In 1987 purple loosestrife was designated a noxious weed, making the sale and transport of this plant illegal in Minnesota. According to a 1975 Minnesota statute, it is the responsibility of the occupant or owner, if unoccupied, of privately owned land or the person in charge of public land to control or destroy noxious weeds to prevent their spread.
There are several species of loosestrife. Lythrum salicaria (purple loosestrife) and L. virgatum are species native to Europe. In nature, purple loosestrife lives where soils are wet or have shallow standing water, including wet meadows, pasture wetlands, cattail marshes, stream and riverbanks, lake shores and ditches. It has also been commonly grown in perennial gardens. Because loosestrife is particularly dangerous when planted near water, it is illegal to grow any of these plants anywhere in Minnesota.
Since purple loosestrife was introduced from Europe it has no natural enemies to keep it in check in North America. When conditions are right, a small clump can spread throughout a marsh in a single season. It is very aggressive and will crowd out native vegetation required by wildlife, while serving no useful purpose for the wildlife.
Economic impact is felt by the need to sustain waterfowl habitat and production, the loss of fisheries (especially northern pike spawning sites), competition in wild rice paddies, and a reduction in resale land value.
Purple loosestrife is a bushy, hardy perennial that grows from 2 to 7 feet tall. The small flowers (one half to one inch in size) are a purple-magenta color with 5 to 6 petals per flower. It blooms on spikes 12 to 24 inches long, from mid-July through the end of August. Purple loosestrife has a stiff, four-sided stem with leaves usually in pairs, attached directly to the stem. Leaves are narrow and more or less hairy with smooth edges. The plants have a woody taproot and a fibrous root system that forms a dense mat.
Several horticultural varieties of loosestrife have been developed with parentage or species of Lythrum salicaria or L. virgatum prior to its designation as a noxious weed in 1987. Recent studies have found ALL these varieties have viable seeds that could be spread by wind, water or animals.
Controlling small populations is possible by removing young plants by hand. Older plants can be dug up, "teasing" the roots loose with a hand cultivator -- try not to break their roots, as they may re-sprout. Plants under water can be cut for three growing seasons, but loosestrife on dry land should be dug out. All parts should be removed, dried, and burned, if possible, or bagged and sent with the trash collectors. Check the area periodically for re-sprouting or new purple loosestrife plants coming in.
Large, dense populations are not as easily controlled...and not a job for amateurs. For detailed spray instructions, herbicide applicator license information, and a list of dealers and licensed commercial applicators phone the DNR Purple Loosestrife Program Coordinator at 651-297-3763. You may also call this number to report new purple loosestrife sites to the DNR. If you plan to treat a wetlands area with an herbicide yourself, a permit is required from the DNR.
Spraying in June or July is most effective as it keeps plants from setting seeds. A follow-up treatment for at least one more season is necessary as new seedlings may sprout, some may be missed, and others may survive the treatment. If the loosestrife is well-established, removing flower spikes and seed heads helps reduce the seed supply.
Addendum: Winged Loosestrife, Lythrum alatum is a non-invasive species native to Minnesota. Though legal to grow, it's difficult to find in the nursery trade. Check with places specializing in wildflowers and native plants.