April will soon be here, bringing the colors and fragrance of spring flowers, but also the unwelcome annual bloom of garlic mustard. While its name may sound like a spicy condiment, garlic mustard is actually one of Ohio's worst invasive weeds.
Native to Europe and Asia, garlic mustard (Alliaria petiolata) was introduced into New York in the 1860s and spread rapidly. It is now abundant in the northeastern and central U.S., including Ohio. It tends to move in initially along rivers, roads and trails and then spread out from there. The seeds may be carried in bits of mud on people's shoes, as well as by floodwaters and roadway mowing equipment.
Garlic mustard grows in a wide range of habitats but especially thrives in moist, shady woodlands, where it crowds out native wildflowers. Producing up to 800 seeds per plant, yielding as many as 1800 seedlings per square foot of ground, it quickly forms dense patches. Once it arrives, a diverse forest understory can quickly become a solid stand of garlic mustard. It has few natural enemies in North America; even the deer won't eat it. Garlic mustard also contains chemicals that suppress the fungal partners (called mycorrhizae) that most plants—but not garlic mustard—depend on to help them absorb water and minerals from the soil. These chemicals even suppress everyone’s favorite spring mushroom, Morels.
Garlic mustard is a biennial. It spends its first year as a low-growing rosette of yellowish-green, wrinkly-veiny leaves. The following spring, the stems rapidly lengthen and produce clusters of small, white, cross-shaped flowers. The flower clusters are initially flat-topped but become elongate as the shoot continues to grow. The flowers develop into narrowly cylindrical green fruits, which eventually turn brown and release their seeds, after which the plant dies. Garlic mustard is easily distinguished by its kidney-shaped to triangular, prominently veiny leaves with a garlicky odor when crushed.
It is important to watch for garlic mustard and remove it as soon as it colonizes a site. Small patches can easily be pulled up by hand, and a weed-whip is helpful with larger infestations. However, this treatment must be repeated annually because the seeds may remain viable in the soil for five to ten years. Plants should ideally be pulled up or cut at ground level before or shortly after they start flowering. If the fruits have already started to form, then either the pulled plants should be bagged and removed or the fruits should be removed from each plant as it is pulled up. Fruits that are left attached to the plant sometimes continue development and release seeds even though the root is no longer in the ground. Larger populations may require the assistance of chemical herbicides.