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Purple loosestrife (Lythrum salicaria)
Wetland perennial that can be 3-7’ tall and have as many stems as 1-50. Stems are green-purple, stiff and generally 4-sided; sometimes they have 5 or 6 sides. Branching stems make the plant look bushy; plants die back each year If stems are clipped, trampled or buried they can take root and produce new plants.
Purple loosestrife is Restricted (Orange counties)
Other names for this plant include:
- Common names: spiked loosestrife
- Scientific names: L. salicaria var. tomentosum; L. salicaria var. vulgare
- Prefers moist soils and shallow waters where it competes with native wetland plants. It will adjust to varying light conditions and water levels.
- Has been widely planted as an ornamental where it escapes to nearby water ways. It is still sold in nurseries as a sterile variety; however, it can still produce viable seeds with wild varieties.
Classification in Wisconsin: Restricted
Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for purple loosestrife was based upon this literature review developed by the department.
Leaves: Simple leaves that are usually opposite but are sometimes whorled. They are lance-shaped and do not have petioles.
Flowers: Flowers have 5-6 petals that are pink-rose colored. They are closely attached to the stem and bloom from the bottom of the flower spike to the top from early July to September. Plants will bloom after several years.
Fruits & seeds: Capsules will burst open when mature in late July-August. A single stem can produce 100,000- 300,000 seeds per year; mature plants with around 50 stems can produce 2 million seeds. These seeds are viable for up to 20 years.
Roots: A large woody taproot with fibrous rhizomes; rhizomes form dense mats.
Similar species: Garden loosestrife (Lythrum vulgaris) is not as invasive but is also an escaped non-native wetland plant that has yellow flowers. Native wing-angled loosestrife (L. alatum) is found in moist prairies and wet meadows; has winged, square stems and solitary flowers in the leaf axils. Lower leaves are paired while the upper leaves are alternate.
Counties in WI where purple loosestrife has been reported (as of July 2011). Both vouchered and unvouchered reports included.
Do you have purple loosestrife in your county but it isn't shaded on the map? Send us a report.
Do you know of purple loosestrife infestations and want to do something about it? Visit the purple loosestrife biocontrol page to learn more.
Mechanical: Small plants can be dug or pulled out. Larger plants can be dug out making sure to remove root fragments. Remove all plant parts and either burn, landfill, or bury deep in the ground. Mowing is not recommended due to plant parts starting new populations and seed dispersal.
Chemical: If near water, permit may be required. Foliar spray triclopyr amine.
Biological: Galerucella beetles have been successful in many parts of the state in controlling purple loosestrife populations. Want to get involved with biocontrol? Find out more on our purple loosestrife biocontrol page.
View purple loosestrife pictures in our photo gallery!
Sources for content:
- Czarapata, Elizabeth; Invasive Plants of the Upper Midwest: an illustrated guide to their identification and control. University of Wisconsin Press. 2005. Pg. 65-68
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