Purple Loosestrife

Common Name: Purple Loosestrife
Scientific Name: Lythrum salicaria L.

Legal Status

Prohibited Control

Efforts must be made to prevent seed maturation and dispersal of plants into new areas. Additionally, no transportation, propagation, or sale of these plants is allowed. Failure to comply may result in enforcement action by the county or local municipality. 

Background

Purple loosestrife is native to Europe and Asia. It was introduced to North America in the early 1800s in ship ballast and as a medicinal herb. It is now found in 40 US states.

Description

• A semi-aquatic perennial species that typically forms a dense bushy growth of many erect stems reaching heights of approximately 4- 7 feet tall. It is highly visible from July through September because of its robust purple flowering spikes.
• Leaves are smooth-edged, slender, pointed and arranged in opposite pairs along ridged stems.
• Showy spikes of flowers develop at the tops of each stem consisting of many individual 5- 7 petaled purple flowers.
• Large roots develop over time and store high levels of nutrients providing the plant with reserves of energy early in the spring or during stressful periods.

Habitat

Purple loosestrife prefers wet soils or standing water. Loosestrife plants are typically found in poorly drained soils of road right-of-ways and trails, drainage ditches, culverts, lake shores, stream banks, and a variety of wetland habitats.

Means of spread and distribution

Purple loosestrife reproduces both by seed and vegetative propagation which allows it to quickly invade new landscapes. Each flower spike can produce thousands of tiny seeds that are easily dispersed by wind, water, snow, animals, and humans. Purple loosestrife is found throughout Minnesota. The Minnesota Department of Natural Resources estimates that infestations have been recorded in 77 of Minnesota's 87 counties, covering 58,000 acres of lake, river, and wetland habitats.

Impact

Purple loosestrife aggressively invades lakes, rivers, and wetlands, creates large monocultures, and significantly decreases the biological diversity of native plant and wildlife populations.

Prevention and management

• A sound management plan will take several years of commitment, especially on older stands that have an established seed bank. Regular follow-up is critical to ensure the population is decreasing.
• Hand pulling or digging is only recommended when a few plants are discovered on a property. To successfully control purple loosestrife in this manner, the entire root system has to be removed from the soil to prevent re-sprouting of new stems. Checking the site periodically for several years is recommended to ensure that new seedlings or re-sprouts can be destroyed.
• Mowing or cutting is not practical for sites where loosestrife is growing in an aquatic or semi-aquatic environment. However, if conditions permit, and if executed prior to flowering, mowing or cutting can reduce seed production. Re-sprouts will vigorously appear following mowing, so follow-up cutting will be necessary to prevent seed production during the growing season. Make sure to wash equipment thoroughly following mowing to prevent spread of seeds to new areas.
• Various herbicides have been used successfully against purple loosestrife in Minnesota. Due to the fact that purple loosestrife is a semi-aquatic to aquatic species, it is IMPORTANT to use only herbicides that are labeled and approved for use in or around water. If treating plants near water with herbicide, please be aware of the state pesticide laws and use only products labeled for aquatic use. If using herbicide treatments, check with your local University of Minnesota Extension agent, co-op, or certified landscape care expert for assistance and recommendations. There are several businesses throughout the state with certified herbicide applicators that can be hired to perform chemical applications.
• Biological control, using host-specific natural enemies of purple loosestrife, is a popular form of management for this species in Minnesota. Biological control agents feed specifically on purple loosestrife plants and have been shown to provide a long-term sustainable management solution. The Minnesota Department of Natural Resources, in cooperation with the Minnesota Association of County Agricultural Inspectors, oversees a statewide biological control program for this noxious weed.
• Purple loosestrife lifecycle and treatment timing graphic