Garlic mustard (Alliaria petiolata) is a plant that is becoming a more common nuisance in much of the Midwest and Nebraska. In the mixed grass and tall grass prairie ecosystems of the state, garlic mustard is currently on the Invasive Plant Watch List and listed as a Category 2 plant. Category 2 plants are species considered top priority for monitoring. Eradication is still possible for new and existing populations but could conceivably become much more of a problem.
Garlic mustard is a member of the mustard family and native to Europe where it is often used in cooking. It was first discovered in the US on Long Island, New York in the late 1860s. It is thought to have come with immigrants when they came to this country and moved with them as they settled across the US. As the name implies, it has a pungent smell and taste reminiscent of garlic.
Garlic mustard is a biennial herbaceous plant that is propagated by seed. The first year of its lifecycle begins as a seed that germinates in the early spring and forms a rosette of small, oblong to round scallop-edged leaves. During the first year much of the plant's energy is spent on developing its root system that includes a taproot. The taproot is used in many cultures as a substitute for horseradish and is prepared in the same way. The second year, the plant develops a weakly branched stalk up to 3 ½ feet tall with white flower clusters at the end of the stems. Some side flowering may occur as well. The stem and the leaf petioles are often hairy. Mature leaves become more triangular, up to 3 inches long. The leaf margin is more serrated as compared to the younger leaves' scalloped edges.
Garlic mustard prefers a shady habitat and often is found in forested areas and riparian waterways. It also can be found in yards, along roadsides, in lightly shaded pastures, and even occasionally in full sunlight. Much of the habitat where garlic mustard is found is also conducive to deer populations. Garlic mustard has no natural enemies. Deer and other wildlife do not consume it because of its garlic taste. Deer will graze the plants surrounding garlic mustard and leave it untouched, reducing the competition between native plants and both 1-year old and mature plants. This allows the garlic mustard to go to seed unscathed.
Flowering begins in June and can extend into August. A cluster of small, white flowers each with 4 petals develop into seed pods. The flowers are self-fertile meaning a single plant can be the source of an outbreak. On average, a plant will produce 22 seed pods. Each individual seed pod, called a silique, can contain as many as 28 seeds. A very vigorous plant can produce over 7,000 seeds but average seed production is in the range of 600 seeds per plant. Many of these seeds will germinate in one to two years but the seed can remain viable up to 5-7 years. The amount of seed this plant produces coupled with the fact that there are no natural enemies makes for a combination that can result in explosive plant populations.
Three options are available for control: mechanical, chemical, and prevention. Mechanical controls include hand pulling for small stands and cutting for medium and large populations. Both are very effective control measures although labor-intensive. All plant material needs to be bagged and removed from the area, especially in the case of more mature plants. The plant can continue to develop even after pulling or cutting, and may produce seed. Control should continue for 5-7 years to keep a handle on any seeds left in the soil that germinate. When cutting plants, it is important to cut the plant as close to the ground as possible, leaving little or no stub. The longer the length of the remaining stem, the greater the opportunity for side flower development and seed production.
Chemical control is also an option. Apply a non-selective herbicide such as glyphosate early in the spring when garlic mustard is germinating and the season's growth is concentrated near ground level. Few other plants are actively growing at this time of year and the impact on other non-target plants in the area is reduced. Chemical applications can be made later in the season also but greater care must be taken to reduce exposure of non-target plants to herbicides.
Prevention is the best control overall for garlic mustard and other weeds. Reduce or eliminate any disturbance of the soil surface which would facilitate plant development from seed. Prevent over-grazing or mowing groundcover too close to the ground. Tall, dense groundcover facilitates greater competition and reduces the chance that garlic mustard will take hold. Reduce traffic wear from foot and vehicle movement to encourage strong groundcover growth and control erosion that opens up areas of bare soil. Regular scouting of susceptible areas is crucial for early detection and removal of garlic mustard populations. If populations are discovered and removed, continue to monitor the area for several years to assure the total elimination of this invasive plant.