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Garlic Mustard: A Vinegary Disposition

2nd year garlic mustard flowering in woods. Photo by: King County, Washington

Earlier this summer, Minnesota Valley Master Gardeners spotted a grove of garlic mustard (Alliaria petiolata) across Locust Street. The plant made its home on the edge of the woods, where it felt most “at home”. The land belonged to the City of Mankato, and we quickly got permission to start the removal process of this noxious weed. It was a great opportunity to teach about invasive species to both Master Gardeners and members of the public. Soon enough we started removing this damaging weed by hand. Common buckthorn (Rhamnus cathartica) was also present in the area, and it was fun teaching the tricks to identifying them with our new Master Gardener interns. One tip is to scour the main stem of a suspected buckthorn with a knife. If the wood just below the bark looks orange, chances are it is indeed that invasive bush. However, it was garlic mustard that was the real goal that day, not the buckthorn. To be honest, early summer was a poor time to combat the mustard. The plant was just setting seed, and doubtless we missed a plant or two. To make matters worse, the seeds from garlic mustard can lay dormant in the soil for years. That is why it is important to remove the whole plant off site, simply mowing the plant risks spreading seeds that could be a headache later.

Ideally, we would venture out again in the fall to help further reduce the weed’s population. Because garlic mustard is a biennial weed, the 1st year of its life is spent as a rosette low in the forest floor. Often, it is confused with either wild ginger (Asarum species) or violets (Viola species) due to its kidney shaped leaves. One trick to find them is to scout in the late fall, as garlic mustard is one of the last green plants in the woods. Herbicide applications are generally the favored technique for reducing garlic mustard 1st year rosettes. If you decide to go this route, a labeled broadleaf herbicide that contains the active ingredient triclopyr is the preferred way to attack them.

One aspect of garlic mustard, aside from its annoying growth habit, is its influence on the soil underneath. The weight of scientific studies seem to suggest garlic mustard is an alleopathic plant, which puts out chemicals that hurt the growth of its neighbors. One of garlic mustard’s favorite “victims” is the false Solomon’s seal (Maianthemum species) which tends to inhabit the same forest areas. The thinking is that garlic mustard’s chemical can not only harm plants, but also beneficial soil fungi called mycorrhizae. These fungal organisms can help provide important nutrients–such as phosphorus–to the plant in exchange for some sugar. Some estimates are that 70% of all land plant species have some sort of arrangement with mycorrhizae.

Species in the mustard family (Brassicaceae), which garlic mustard is a part of, are notorious for having many of their members shun this fungal relationship. Since garlic mustard can do fine without these beneficial fungi, it stands to gain by depriving its plant neighbors from receiving the extra nutrients from them. Some mustard species (including garlic mustard) can produce a group of chemicals called glucosinolates, which decomposes into a toxic gas for plants, nematodes, and fungi. Botanists think it is this chemical which makes garlic mustard alleopathic. In its native European habitat, garlic mustard’s plant competitors and soil microbes have adapted somewhat to glucosinolates. In new or disturbed areas, such as near Glenwood Gardens, the environment is caught off guard by the chemical.

Glucosinolates are not all bad, however. It is responsible for the spicy or bitter tastes we enjoy with horseradish or Brussel sprouts. There are also cases where this chemical’s effects in soil can be leveraged by humans. While not a slam dunk scientifically speaking, glucosinolates can be used as a soil treatment for disease carrying fungi. Specially bred mustards (note, not garlic mustard) are finely chopped up at flowering, and then buried. The reason for the fine chopping is that the plant’s sap contain the highest amount of glucosinolates, and by burying them, we can get good contact with disease causing fungi. Organic vegetable farms that use this method plant mustard in late summer/early fall, and sometimes these plants are fertilized with nitrogen and sulfur to increase the amount of chemical produced. While there is a chance to reduce the amount of soil borne disease by using this method, it is no substitute for plant rotation, which is the surest way to head off fungal issues. Practice a 4 to 5 year rotation based on plant families. If you do not have enough space in the garden to have tons of tomatoes, consider container planting with adapted varieties. All ketchup and no mustard is no fun!

Photo by: King County, Washington. 2nd year garlic mustard flowering in woods.

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