Garlic Mustard (Alliaria petiolata)

Jerry Doll

Garlic mustard has gained much attention in recent years for its ability to rapidly invade once introduced into a site. It is of particular concern once it appears in wooded habitats because it rapidly spreads and displaces native woodland wildflowers in Wisconsin. It can dominate the forest floor and can displace most native herbaceous species within ten years. Unlike other plants that invade disturbed habitats, garlic mustard readily spreads into high quality forests.

Garlic mustard is a major threat to the survival of Wisconsin's woodland herbaceous flora and the wildlife that depend on it. There are two modes of spread: an advancing front, and satellite population expansion possibly facilitated by small mammals. Knowing more about its life history, how to identify it and subsequently manage it are becoming increasingly important.

Life History

Garlic mustard is a biennial that produces hundreds of seeds per plant. In our areas, seeds lie dormant for 20 months prior to germination, and may remain viable for five years. Seeds germinate in early April. First-year plants appear as basal rosettes in the summer season. First-year plants remain green through the following winter, making it possible to check for the presence of this plant in woods throughout the year. Garlic mustard begins vegetative growth very early in the spring, and blooms in southern Wisconsin from May through early June. Fruits begin to ripen in mid-July, and are disseminated in August. Viable seeds are produced within days of initial flowering. The seeds are believed to be dispersed on the fur of mammals such as deer, horses, and squirrels, by flowing water, and by human activities.

Description

Garlic mustard is a cool-season biennial herb that ranges from 12 to 48 inches in height as an adult flowering plant. Garlic mustard is the only plant of this height in our woods with white flowers in May. Leaves and stems emit the distinctive odor of onion or garlic when crushed (particularly in spring and early summer), and help distinguish the plant from all other woodland mustard plants. First-year plants consist of a cluster of three or four round, scallop-edged, dark-green leaves rising 2 to 4 inches in a rosette.

Second-year plants generally produce one or two flowering stems with numerous small white flowers that have four separate petals. Stem leaves are alternate and triangular in shape, have large teeth, and can be 2 to 3 inches across in flowering plants. Petioles are longer on the leaves toward the base. Fruits are slender capsules 1 to 2 2 inches long that produce a single row of oblong black seeds with ridged seed coats. Garlic mustard can also be distinguished by its taproot, which is slender, white, and "s"-shaped at the top of the root.

Distribution and Habitat

Garlic mustard is an exotic species introduced from Europe presumably by early settlers for its supposed medicinal properties and for use in cooking. It is widely distributed throughout the northeastern and Midwestern U.S. from Canada to South Carolina and west to Kansas, North Dakota, and as far as Colorado and Utah. In Wisconsin, the plant is currently concentrated in the southeastern and northeastern counties, although distribution records indicate its presence is nearly statewide.

Garlic mustard grows in upland and flood plain forests, savannas, yards, barnyards, and along roadsides. It is shade-tolerant, and generally requires some shade; while not commonly found in sunny habitats, it will grow and spread in sunny areas. It cannot tolerate acidic soils. The invasion of forests usually begins along the wood's edge, and progresses via streams, campgrounds, and trails.

Controlling Garlic Mustard

Mechanical Control: Minor infestations can be eradicated by hand-pulling at or before the onset of flowering, or by cutting the flower stalk as close to the soil surface as possible just as flowering begins (cutting a couple inches above ground level is not quite as effective). Cutting prior to this time may promote resprouting. Cutting flowering plants at the ground level has resulted in 99% mortality and eliminates seed production. A scythe, monofilament weed whip, or power brush cutter may be helpful if the infestation covers a large area. When hand-pulling garlic mustard, the upper half of the root must be removed to prevent buds at the root crown from sending up new flower stalks. Pulling can result in soil disturbance, damaging desirable species, and bringing up garlic mustard seeds from the seed bank which then germinate. This can be partially prevented by thoroughly tamping the soil after pulling. If, however, seed bank depletion is desired, leave the soil in a disturbed state to encourage further germination, and return annually to remove plants.

In general, cutting is less destructive than pulling as a control method, but can be done only during flower stalk elongation. Pulling can be done at any time when the soil is not frozen. If flowering has progressed to the point that viable seed exists, remove the cut or pulled plants from the area. Because seeds remain viable for five years, it is essential that an area be monitored and plants removed for at least five years after the initial control efforts.

Burning: For larger infestations, fall or early spring burning may be effective. First-year plants are killed by fire, if the fire is hot enough to remove all leaf litter. However, the bare soil enhances survival of seedlings that germinate after the fire, and the total population may increase after burning. Dense populations may be controlled most effectively by fall burning, when leaf litter provides adequate fuel. Spring burns should be conducted early enough to minimize possible injury to spring wildflowers. Three to five years of burning are required, and should be followed by hand-pulling or cutting of small populations produced from the seed bank. Garlic mustard plants hit by fire are generally killed. Because most woodland fires are patchy, flame torches may be useful in areas not burned in entirety.

Chemical Control: Severe infestations can be controlled by applying a 1 to 2% solution of Roundup or Touchdown (glyphosate) to the foliage of individual plants and dense patches during late fall or early spring. At these times most native plants are dormant, but garlic mustard is green and vulnerable. Glyphosate is a nonselective herbicide that will kill or injure all green nontarget plants if it comes into contact with them. Use caution during application, and do not spray so that herbicide neither drips from the garlic mustard leaves nor drifts onto adjacent desired vegetation. Other herbicides that control mustards are expected to also control garlic mustard. This includes 2,4-D, triclorpyr (Garlon) and the combination of these products (sold as Crossbow). Herbicide use is safest for native plants if done during the dormant season, as garlic mustard will grow as long as there is no snow cover and the temperature is greater than 35 degrees F.

Biological Control: Scientist at Cornell University plan to do research on biological control of garlic mustard but at this time there are no known biological control agents to suppress this weed.

- Adapted from pages 27-29 of the " Wisconsin Manual of Control Recommendations for Ecologically Invasive Plants" published by the Bureau of Endangered Resources, Wis. Dept. Natural Resources, Madison. 1997. Eds. R. Hoffman and K. Kearns. 

May, 1999.