CU Herb Society Herb of the Month

"Garlic mustard is an exotic weed that was introduced to North America from Europe in the early 1800s. It occupies and spreads in forest floors and other natural areas, such as parks and natural preserves. Garlic mustard can be distinguished from other woodland plants by the characteristic odor of garlic that the plants give off when crushed. Garlic mustard is an invasive biennial herbaceous weed. The first-year plants consist of three to four rounded leaves rising to about four inches from the ground. The plants grow as a roseate in the summer and remain green the following winter. Because of the green plants in winter, it is relatively easy to spot this pesky weed during the winter months. The second-year plants bloom early in May to early June and are about two feet tall when in bloom. They produce numerous white flowers with four separate petals. Fruits, consisting of slender capsules up to two inches long containing a single row of black ridged seeds, are developed in July. The ridged seed coat is believed to help its spread by attaching to animal fur and human clothes or by helping the seed float in water. Consequently, the usual distribution pattern of garlic mustard is along streams, edges of woods, and trails.
"Numerous features of garlic mustard give the plant advantages in competing against our native flora. These features include monoecious reproduction (a single isolated plant can produce numerous seeds), tolerance to cold weather (an escape from competition of native plants), varied periods of seed dormancy (giving unsynchronized germination to escape detrimental environmental conditions), and offensive odor and chemicals within the plants protecting them from herbivores."
"Garlic mustard invades woodland communities in the midwestern U.S. and adjacent Canada. It is now observed in at least 31 states and three provinces. Within individual communities garlic mustard population size may fluctuate widely from year to year, reflecting the biennial nature of this plant and the varied periods of seed dormancy. Across a region, the presence of garlic mustard consistently increases over time. In Illinois, garlic mustard is now widespread through the northern two-thirds of the state and occurs in state parks, natural preserves, and other natural areas in at least 41 counties. Garlic mustard is so aggressive that monospecies stands have become established in many natural areas and now pose a threat to the natural quality of Illinois forests and significantly reduce biodiversity of our native plant communities. It is considered one of the critical exotic weeds in Illinois natural areas.
"The rapid spread of garlic mustard threatens the floristic structure, particularly the herbaceous layer. Current control measures of garlic mustard are mechanical cutting, prescribed fire, and application of glyphosate-containing herbicides. The most effective control of garlic mustard at initial or minor infestations is by hand pulling or by mechanical cutting before flowering begins. If viable seeds are already set in the pulled or cut plants, they should be removed from infested areas preferably to compost piles. Prescribed burning in the fall or early spring is applicable to large infestations. Plants in their green vegetative state cannot tolerate fire and, therefore, they are likely killed during burning. Fall burning can take advantage of the new leaf fall as fuel, but early spring burning can minimize fire injury to spring wildflowers. Due to the biennial nature of garlic mustard and the varied dormancy of its seeds, several years of burning may be required and should be followed with hand pulling and mechanical cutting in order to provide effective control. The control measures can also be combined with application of the glyphosate-containing herbicides. The herbicide can be applied in the fall and early spring to minimize injury to native flora when most native plants are dormant and garlic mustard is green and vulnerable.
"Research efforts in the biological control of garlic mustard have been under way using fungal pathogens to control this invasive weed so that this biocontrol measure can be incorporated into integrated management practices....Garlic mustard has spread rapidly over recent years and this stinking weed will not go away by itself. There are no quick solutions available for this invasive weed. To preserve our native woodland herbaceous flora and the wildlife dependent upon it, concerted efforts are required to work on garlic mustard to discover an applicable and effective integrated approach in order to suppress its rapid spread."

Thanks to Eleanor Sheffield for her report last month on gardening publications, which inspired the presentation of the above article sent in by Jane Myers. It was written by Weidong Chen. Center for Biodiversity, and appeared in the November/December 1998 Illinois Natural History Survey Reports.