University of Wisconsin - Extension

Though you may have seen these purple flowering plants on area wetlands and shorelines as a welcome spot of color, they are a major invasive threat to Wisconsin's wetlands. A close watch should be kept to monitor and treat infestations of the plant, according to Paul Hartman, UW-Extension Brown County horticulture agent and an invasive plants specialist for the Urban Agriculture/Horticulture Team.

Purple loosestrife is native to Europe and Asia, where specialized insects and diseases keep it in check. Since being introduced to North America, where it is free of these natural controls, it has gained a competitive edge over native wetland plants. It has invaded many wetlands to the near total exclusion of most other vegetation; it literally shades everything else out. It threatens Wisconsin's diverse wetland vegetation and most wildlife that depend upon native vegetation for food and shelter.

Purple loosestrife spreads primarily by seed. According to Harman, a single plant can produce over 100,000 seeds a year, and seeds can remain viable in the soil for many years. A loosestrife invasion usually begins with a few pioneering plants. These first plants may not spread for several years as they build up a large seed bank in the soil. When the right soil disturbance occurs, the loosestrife often spreads rapidly and can eventually take over an entire wetland.

The plant can grow from three to seven feet in height, with purple flowers on a spike, closely attached to the stem. There are five to six petals per flower, a stiff, four-sided stem, and paired leaves opposite each other on the stem.

"To prevent the spread of purple loosestrife, look for pioneering plants or small colonies and remove them immediately," said Hartman.

He also recommends rinsing off any equipment, boats, trailers, clothing and footwear used in infested areas. Hartman said property owners should remove and destroy purple loosestrife planted in lawns and gardens, as it is illegal to cultivate purple loosestrife in Wisconsin.

There are several methods for controlling the spread of purple loosestrife. Small young plants can be hand pulled, but be sure to get the entire root. Older plants may be too difficult to dig up. Avoid disturbing the soil too much. Handle plants before the seeds develop in early August, or cut and bag the seed heads to avoid spreading the seeds. Removed plant parts should be dried and disposed of properly. Don't put them in a compost pile. Mowing is not an effective way to destroy purple loosestrife unless the plants are cut below a level that water will cover for about 12 months.

If herbicides are used, glyphosate is the most effective active ingredient for killing loosestrife. Glyphosate comes in two forms:

Roundup, for use on dry sites, and Rodeo for use on wet or standing water sites. Apply in July or August to be most effective. Spray onto loosestrife foliage in a one-percent solution. Glyphosate is a nonselective herbicide and will kill any green foliage, so avoid treating non-target plants. A more selective method of applying glyphosate is to cut the loosestrife stems near the ground and paint or dab the fresh cut surface with a 50 % solution of the herbicide. The cut stems should be removed from the wetland or they may take root.

Galerucella beetles have been used as a biological control method. These beetles are dependent on purple loosestrife and are not a threat to other plants. Beetles are placed in large populations of purple loosestrife and within four years they will take the population down.

For more information about purple loosestrife and other invasive plants, contact your http://www1.uwex.edu/ces/cty">UW-Extension county office or the Wisconsin Department of Natural Resources. The bulletin, "Purple Loosestrife: What You Should Know, What You Can Do," is available from UW-Extension county offices and the DNR.