Purple loosestrife
(Lythrum salicaria)

Name and Family

Purple Loosestrife, Lythrum salicaria, is a member of the loosestrife (Lythraceae) family.


  • Purple loosestrife was first reported in North America in the early 1800s.
  • The mode of introduction is not certain, but it is possible that it was introduced via ballast water or as an ornamental.
  • Since the initial introductions this plant is often shared by gardeners or spread in waterways.

Identification Guide

  • Lythrum salicaria is an herbaceous wetland perennial that can grow 0.5-1.5 m tall.
  • The leaves are normally opposite and in pairs, however they can be alternate and they can be found in whorls of three.
  • Leaves are lance-shaped and 3-10 cm long.
  • The flowers are purple to pink and closely attached directly to the stem.
  • They are numerous and borne on spikes that are between 10 and 40 cm long.
    • Each flower has 5-7 petals.
    • The flowers are in bloom from July to September.
  • The fruits are capsules, each containing numerous reddish-brown seeds.

What it does in the ecosystem

  • Purple loosestrife invades and destroys habitat along rivers, streams, and wetlands.
  • It grows in dense patches that choke out native plants and deter wildlife.
  • Purple loosestrife is a prolific seed producer and its light seeds are easily carried by wind and often take hold in nearby wetlands.



  • If managing purple loosestrife populations near a watercourse, it is necessary to apply for a Watercourse, Wetland and Buffer Zone Activity Permit from the Department of Environment, Energy, and Climate Action. You should apply for the permit at least six weeks before beginning work. 
  • Populations found upstream should be controlled first, as upstream plants will release seeds that ultimately travel downstream and could recolonize a downstream control area with ease. 
  • When managing, focus on smaller satellite populations first, as these are the most manageable and are most likely to expand the infestation’s boundaries.
  • Before managing, it is important to confirm the identity of the plant to ensure that you are indeed dealing with purple loosestrife.
    • Fireweed, a native flower, is commonly mistaken for purple loosestrife. 
    • See our purple loosestrife ID Factsheet for more information on how to distinguish between the two plants. 
    • Other native lookalikes include blue vervain and swamp loosestrife, both considered vulnerable on PEI by the Atlantic Canada Conservation Data Centre. 


  • Digging is a labour-intensive control method, but it is considered the most effective way to control purple loosestrife. 
  • It is best to dig purple loosestrife before the plant produces any seed. 
  • When digging, try to remove as much of the root system as possible to reduce re-growth. 
  • Digging plants disturbs the soil and may promote the germination of seeds from the soil seed bank. Therefore, yearly monitoring should be carried out on all treated sites. 


  • Cutting or mowing the aboveground vegetation has not proven to be an efficient control measure for purple loosestrife, due to the plant’s highly regenerative roots.
  • Often, the removal of aboveground vegetation results in re-sprouting. 
  • Mowing may cause scattering of vegetative tissue pieces, promoting the spread and further establishment of purple loosestrife in treated areas. 


  • Purple loosestrife has no natural pest insects or diseases in North America to keep its population under control, therefore extensive research has been done on biological control agents that specifically target and feed on purple loosestrife. 
  • Ducks Unlimited has successfully released the two species of Galerucella beetles on PEI.
    • Both the adults and larvae of these beetles forage on purple loosestrife plants causing severe damage and often killing the plant. 


  • The PEIISC does offer advice on chemical control methods at this time. 
  • If chemical control is used, product labels should be strictly followed and considerations for possible environmental damage should be taken into account. 
  • Herbicide use is prohibited around wetlands and watercourses in PEI.


  • It is key that all equipment be cleaned and that all plant parts be removed from your person, vehicles, and tools before leaving the area to prevent the spread of purple loosestrife. 
  • The plant will resprout from left-behind roots, nodes, or seeds. 
  • Each time you manage the site, collect, bag, mark, and dispose of all plant parts properly.
    • Bag the plant material in clear plastic bags.
    • Mark the bags boldly with a permanent marker. Write “INVASIVE PLANT” or “PURPLE LOOSESTRIFE” on the bags. 
    • If you have a place away from foot and wildlife traffic, you can dry out the material, making it much easier to move.
      • Leave the bags open and in the sun for a week to dry. 
      • Only do this if you can guarantee that the plant will not be spread from where it is drying
    • If the bags are thin, double bag to prevent accidental dispersal.
    • Tie the bags tightly and place them in your usual residential waste collection (THE BLACK BIN)
      • Never place invasive plants into the compost. This may allow them to become established once they reach the heap. 
  • For loads larger than what will fit into a ½ ton truck, reach out the the PEIISC for specific disposal instructions. 
  • Repeated and regular visitation to the site will be required to complete management, as new growth is likely to arise after control efforts have been completed. 
  • Replanting the disturbed site with native plants may prevent recolonisation by invasive species and will generate competition for any invasives that do arrive in the area.


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The biology of Canadian weeds. 100. Lythrum salicaria (cdnsciencepub.com)

Purple Loosestrife – Ontario Invasive Plant Council (ontarioinvasiveplants.ca)

Purple Loosestrife – Profile and Resources | Purple Loosestrife (invasivespeciescentre.ca)

PurpleLoosestrifeFactSheet.pdf (metrovancouver.org)

Purple Loosestrife – SSISC

Purple loosestrife (Lythrum salicaria) | Fraser Valley Invasive Species Society (fviss.ca)