Ever since humans invented agriculture and started moving from continent to continent, they have taken plants with them. In most cases imported, non-native plants do not spread much beyond the bounds of horticulture. But the exceptions are increasingly worrisome to biologists. Removed from the pests and diseases that kept them in check in their natural habitats, some plants multiply explosively. They can smother native ecosystems in a matter of a few years.

Some of these invasive plants, such as bush honeysuckle, Japanese knotweed, phragmites reed, and purple loosestrife, are all too familiar in our region. As if that’s not enough, we must now add a new menace to the list. The latest member of this rogues’ gallery is garlic mustard, a pungent herb in the cabbage family.

Interestingly, garlic mustard has been in the United States for a long time. It was brought from Europe to Long Island in 1868 as a remedy for gangrene and ulcers. For a century, it spread relatively slowly, by an estimated 140 square miles a year. However, from the 1970s onward, garlic mustard’s rate of expansion skyrocketed to 2,500 square miles a year. Some biologists think it is no coincidence that white-tailed deer became increasingly numerous at the same time that garlic mustard took off. Deer avoid bitter garlic mustard but love to browse native herbaceous plants, conveniently clearing space for garlic mustard to take hold.

Unlike some invasive plants, garlic mustard has few growth limitations. Many invasives take hold primarily in disturbed soil (for example, a sunny roadside bank), but garlic mustard isn’t as picky. It likes any site in shade or partial sun, even fairly dry sites. It is particularly aggressive in rich, moist, forest understories, where it out-competes other herbaceous plants -- and this is what has biologists particularly alarmed. These shaded woodland areas are the stronghold of most northern wildflowers, beloved species like trillium, spring beauty, bloodroot, and Jack-in-the-pulpit. Huge swaths of these plants are lost annually to garlic mustard advances.

The damage isn’t limited to wildflowers. Most native insects are adapted to eat only certain plants, so a loss of plants means a loss of insects as well. The importance of insects as food for songbirds and soil dwellers like salamanders cannot be overstated.

Unfortunately, there are no native insects that successfully eat garlic mustard. In short, when this weed invades, insects, birds, and salamanders all lose ground.

The success of garlic mustard is amplified by chemical warfare. To ensure distastefulness, the leaves contain cyanide, insufficient to harm people but enough to knock down insects. Its roots exude compounds called glucosinolates that prevent other species’ seeds from germinating and kill beneficial soil fungi. Since native trees depend on root associations with fungi for growth, killing fungi enables garlic mustard to outcompete hardwood tree seedlings.

So what does garlic mustard look like? It takes on two appearances, depending on its age. In its first year it forms a ground-hugging rosette of green, kidney-shaped leaves, two and half to six inches across, with scalloped edges. Second-year rosettes rapidly produce a tall, central stem with leaves in an alternating pattern. The leaves on hairy stalks are triangular with toothed edges and are up to three and a half inches across. Young leaves emit a garlic odor when crushed. By May of its second year, garlic mustard is usually the tallest blooming forest plant (often waist high or more) with a terminal cluster of white flowers, each with four petals and six yellow stamens, four short ones and two long.

Each plant produces up to 8,000 seeds in long wiry capsules. The seeds look like miniature, dark, rice grains and they are flung up to ten feet when the ripe capsules explode. Most seeds germinate in the first two years and seedling densities of 24,000 per square yard have been reported. Some seeds remain dormant and can germinate up to 10 years later.

While it’s easy to become discouraged learning about the challenges of this invasive, it’s worth noting that, in our region, garlic mustard has not reached the plague proportions of Midwestern states. We have time to control this menace. Control is labor intensive but simple. Identify patches of garlic mustard in your town. Pull them up in May before they set seed. Pulled plants can still ripen and disperse seed, so bag and send them to the landfill. Here’s the hardest part: Repeat yearly till the patch is gone.

Li Shen is an adjunct professor at the Dartmouth Medical School and the chair of the Thetford Conservation Commission. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: wellborn@nhcf.org.