Purple Loosestrife

Description

Purple loosestrife (Lythrum salicaria) is an invasive perennial plant native to Europe and Asia that was introduced to North America in the 1800s. Known for its showy spikes of purple flowers, it invades wetlands and other moist habitats where it can form dense stands that crowd out native vegetation and reduce suitable food, shelter, and nesting habitat for wildlife. Purple loosestrife is a prolific seed producer, with large plants capable of producing millions of seeds each year, allowing it to spread rapidly into disturbed areas.

  • Scientific name: Lythrum salicaria

  • French name: Salicaire commune

  • Common name(s): Spiked loosestrife

  • Regulatory Status: Purple loosestrife is federally regulated as a Class 2 Primary Noxious Weed under the Canadian Weed Seeds Order under the Seeds Act. It is provincially regulated as a noxious weed in British Columbia, Saskatchewan, Manitoba, and Alberta. 

Image of Purple Loosestrife

© Donna Pomeroy via iNaturalist.org, used under CC-BY-NC

Introduction and Spread

Purple loosestrife is native to Europe and Asia. It was brought to North America in the 1800s through several pathways including in ship ballast, imported livestock, sheep fleece and as ornamental plant for gardens and bee keeping. It can spread through natural pathways (i.e, wind, water, birds or other wildlife) or human-made pathways (i.e, in seed mixes, contaminated soil and equipment, clothing or footwear or intentional planting in gardens). Where it was once intentionally planted in gardens, it escaped into natural areas. Disturbance created by road maintenance or construction can encourage the germination of purple loosestrife seeds. In addition, improper cleaning of equipment and transportation vehicles can also carry plants to new areas. 

Purple loosestrife occurs in all US states with the exception of Florida. In Canada, purple loosestrife is found in all provinces except for the Yukon, Northwest Territories and Nunavut. 

Habitat

Purple loosestrife can colonize a variety of habitats. However, it prefers recently disturbed areas with exposed soil and abundant light. Once established, plants can thrive in a wide range of soil pH, soil types and climatic conditions. 

Identification

  • Height and Stem: Purple loosestrife can grow between 60 – 120 cm tall, however, some plants can grow over 2 m tall. On average, plants have between 1 – 15 flowering stems. Its annual stems arise from a woody rootstock. They are woody, stiff, covered in fine hairs and are square shaped (4 – 6 sides). Actively growing shoots are green whereas older stems are reddish brown or purplish. 

  • Leaves: The leaves of purple loosestrife are simple, narrow, lance-shaped and taper towards the top. They have smooth margins and are covered in fine hairs. They typically measure between 3 – 12 cm long. Leaves are arranged opposite, except for upper leaves that are arranged alternate. The leaves lack a petiole (leaf stalk) and are attached directly to the stem. 

  • Flowers: The flowers of purple loosestrife are arranged in a dense terminal spike. They are very showy, deep pink to purple with 5 – 7 petals surrounding a small, yellow centre. The petals have a wrinkly appearance. 

  • Fruits: Each flower produces a tiny, rounded seedpod/capsule that measures 3 – 6 mm long and 2 mm wide. They contain many, small seeds. Larger plants can produce up to 2.7 million seeds annually. 

  • Roots: Purple loosestrife has a persistent taproot that comes woody with age. 

Image of purple loosestrife stem

Purple loosestrife

Image of purple loosestrife flower

Purple loosestrife flower

Image of purple loosestrife leaves

Purple loosestrife leaves

Impacts

  • Purple loosestrife leaves decompose faster and earlier than native plant species. Because of this, nutrients from decomposition are flushed from wetland ecosystems faster and earlier. This alters decomposition rates, timing and nutrient cycling, changing the chemistry in wetlands. 

  • Purple loosestrife forms dense monoculture stands that outcompete native species, including species at risk.  

  • The dense monocultures formed by purple loosestrife can also result in a reduction is suitable nesting sites, shelter and food for birds and other wildlife (Tavernia and Reed 2012).  

  • Dense stands of purple loosestrife can clog irrigation canals, degrade farmlands and reduce the forage value of pastures. They can also impede recreational activities such as boating, hunting, trapping, and fishing.  

Life cycle

Purple loosestrife is a perennial herbaceous plant that regenerates from a persistent taproot and root stock. Reproduction occurs primarily through seeds which can remain dormant for several years. Germination takes place in the late spring and early summer in open, sunny areas once the soil temperature reaches between 15 – 20 °C. Purple loosestrife is one of the fastest growing wetland plants and is able to grow over 1 m tall in its first year. It flowers in late June and produced seed pods in early August (McCaughey and Stephenson 2000). One plant can have 900 – 100 capsules each with 83 – 100 seeds. In the fall, the aboveground parts of the plant will dieback (Mai et al. 1992). 

What You Can Do

You can help prevent the spread of purple loosestrife by:

  • PlayCleanGo: To prevent the spread of invasive species, stay on designated trails, clean equipment, vehicles and clothing after visiting infested areas, and dispose of plant material properly. Cleaning equipment and vehicles that have come into contact with purple loosestrife, and educating the public about the importance of not moving plant materials, can help prevent the spread to new areas.  

  • PlantWise: Do not plant purple loosestrife or other invasive species. Choose native or non-invasive alternatives. Dispose of yard waste properly and do not dump it into natural areas. 

  • Educate: Learn how to identify purple loosestrife and spread awareness about the ecological risks posed by invasive plants and encourage responsible practices in your community.  

  • Reporting: Report purple loosestrife and any invasive species to the appropriate provincial authorities such as provincial invasive species councils or report using free tools such as EDDMapS and iNaturalist.  

Other Resources

  1. Mai, T. K. et al. 1992. The Biology of Canadian Weeds. 100. Lythrum salicaria. Canadian Journal of Plant Science 72(4): 1305-1330. 
  2. McCaughey, T. L. and G. R. Stephenson. 2000. Time from Flowering to Seed Viability in Purple Loosestrife (Lythrum salicaria). Aquatic Botany 66: 57-68. 
  3. Tavernia, B. G. and J. M. Reed. 2012. The Impact of Exotic Purple Loosestrife (Lythrum salicaria) on Wetland Bird Abundances. The American Midland Naturalist 168 (2): 352-363. 
  4. Wilson, L. M. et al. 2004. Biology and Biological Control of Purple Loosestrife, Technology Transfer/Forest Health Technology Enterprise Team. Morgantown, West Virginia: Forest Health Technology Enterprise Team, U.S. Dept. of Agriculture, Forest Service. 

Invasive Species Categories

Stop The Spread: Report Invasive Species

How to Prevent and Manage Invasive Species

The Impact of Invasive Species on Canada’s Ecosystems and Economy

Invasive Species Resources: Guides, Manuals, and Best Practices