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Purple loosestrife (Lythrum salicaria)
Wetland perennial, 3’-7’ tall, with up to 50 stems topped with purple flower spikes. One main leader stem, but many side branches often make the plant look bushy. Clipped plants grow back, and cut stems readily re-root in soil to produce new plants. Many areas of the state use safe biocontrol beetles that feed on the loosestrife to keep it in check and allow other plants to grow.
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, lance-shaped and do not have petioles. Usually opposite and rotated 90 degrees from those below, but are sometimes whorled.
Flowers: closely attached to the stem with 5-6 pink-rose colored petals. Blooms from the bottom of the flower spike to the top from early July to September. Plants can bloom the first year after seeds germinate.
Fruits & seeds: capsules burst open when mature in late July-September. A single stem can produce 100,000-300,000 seeds per year. Mature plants with many stems can produce 2 million seeds. Seeds are viable for at least 7 years.
Roots: large woody taproot and many side roots. Plants intertwine to form dense clumps.
Stems: green, sometimes tinged purple, stiff, erect, and generally 4-sided (older stems, 5 or 6 sided).
Similar species: Garden loosestrife (Lysimachia vulgaris) is a non-native, wetland garden escapee with yellow flowers. Smaller, native winged loosestrife (L. alatum) is found in moist prairies and wet meadows, has winged, square stems, solitary flowers in separated leaf axils, paired lower leaves and alternate upper leaves. Swamp loosestrife (Decodon verticillatus) arches out from shorelines, has mostly whorled leaves, and flowers in well-separated leaf axils.
Identifying purple loosestrife in spring
Spring purple loosestrife stem tops and seed pods. Click image to enlarge.
Spring purple loosestrife (PL) and native wetland look-a-like stems from left: 2 year old PL-2, one year old PL-2, Steeplebush (Spiraea tomentosa), Swamp Loosestrife (Decodon verticillatus), Great Water Dock (Rumex britannica). Click image to enlarge.
Spring purple loosestrife clumps without leaves or flowers. Click image to enlarge.
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: Young, small plants can be dug or pulled. Larger plants can be dug if all root fragments are removed. Burn, landfill, or bury all plant parts deep in the ground. Mowing is not recommended as plant parts may re-sprout and seeds may be dispersed.
Chemical: Imazapyr or glyphosate work well against purple loosestrife. If near water a permit may be required and aquatic-use formulas of these herbicides should be used.
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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