Wild Side : Garlic mustard threatens to take hold on Martha's Vineyard
Photo by Matt Pelikan
Anyone interested in conservation probably knows of the problems posed by non-native invasive plants: freed from the factors that control their population in their original range, invasives out-compete and stifle our native species, reducing plant diversity and depriving other native wildlife of the plants they've evolved to depend on. Over the last few years, garlic mustard, a weed only recently established on the Vineyard, has put on a clinic on the tactics of invasive plants, shocking even conservation professionals with its explosive expansion.
Garlic mustard isn't even listed in the Flora of Martha's Vineyard, a botanical catalog for the Island last updated in 1999. According to Liz Loucks, a land steward for The Nature Conservancy here on the Vineyard, the first Island populations were noticed about six years ago, all of them along roadsides. Island conservation groups, optimistic that early action would nip this invasion in the bud, began organizing "pulling parties" to yank the stuff out by its roots. The battle seemed to be going well.
Until this spring. Perhaps aided by high survival over the mild winter, maturing garlic mustard plants seemed to erupt everywhere. Ms. Loucks estimates that between 50 and 100 stands now exist, along roads and driveways, in compost piles, in yards, and along woodland edges.
Garlic mustard, recognized by its heart-shaped, coarsely toothed leaves and four-petaled white flowers, was introduced into the U.S. about a century and half ago. It has colonized most of the eastern states and southeastern Canada and is rapidly gaining traction in the west as well. Under suitable conditions, it can totally dominate expanses many acres in size, choking out all other herbaceous species and even preventing the regeneration of forest trees.
One can almost admire its vigor and competitive abilities. It's a widely adaptable biennial, growing as a low rosette through its first year and then bolting into a seed-producing machine early the following season, overshadowing native plants that are slower to break dormancy. The seeds pop free of the ripe seed pods, springing as much as several yards away, and then the plant senesces and dies by late summer.
As its name suggests, garlic mustard produces bitter chemicals that discourage our insects from eating it (though in its native range, many herbivores have evolved a taste for this plant). Other chemicals it produces appear to suppress the soil fungi that benefit other plant species. Its flowers can either self- or cross-pollinate, and in either case, a very high percentage of the hundreds or even thousands of seeds a plant produces will usually be viable. Garlic mustard seeds can persist for several years, waiting for the right conditions to germinate, and they spread readily as hitchhikers on clothing or animal fur, or in mud attached to vehicles and equipment. I suspect that the slipstreams of moving automobiles help disperse seeds along roads.
Researchers consider deer a major ally for this plant, carrying its seeds on their fur, disturbing soil with their hooves, and preferentially grazing on native plants that compete with garlic mustard. But disturbingly, it also appears that garlic mustard may have received a big, unintentional assist from humans on the Vineyard. Several Island landscaping operations, Ms. Loucks reports, are heavily infested with garlic mustard, and it seems likely that seeds have been transported in soil or on equipment to numerous sites around the Island. This may account for the rapid spread of this plant.
The good news is that small populations of garlic mustard are fairly easy to control by nontoxic, mechanical means. Simply yank the plants out by the roots before they produce seeds, and repeat in subsequent years if needed. Dispose of the pulled plants like trash; the seeds on a dying garlic mustard plant will continue to mature, and if you compost or just leave the plants you've pulled, you'll make the problem worse.
Biological factors may help limit the damage this plant can do in some portions of the Island. Garlic mustard relies on exposed or disturbed soil to get established and hence has a hard time invading intact native plant communities. And it grows a little less vigorously in the dry or acid soils that characterize much of the Island. But this plant appears determined to dominate up-Island roadsides, where, Kristen Fauteux of Sheriff's Meadow Foundation notes, the semi-open conditions supports a surprising number of rare of unusual native plants. And garlic mustard could dominate stream drainages, red maple swamp, and other damp habitats, especially where some kind of disturbance has exposed soil.
Given the explosive spread of this plant, and its potential to alter the Island's ecology, a grassroots effort to control it is vital. Learn to recognize garlic mustard (if the pictures accompanying this column aren't sufficient, search for the species online). Pull it without mercy wherever you find it, making sure it can't produce seeds. Check the sites again next year in early spring, again pulling any bolting specimens, and repeat the process until the seed supply is depleted.