Invasive Plant Management
In November of 2007 the Des Moines Park and Recreation Department began measures to control garlic mustard, an aggressive biennial plant that is an increasing threat to the aesthetics and ecological integrity of the woodlands in the city’s Greenwood and Ashworth Parks, located from 45th to 49th Streets, between Grand Avenue and the Raccoon River. Initial eradication efforts will involve herbicide application when the temperature is above freezing this fall through early spring; native plants will be dormant, but the garlic mustard plants will remain green and photosynthesizing. Native plants, which emerge later in the spring, will not be affected by the herbicide because the glyphosate herbicide, known on the retail market by such brand names as Roundup, is not residual in the soil. As is typically the case with herbicide applications, people are asked to stay off treated areas for 24 hours following treatment; dated signs along major entries to the park areas indicate when those portions will be again be available for use.
Intensive follow-up efforts using multiple control techniques over the course of the next four to five years are necessary in order to deplete the seed bank of the garlic mustard in the soil. Measures will include spring prescribed burns, further herbicide applications and cutting of plants. Thereafter, continued monitoring and routine maintenance as well as eradication by property owners adjacent to the parks will be necessary to prevent another outbreak of garlic mustard.
A native of Europe, the rapidly spreading plant was introduced in the United States in the mid-1800s for medicinal and herbal purposes and has no native predatory insects or other natural controls to curb its rapid spread in this country. Growing tall and dense within a few years, it crowds out woodland understory plants such as wildflowers, ferns and tree seedlings as well as wildlife habitat. Garlic mustard prefers shaded and semi-shaded areas, and spreads most rapidly in highly-disturbed areas such as along trails, waterways and forest edges.