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Purple loosestrife

(Lythrum salicaria)

Photo of purple loosestrife
Photo credit: S. Kelly Kearns

Wetland perennial, three to seven feet 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 the 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.

Overview

Other names for this plant
  • Common names: spiked loosestrife
  • Scientific names: L. salicaria var. tomentosum; L. salicaria var. vulgare
Ecological threat
  • 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 waterways. It is still sold in nurseries as a sterile variety; however, it can still produce viable seeds with wild varieties.
Overview map of purple loosestrife classification in WI
Restricted (orange) counties

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 [PDF] developed by the department.

Identification

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 five to six pink-rose colored petals. Blooms from the bottom of the flower spike to the top from late June to September. Plants can bloom the first year after seeds germinate.

Fruits & seeds: Capsules start bursting open from the bottom of the inflorescence upwards from July through October, often while still flowering above. A single stem can produce 100,000-300,000 seeds per year. Mature plants with many stems can produce two million seeds. Seeds are viable for at least seven years.

Roots: Large woody taproot and many side roots. Plants intertwine to form dense clumps.

Stems: Green, sometimes tinged purple, stiff, erect, and generally four-sided (older stems, five or six-sided).

Similar species: Garden yellow 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 stem tops and seed pods.

Wetland look-alikes to purple loosestrife
(click image to enlarge)

Spring purple loosestrife and native wetland look-a-like stems from left: two-year-old plant, one-year-old plant, Steeplebush (Spiraea tomentosa), Swamp Loosestrife (Decodon verticillatus), Great Water Dock (Rumex britannica).

 
Spring purple loosestrife clumps
(click image to enlarge)

Spring purple loosestrife clumps without leaves or flowers.

Distribution

See the reported locations of purple loosestrife in Wisconsin.

Do you know of additional populations? Send us a report.

Would you like to do something about purple loosestrife infestations? Visit the purple loosestrife biocontrol page to learn more.

Control

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 works 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.

Photos

View purple loosestrife pictures in our photo gallery.

Resources

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
Links for more information