eGarden © 2007 Ed Mues

Garlic Mustard: Invader from Abroad.

I believe a weed is a lot more than a plant growing where it's not wanted. It is a plant that is having a negative impact on the economy or health of another ecosystem, be it human, animal, botanical or environmental. This influence can be far reaching. It might include food or forage crops, landscapes, aesthetics of open spaces, gardens, nurseries, golf courses, wildlife habitat, and more.

I've written about invasive plants a few times in the past. Twice about very conspicuous, showy, and lovely (in an aesthetic sense) blooms. There in lay some of the problems. Citizens joyfully transplanted these attractive looking plants and their seeds to new locations in a desire to see them more closely, to incorporate them into their personal landscape's color scheme.

Sometime around the mid-1800s, European immigrants introduced one of their natives to our shores for culinary and medicinal uses. That plant, Alliaria petiolata, a.k.a. Garlic mustard, is actually native to Europe, Asia, Africa, the British Isles, Scandinavia, India, and China. Both its nickname and its genus name refer to the characteristic garlic-like odor given off when the foliage is bruised or crushed. All of the plant's parts are edible.

It's a far cry from the Japanese knotweed, purple loosestrife, and the French Impatiens (Policeman's helmet) that won their way into the hearts of unwitting gardeners and homeowners just by dint of their blossom displays.

It was an email from a reader that prompted me to write today's column. He first took note of garlic mustard near the Beaverkill area six years ago, and after observing its rapid and seemingly uncontrollable spread a couple of years later took it upon himself to positively identify it and begin to eradicate it.

He writes,

"Last year I spent at least 100 hours pulling every adult plant on the property, which includes about a mile of roadside, probably the worst infestation I've seen around here. Snowplowing and roadside mowing spreads it, and my mowing the edges for several years scattered the seeds down a rich fertile slope and accelerated the spread and made my current work much harder. The areas where I worked two and three years ago are in much better shape now, which is encouraging.

The urgent problem is we have many, many miles of small back roads in the woods where GM has begun to be established in the last two or three years, exacerbated especially by the Sept. 2005 flooding rains, which carried seeds down the watercourses below culverts. The seeds are relatively dense and heavy and don't fall far from the plant normally. All the roads I've looked at around here have some infestation here and there but the great bulk of the roadside and all of the woods are clear. I found only two small patches in one nearby road, which is very encouraging.

It would be really nice if people would become aware of GM and would educate others about it. Pulling the plants is easy enough and people need to get more involved. As always, Bailey's Principle of weed control applies: survey the areas and snag the new outbreaks on the periphery before you tackle the well-established patches.

My thought is that if the total number of hours (people spend clearing GM) grows faster than the plant spreads, it can be held in check or even controlled. That depends on lots of people being educated and eventually on people adopting sections of uninhabited roads. But first people just need to get started, and right now is an ideal time because the plants are very easily visible."

There are thousands of weed pests that are not native to America. The federal agencies have named nearly 1400 as exotic and pests. Three hundred of these have been positively classified as invasive plants. Of these, ninety-four are officially categorized as Federal Noxious Weeds that infest (are you ready for this?) in the neighborhood of 100 million acres coast to coast.

Prof. Larry Kuhns from Penn State's College of Agricultural Sciences as early as 1999 suggested that "... invasive plants may be the top environmental issue of the 21st century."

Infestations of garlic mustard here in Sundown are just as serious. A comprehensive plan for managing garlic mustard via conventional means includes the following elements adapted from Nuzzo (1991). Because garlic mustard is a disturbance-adapted plant, all management efforts should strive to reduce soil and vegetation disturbance to prevent giving further advantage to garlic mustard.

Garlic mustard spreads from established (core) infestations along an invasion front. Here in Sundown, as in other areas, every time the highway department cleans some roadside ditches, hundreds of thousands of seeds are re-distributed along the roadside. There is no accounting for what numbers of seeds are moved by the tires and rest of the heavy equipment.

Where garlic mustard is not well established, efforts should focus on detecting and eradicating new satellite infestations before a seed bank develops (i.e. dormant seeds in the soil). Monitoring should focus on areas where garlic mustard seeds are likely to be dispersed and find disturbed areas suitable for germination. Trails, parking areas, transportation corridors and recreation sites in suitable habitats are known sites of early infestation.

Once garlic mustard has established an invasion front (several years of flowering plants), the goal is to prevent further seed set until the seed bank is exhausted; a period of up to five years. Depending on the site characteristics and infestation level, pulling, cutting, applying herbicide or repeated fire will be required.

Pulling individual garlic mustard plants by hand is the simplest and most effective approach to managing small or isolated infestations. This is most successful after a rain event or while the soil is still moist. When pulling plants, it is important to remove the upper portion of the roots as well as the stem, since buds in the root crown can produce additional stems. All pulled plants should be removed from the site as seed ripening continues even after plants are pulled.

Because seeds remain viable in the soil for up to five years (some sources state 11 years), it is important to pull all garlic mustard plants in an area every year until the seed bank is exhausted and seedlings no longer appear. This will require multiple efforts each year as rosettes can continue to bolt and produce flowers over an extended period.

About the plant: Garlic mustard is a cool season biennial herb with stalked, triangular to heart-shaped, coarsely toothed leaves that give off an odor of garlic when crushed. First-year plants appear as a rosette of green leaves close to the ground. Rosettes remain green through the winter and develop into mature flowering plants the following spring. Flowering plants of garlic mustard reach from 2 to 3 feet in height and produce button like clusters of small white flowers, each with four petals in the shape of a cross.

Seeds are produced in erect, slender pods and become shiny black when mature. When most garlic mustard plants have died, they can be recognized only by the erect stalks of dry, pale brown seedpods that remain, and may hold viable seed, through the summer.

Garlic mustard poses a severe threat to native plants and animals in forest communities in much of the eastern and midwestern U.S. Many native widlflowers that complete their life cycles in the springtime (e.g., spring beauty, wild ginger, bloodroot, Dutchman's breeches, hepatica, toothworts, trilliums, and ramps) occur in the same habitat as garlic mustard. Because it persists through the first winter as a green rosette, it can overrun and eliminate many native plants. Consequently, it would deprive us of the vibrant display of native spring wild flowers.

Once introduced to an area, garlic mustard out competes native plants by aggressively monopolizing light, moisture, nutrients, soil and space. Wildlife species that depend on these early plants for their foliage, pollen, nectar, fruits, seeds and roots, are deprived of these essential food sources when garlic mustard replaces them. Humans are also deprived of the vibrant display of beautiful spring wildflowers.

Garlic mustard ranges from eastern Canada, south to Virginia and as far west as Kansas and Nebraska. Garlic mustard frequently occurs in moist, shaded soil of river floodplains, forests, roadsides, edges of woods and trails edges and forest openings. Disturbed areas are most susceptible to rapid invasion and dominance. Though invasive under a wide range of light and soil conditions, garlic mustard is associated with calcareous soils and does not tolerate high acidity. Growing season inundation may limit invasion of garlic mustard to some extent.

A single plant can produce thousands of seeds, which scatter as much as several meters from the parent plant. Depending upon conditions, garlic mustard flowers either self-fertilize or are cross-pollinated by a variety of insects. Self-fertilized seed is genetically identical to the parent plant, enhancing its ability to colonize an area. Although water may transport seeds of garlic mustard, they do not float well and are probably not carried far by wind. Long distance dispersal is most likely aided by human activities and wildlife. Additionally, because white-tailed deer prefer native plants to garlic mustard, large deer populations may help to expand it by removing competing native plants and exposing the soil and seedbed through trampling.

Garlic mustard reproduces exclusively by seed. It is an obligate biennial. The first year the seeds germinate and develop a basal rosette of kidney-shaped leaves, and the following year it produces an upright stalk with triangular leaves. The stalk bears terminal clusters of small white flowers. After flowering and producing large quantities of seeds in elongate seed capsules, the plant dies.

Second year garlic mustard plant with terminal white flowers, elongate seed capsules, and coarsely toothed triangular or heart-shaped leaves are the ones to go after.

If you are at all interested in helping eradicate this pernicious plant pest, maybe you can form a small group of friends and nature lovers and begin to scour your area while taking strolls and talking about nature. They come easily out of the ground after a rainfall or while the soil is still damp. Carrying a plastic bag or cardboard box will remove them and their seed potential from the area they are collected.

From The Garden of Ed. Submitted for publication in The Towne Crier on June 6, 2007

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