some experts recommend tamping the soil after removing each plant. In areas of light infestation, this may be doable. In areas of garlic mustard mania, it's impractical.

The next option is to cut the plants with a weed whip or scythe. This method applies to areas where there's a monoculture of garlic mustard; otherwise native plants will get the ax along with the invasive ones. I find that whacking the plants with a scythe is somewhat therapeutic when I have the invasive species blues. If you try this, watch your shins! And take care to cut the plants as close to the ground as possible, or flower stalks will bolt up and quickly produce new flowers — and new seed.

Fire is another management tool. Fall or early spring burning will kill first year plants. The downside to this tactic is that the bare soil resulting from a burn is conducive to germination of yet more garlic mustard seeds. The emerging seedlings can be zapped with an herbicide such as Roundup.

Because garlic mustard is not completely killed by any of these methods alone, the best plan of attack is to employ these methods in concert. The Forest Preserve District of Kane County has taken this multipronged approach in several high quality natural areas. According to Ben Haberthur, the district's Restoration Ecologist, a combination of weed-whipping, seed head removal, and prescribed burning has shown some results at Johnson's Mound and Bliss Woods forest preserves.

“We're in the third year of control,” Haberthur said. “And instead of seeing just a few wildflowers in a sea of garlic mustard, we're now seeing a few garlic mustard plants among lots of native wildflowers.”

There's one more weapon in the arsenal: biological control. This method pits an herbivore against an invasive plant. The challenge is to find just the right herbivore, and in the case of garlic mustard the most likely candidates are insects from Europe. The creature must be host-specific — in other words, it has to be a very picky eater that won't eat anything but garlic mustard.

Biological control is a somewhat daring tactic, considering the risk. If an insect brought over from overseas takes a liking to one (or more) of our native plants, an ecological nightmare could ensue. But biological control has proved to be effective in the battles against some invasive plants, including purple loosestrife.

The search for a biological control agent for garlic mustard management is on in Minnesota where insects are being carefully evaluated for their potential to reduce both garlic mustard and another invasive species, buckthorn. It's a daunting, tedious, and expensive endeavor. Many people are waiting with bated breath for the results.

No matter how you cut it, garlic mustard is a monumental challenge, and it's a challenge we will face for quite some time. The control of garlic mustard takes vigilance, persistence, and a fair amount of optimism.

For Haberthur, there's reason for optimism. After years of work, he's inspired by the reappearance of such native gems as white trillium, blue cohosh, and woodland phlox in areas formerly overrun with garlic mustard.

So take heart and keep the faith, even if it's only as big as a garlic mustard seed.

Valerie Blaine is a naturalist with the Forest Preserve District of Kane County. She welcomes your comments and questions via email blainevalerie@kaneforest.com.