By: Bill Reichenbach, Certified Arborist WI-0188A
In the past 15 years the exotic invasive garlic mustard (Alliaria petiolata) has made huge inroads into southeast Wisconsin. It grows in upland and floodplain forests, along roadsides and in our home landscapes; it generally grows in shaded areas. Left uncontrolled it can cover large areas in a few years, displacing Wisconsin’s native woodland plants and is becoming an ugly menace in our home’s landscape.
Garlic mustard is a cool season biennial (grows early and late in the cool spring and fall seasons, and has a two-year life cycle). The leaves and stems emit the distinctive odor of onion or garlic when crushed. Firstyear plants grow in very early spring from tiny seedlings and consist of a cluster of round, scallop edged leaves rising 2 to 4 inches in a rosette. Second-year plants’ foliage is more triangular and toothed, and taller than firstyear plants. Second-year plants produce 2- to 3-foot flowering stems with white flowers in May and June. Very quickly, seeds are produced in slender capsules on the tops of the plants. After producing seeds the 2-year-old plant dies. However, each plant can produce hundreds of seeds which are spread by wildlife and human activity. Seeds can remain viable for 5 years in the soil, re-infesting and spreading throughout the landscape.
Because of its life cycle, garlic mustard has an Achilles heel. Since it is a cool season plant, it is green and growing very early in spring before other desirable plants are up. A glyphosate herbicide can be used to kill garlic mustard before it goes to flower and seed. In certain situations, the use of fire (a propane torch) can be used to kill seedlings or early first-year plants. Care must be taken to treat it early in spring to avoid damage to non-target plants. Small infestations can be controlled by careful hand pulling. If plants are not controlled in early spring, it is imperative that at the onset of flowering, plants be cut as close to the ground as possible, thus eliminating seed production. Any stems that have flowered and developed seed must be picked up, bagged and properly disposed.
Wachtel Tree Science and Service can help you control garlic mustard. Our trained staff can identify and develop control strategies and provide services that can help you win the battle over garlic mustard. Keep in mind that it is important that you call early in the spring. Also, as with most pests, 100% control is rarely achieved. It will be important to have continued monitoring of the site with continued control measures implemented over time.
Consider planting native trees, shrubs, and herbaceous perennials to fill space and compete with garlic mustard. Mulching around new plantings with shredded bark mulch will aid in their establishment and help limit garlic mustard infestations.
Garlic mustard is here to stay, but over time, with diligent management, we can win the battle over this nasty weed.
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