Purple Loosestrife – Oneida County Aquatic Invasive Species Program

Purple Loosestrife (Lythrum salicaria)

PicturePhoto by Paul Skawinski

Purple loosestrife is an aggressive perennial aquatic invasive species that is a threat to the integrity and function of wetlands, shorelines, and roadside ecosystems. Purple loosestrife forms dense, monotypic stands resulting in suppression of native plant communities, loss of wildlife habitat and food sources, and reduces spawning and breeding habitat. Birds and animals do not consume the tiny hard seeds. The plant’s growth is generally too compact to offer shelter for wildlife and nesting habitat for waterfowl. In fact, waterfowl avoid wetlands dominated by purple loosestrife.
Large infestations can reduce a wetlands flood prevention and pollution control abilities, change hydrology and overall ecological function of a wetland.  Purple loosestrifes dense, snarled root system can clog irrigation ditches and decrease water flow in irrigation canal resulting in decreased crop yield and quality. Furthermore, the plant also reduces recreational opportunities such as boating, wildlife viewing, fishing, and hunting. This, in turn, can hurt local economies.
Although purple loosestrife prefers moist, organic soils and full sun, it can survive and multiply in many soil types, varying light, and water levels, poor water quality and is often the first species to invade a disturbed site. It has evolved to adapt to short growing seasons and cold weather. As a perennial plant, purple loosestrife sends up numerous flowering stems year after year, each with tremendous seed production. In one year, a mature plant can produce over 1 million seeds with a generally high germination rate.
Native to Europe and Asia, Purple Loosestrife was first reported from the northeastern coast of North America in the 1800’s. Settlers brought it for their gardens and medicinal use.  Seeds may have accidentally arrived in sheep fleece, imported livestock feed and bedding, and in discarded soil used for ship ballast. It has since escaped from cultivation and nurseries; it is now an at least 40 states and Canada. 
Purple loosestrife spreads easily by water, wind, wildlife, and humans. When flowers drop off, capsules containing seeds the size of ground pepper appear in their place. People spread purple loosestrife primarily through the movement of water-related equipment and uninformed release of garden plants. Seeds can hide in mud and debris, and can stick to boots, waders, and other fishing and hunting gear. Roadside maintenance equipment can also spread this plant and its seeds.
When working with this plant, please remove soil, seeds, plant parts and other debris from your shoes and clothing prior to leaving an area.
By law, purple loosestrife is a restricted invasive species in Wisconsin. It is illegal to sell, buy, give away, barter, transport, introduce, cultivate, and transfer. Do not use regulated species in plantings or projects. In addition, purple loosestrife seeds may be present in some wildflower seed mixes; check the label before you buy any seed packages.


Plants range from two to nine feet tall, with several half to one foot long flower stalks on a single plant. Established plants can have dozens of stems and take on a bushy appearance.
Flowers:  Individual flowers have 5 – 6 pink-purple petals surrounding small, yellow centers, and closely attached at the stem. Single flowers make up flower spikes, which can be up to one foot tall and blooms from the bottom of the flower spike to the top. Flowers bloom from early July through September, and then go to seed.
Seeds: Capsules burst open when mature in late July-September. A single stem can produce 100,000-300,000 seeds per year and mature plants with many stems can produce 2.5 million seeds per year. Germination can occur the following season, but seeds can also lay dormant for several years before sprouting.
Leaves: Simple lance-shaped leaves with smooth edges and grow up to 4” long. Leaves are usually opposite each other on the stem or occasionally whorls of 3 .  Pairs alternate at 90 degrees from the pair below. Leaves are attached directly around the stem.
Stems: A recognizable feature is the square-shaped stem, which is generally four to six-sided. Stems are green, sometimes tinged purple. A single plant is made up of multiple woody stems.
Roots: Purple loosestrife produces thick, woody roots. On mature plants, roots are extensive and can send out 30 to 50 shoots, creating a dense web. Pieces of the roots and stem fragments can also produce new plants.
Native Look-Alikes
Blue Vervain (Verbena hastate)
Gayfeather/Blazing Star (Liatrus pycnostachya)
False Dragonhead (Physostegia virginiana)
Fireweed (Epilobium angustifolium)
Joe-Pye Weed (Eupatorium maculatum)
Lupine (Lupinus perennis)
Pickeral Weed (Pontederia cordata)
Smartweed (Polygonum sp)
Steeplebush (Spiraea tomentosa)
Swamp Loosestrife (Decodon verticillatus) 
Winged Loosestrife (Lythrum alatum)



To reduce seed dispersal while handling any part of the plant, remove all of the flowering spikes first by bending them over a plastic bag and cutting them off into the bag. Further cutting of stems or pulling can now take place without fear of spreading the tiny seeds.

Ensure that the plastic bag is tightly secured to prevent any part of the plant from spreading. Do not compost the plants! Destroy the plants by burning or visit the Wisconsin DNR’s website for additional disposal options at  https://dnr.wi.gov/topic/Invasives/control.html#disposal.

Manual removal:  Removal of small stands of purple loosestrife can be very effective! Individual plants, small plants, and plants in moist, soft substrate (such as in the water or along the shoreline) can be removed easily by hand or with a shovel. Very large stands may require using a shovel to remove the entire plant and roots, but can be labor intensive.
Mowing is not recommended as plants will likely re-sprout and seeds may be spread.
Chemical: For the current regulations and permits, please refer the DNR’s website at https://dnr.wi.gov/topic/Invasives/fact/purpleloosestrife.html
Biological control using Galerucella beetles: Many areas of the state, including Oneida County, use safe biocontrol beetles that feed on the loosestrife to keep it in check and allow other plants to grow.
Big Carr Lake, Buckskin Lake, Chain Lake, Crescent Lake, Hemlock Lake, Island Lake, Katherine Lake, Kawaguesaga Lake, Lake Nokomis, Little Carr Lake, Lower Kaubashine Lake, Madeline Lake, Manson Lake, McNaughton Lake, Mercer Lake, Minocqua Lake, Pelican Lake, Planting Ground Lake, Sand Lake, Spirit Lake, Squaw Lake, Sureshot Lake, Sweeny Lake, Tomahawk River, and Upper Post Lake.

Seed Head Cutting: As a management effort, we get out into the field and cut the flowering heads off of the plants. This helps by reducing the number of seeds that are added to the area. In addition to reducing the seeds in an area, cutting the flowering heads also stresses the plant. It takes a lot of stored energy from the plant to regrow the flower. If we can stress the plant enough, it may not return the next year.

Biocontrol: Oneida County AIS team raises Galerucella beetles to use as a biocontrol management method to fight purple loosestrife. These beetles originally were introduced to the US along with the purple loosestrife plant. They occur naturally with the plant and they spend their entire life cycle on the purple loosestrife plant consuming the leaves. The goal of this management method is to have enough beetles living on and consuming the plant to stress the plant. The stress will cause the plant to be smaller, if there is enough stress on a particular plant it may cause the demise of that plant! Beetles are harvested in late spring from purple loosestrife stands in Oneida County and brought back to an environment where they are given prime habitat to reproduce. Once the beetle larvae has hatched, they are released into a dense area of purple loosestrife to do their damage.

If you have questions, need help identifying suspected plants on your property, or help with removal, please contact Stephanie Boismenue, Oneida County AIS Coordinator at sboismenue@co.oneida.wi.us

Purple Loosestrife and its Native Lookalikes
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