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Purple Loosestrife

(Lythrum salicaria)

Purple Loosestrife

(1B, limited presence in Montana)

(Lythrum salicaria)

Quick ID
  • Stems can be felt to be distinctly square, five or six-sided, and up to fifty stems per plant
  • Showy pinkish purple flowers on long spikes at the top of the plant, blooming late in the summer, with individual flowers having five or six petals
  • Opposite or whorled, lance-shaped, stemless leaves with smooth margins
  • Plants are often head height or taller at full maturity, although they are regularly smaller too

 Map Courtesy of Montana Natural Heritage Program, Purple Loosestrife – Low Suitability: 9% of Montana, Moderate: 3%, Optimal: 2%. Click here for larger image.

Purple Loosestrife Plant

Video Information

Weed Images

Purple Loosestrife plant
Purple Loosestrife flower
Purple Loosestrife leaf
Purple Loosestrife Seedling - Photo by Ohio State Weed Lab, The Ohio State University,
Purple Loosestrife stem
Purple Loosestrife Immature - Photo by Carol Bell Randall, USDA Forest Service,
Purple Loosestrife Inflorescence - Photo by Linda Wilson, University of Idaho,
Purple Loosestrife Invading - Photo by Eric Coombs, Oregon Department of Agriculture,

Weed Specifications

Weed Info
Type Information
Toxicity Non-toxic
Best Management Practices

Biological for larger infestations; chemical, but specialized aquatic herbicides must be used and used with appropriate precautions; mechanical, clipping and bagging seed heads and pulling smaller plants, although it is difficult to remove entirety of root system and plants can resprout from stem fragments; it is an escaped ornamental – do not buy seeds or plant in gardens

*See additional documents below
Habitat Canals, ditches, ponds, wetlands, marshes, roadsides, islands, moist soil, shallow water, frequently flooded areas below high-water mark, shorelines of any waterbody
Root Taproot and rhizomes
Leaves Smooth-margined, lance-shaped, stemless and opposite or whorled arrangement on the main stem
Lifespan Perennial
Similar Looking Plants Fireweed, Dame’s Rocket, Dotted Blazing Star
Important Information A single plant can produce more than 2 million seeds annually, found in almost every state and Canadian province, can survive 20 months underwater with no plants parts above the water surface; often grows in difficult to access sites, requiring boats and wading; hard to find among other plants in riparian habitat, often intermixed with tall and dense stands of willows or cattails, especially before it flowers, easily being mistaken for plants like wild mint when small; creates monocultures that can impede waterflow and recreation and diminish wetland plant diversity and wildlife habitat

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