Purple Loosestrife

Purple loosestrife (Lythrum salicaria) is a nuisance exotic weed that is extremely invasive in Wisconsin’s wetlands. Although colorful, this plant is extremely undesirable because it prevents many desirable native wetland plants from becoming established.

Purple loosestrife is a perennial herb 3-7 feet tall with a dense bushy growth of 1-50 stems. Showy flowers vary from purple to magenta, possess 5-6 petals aggregated into numerous long spikes, and bloom from July to September. Leaves are opposite, nearly linear, and attached to four-sided stems without stalks.

Purple loosestrife was first introduced as a garden perennial from Europe during the 1800's. The plant was first detected in Wisconsin in the early 1930's, but remained uncommon until the 1970's. It is now widely dispersed in the state, and has been recorded in 70 of Wisconsin's 72 counties. Low densities in most areas of the state suggest that the plant is still in the pioneering stage of establishment. Areas of heaviest infestation are sections of the Wisconsin River, the extreme southeastern part of the state, and the Wolf and Fox River drainage systems.

Purple Loosestrife Infestation

This plant's optimal habitat includes marshes, stream margins, flood plains, sedge meadows, and wet prairies. It is tolerant of moist soil and shallow water sites such as pastures and meadows, although established plants can tolerate drier conditions.

Purple loosestrife spreads mainly by seed, but it can also spread vegetatively from root or stem segments. A single stalk can produce from 100,000 to 300,000 seeds per year. Seed survival is up to 60-70%, resulting in an extensive seed bank. The absence of natural predators, like European species of herbivorous beetles that feed on the plant's roots and leaves, also contributes to its proliferation in North America.

The plant's ability to adjust to a wide range of environmental conditions gives it a competitive advantage; coupled with its reproductive strategy, purple loosestrife tends to create monotypic stands that reduce biotic diversity. Purple loosestrife displaces native wetland vegetation and degrades wildlife habitat. As native vegetation is displaced, rare plants are often the first species to disappear. Eventually, purple loosestrife can overrun wetlands thousands of acres in size, and almost entirely eliminate the open water habitat. The plant can also be detrimental to recreation by choking waterways.

By law, purple loosestrife is a nuisance species in Wisconsin. It is illegal to sell, distribute, or cultivate the plants or seeds, including any of its cultivars.

Control

The primary control season is mid-July through mid-August.

Plants are treated with the herbicide Rodeo, with a concentration of active ingredient of 10% in water. (Rodeo is a form of the herbicide glyphosate that has been formulated for use in and around water.)

The method preferred by the Wisconsin Department of Natural Resources is to traverse the wetland on foot. Old shoes and pants should be worn. In cases of extreme muck these articles of clothing may need to be discarded afterwards.

Once the target (Purple Loosestrife) is located, tear or cut off the inflorescence (flower cluster). If early enough in July, and the plants are not beginning to make seed, the inflorescence can be discarded on the ground. If seed production has already begun, the inflorescence must be bagged and removed from the site.

A wooden-handled paintbrush is affixed with a screw inside a wide-mouth plastic water bottle, such as a Nalgene (available at an outdoor store). Cut the top of the paintbrush square and to a length so that it just reaches the bottom of the bottle. (See photo) It is important to seal around the screw and washer with glue so that the herbicide cannot leak out. In use, hold the paintbrush containing herbicide by the plastic top and wipe off the excess liquid before painting the loosestrife stems.

Paint brush fixed inside lid of plastic bottle. Be sure to use a Nalgene bottle because it does not require a lid liner for tight seal.

Screw arrangement for attaching the paint brush.

Close-up of the inside of the lid, showing how the paint brush handle is positioned.

 

 

 

 

 

 

 

 

 

 

With the paintbrush herbicide applicator, paint each stem of the loosestrife plant from the top down about 3 feet. Be careful to get as many of the stalks painted as possible. Applying herbicide with a paintbrush (instead of a spray bottle) ensures that the chemical is confined to the stem. It is important to recheck the eradication area again approximately two weeks later and treat any plants that were missed and are still alive.

- Contributed by Richard Staffen, Bureau of Endangered Resources, Wisconsin Dept. of Natural Resources

 

See Cella Chow!
A Purple Loosestrife Biological Control Manual for Educators (WDNR)

 

 

Also see Wisconsin DNR site.