Vectors of spread:
General: Purple Loosestrife is a shrub-like perennial with rhizomatous roots that lives in wetlands. It is up to 3 m tall and 1.5 m wide, and mostly recognized by its showy flowers.
Flowers: It has small, purple flowers arranged in dense, showy spikes. Flowers are usually 7-10 mm in size. Purple Loosestrife flowers from late July to September.
Stem: Purple Loosestrife stems are square and numerous – sometimes more than 30 per plant!
Leaves: The leaves are hairy and lance-shaped.
Hardhack (spirea douglasii): Can be distinguished from Purple Loosestrife by its oval leaves and reddish brown, round stems. It is native in the Sea to Sky, but it grows in much drier soils.
Fireweed (Epilobium angustifolium or Chamaenerion angustifolium) is another native often mistaken for Purple Loosestrife. Its long (up to 20cm) leaves and unbranched stems are a great ID characteristic. In addition, purple-pink flowers have 4 petals, and its leaves have wavy edges.
Blue Vervain (Verbena hostata) is sometimes mistaken for Purple Loosestrife, but its leaves have a rougher texture and doubly-serrated margins.
Dotted Blazing Star (Liatris punctata) flowers are on a long curving stem and stiffer, more narrow leaves.
Lastly, Winged Loosestrife (Lythrum alatum) is in the same family as Purple Loosestrife but it is native to BC. It is recognized by its winged bark, with edges that run parallel to the edge of the stem.
Habitat and Origin
In the early 1800s, Purple Loosestrife was accidentally introduced to eastern North America in the water ballast of a ship from Europe. The plant was also intentionally introduced as a medicinal plant, and at the hands of beekeepers, Purple Loosestrife yields a popular, quality honey.
As many other attractive invaders, Purple Loosestrife also arrived via garden centres that stock the plant as an ornamental.
Propagation and Vectors of Spread
- Purple Loosestrife seeds can travel in the wind as well as float on water, so it is common to see new infestations appear downstream of established ones.
- Its seeds are also spread by birds and wildlife.
- Purple Loosestrife can also spread vegetatively, through plant fragmentation.
Ecological and Economic Impacts
Purple Loosestrife will outcompete plant species for light, space and pollinators, and choke other species under a sea of purple flowers and dense, tight strands.
In the United States, it is estimated that 200,000 hectares of wetlands are lost each year due to Purple Loosestrife.
Purple Loosestrife clogs irrigation systems and obstructs waterways used by recreational boaters.
What Can I Do?
What to do if you spot it: You can report any Purple Loosestrife sighting by clicking here.
The best approach to controlling the spread of Purple Loosestrife is PREVENTION.
- Maintain riparian and aquatic environments in a healthy condition to ensure a productive natural plant community
- Regularly patrol your property for Purple Loosestrife plants and immediately control new infestations
- Re-vegetate disturbed soils with native plants that provide dense, early colonization to prevent weed invasion
- Clean boats of plant material and soil prior to leaving a loosestrife-infested area
Mechanical control: Small, isolated infestations of Purple Loosestrife can be hand-pulled. The entire root of the plant should be removed to avoid re-growth from root fragments. Pulling is easiest when plants are young. Older plants have larger root systems that are better removed with a garden fork. Pulling should also occur before seed production. Purple Loosestrife reproduces through fragmentation, so special attention must be paid to ensure all plant parts are removed. This method should be applied before seed set to prevent scattering. Mechanical control for large infestations has been unsuccessful.
- Purple Loosestrife is often found growing in wet soils or on the edge of lakes, rivers and streams, so herbicide application is often not possible (under Canadian and BC regulations).
- In other countries, aquatic formulations of glyphosate have been shown to provide effective control for Purple Loosestrife, however these formulations are not currently registered for use in Canada.
- We recommend that any herbicide application is carried out by a person holding a valid BC Pesticide Applicator Certificate. Before selecting and applying herbicides, you must review and follow herbicide labels and application rates; municipal, regional, provincial and federal laws and regulations; species-specific treatment recommendations, and site-specific goals and objectives.
- If site conditions permit the use of herbicides, best results are achieved by applying herbicide before flowering, in order to minimize seed production.
Biological control: Two species of leaf-eating beetles, Galerucella clamariensis and G. pusilla have been released, and have proven effective against Purple Loosestrife in other areas of BC, including the Okanagan and the Lower Mainland. These beetles feed on the foliage and flower heads, stunting growth and reducing seed production of the plant. Please contact SSISC to find out more about biocontrol.
- BC Government, Biological Control Agents & Host Plants, https://www2.gov.bc.ca/gov/content/environment/plants-animals-ecosystems/invasive-species/plants/biological-control/biological-control-agents-host-plants
- BC Government, Hylobius tranversovittatus Goeze, https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/forestry/agents/hylobius_transversovittatus.pdf
- Invasive Species Council of BC, Purple Loosestrife Information Page, https://bcinvasives.ca/invasive-species/identify/invasive-plants/purple-loosestrife
- Invasive Species Council of BC, Purple Loosestrife TIPS Factsheet, https://bcinvasives.ca/documents/Purple_Loosestrife_TIPS_2017_WEB.pdf
- Lythrum salicaria, Electronic Atlas of the Flora of British Columbia, http://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Lythrum%20salicaria
- Ontario Invasive Plant Council, Purple Loosestrife Best Management Practices in Ontario, http://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/07/Purple-Loosestrife-BMP-April-2016-final.pdf
- Regional District Okanagan-Similkameen, Purple Loosestrife, http://www.rdosmaps.bc.ca/min_bylaws/legislative_services/weed_control/FACTSHEET_purple_loosestrife.pdf