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 June to August.
Stem: Purple Loosestrife stems are woody, square and numerous – sometimes more than 30 per plant! New growth is green, but older stems are reddish-brown to purplish.
Leaves: The leaves are narrow, hairy and lance-shaped (broad near the base and tapering towards the tip). They grow opposite each other along the stems, or in whorls. Purple Loosestrife leaves are small – typically 3 to 12 cm long.
- Dame’s Rocket (Hesperis matronalis), which has 4 petals, alternate leaves, and seed pods that are longer than Purple Loosestrife’s.
Butterfly Bush (Buddleja davidii),which is a deciduous shrub that lives in dryer conditions than Purple Loosestrife. Its leaves are also bigger (3-25 cm long), and its flowers have an orange eye.
- 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 hastata) 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.
As many other attractive invaders, Purple Loosestrife also arrived via garden centres that stock the plant as an ornamental.
Purple Loosestrife thrives in wet areas, such as brackish or fresh standing water, including marshes, stream banks, pond edges, ditches, irrigation canals and lake or river shorelines. It prefers full sun, but can also tolerate partly-shaded conditions.
Propagation and Vectors of Spread
Purple Loosestrife reproduces by seed, with a single plant producing over 2.5 million seeds in a season. The seeds have an extremely high germination rate, and remain viable in the soil or underwater for many years. Purple Loosestrife can also spread vegetatively, through plant fragmentation.
Purple Loosestrife seeds are extremely small and abundant; they can be dispersed by the wind , or travel in waterbodies, so it is common to see new infestations appear downstream of established ones. Its seeds are also spread by birds and wildlife, and pollinated by bees.
Like many other attractive invaders, Purple Loosestrife is also spread through the horticultural trade, as it is sold as an ornamental plant.
Ecological and Economic Impacts
- Outcompetes plant species for light, space and pollinators, and choke other species under a sea of purple flowers and dense, tight strands.
- Reduces habitat and food availability for wildlife.
- Dense stands trap sediment, which can alter the landscape and lead to a rise in the water table.
- In the United States, it is estimated that 200,000 hectares of wetlands are lost each year due to Purple Loosestrife.
- Clogs irrigation systems and obstructs waterways used by recreational boaters.
- Honey produced from loosestrife-feeding bees is of low quality.
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
- Small, isolated infestations of Purple Loosestrife can be hand-pulled or dug up.
- 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.
- Cutting plants at the base may prevent seed spread and inhibit growth but should be used as a stop-gap measure only.
- 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 be 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.
- Two species of leaf-eating beetles, Neogalerucella clamariensis and N. 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.
- Government of BC, Hylobius tranversovittatus Goeze, https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/forestry/agents/hylobius_transversovittatus.pdf
- Government of BC, Purple Loosestrife, https://testwww.for.gov.bc.ca/hfp/noxious/plants/purple-loosestrife.htm
- Government of BC, “Purple Loosestrife” in A Guide to Weeds in British Columbia, https://www2.gov.bc.ca/assets/gov/environment/plants-animals-and-ecosystems/invasive-species/alerts/purple_loosestrife.pdf
- Canadian Wildlife Federation, Root out Purple Loosestrife, https://cwf-fcf.org/en/resources/DIY/habitat-projects/map-your-backyard/root-out-purple-loosestrife.html?gclid=CjwKCAjwsNiIBhBdEiwAJK4khn9qe32yzorRMuDfYyTg3ELGSFZT12iYNa7z9uY17DTdPrR_UbQO0hoCj34QAvD_BwE
- Coastal Invasive Species Committee, Purple Loosestrife, https://www.coastalisc.com/priority-invasive-plants/purple-loosestrife
- Electronic Atlas of the Flora of British Columbia, Lythrum salicaria, https://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Lythrum%20salicaria&redblue=Both&lifeform=7
- Global Invasive Species Database, Lythrum salicaria, http://www.iucngisd.org/gisd/species.php?sc=93
- Invasive Species Council of BC, Purple Loosestrife, https://bcinvasives.ca/invasives/purple-loosestrife/
- Invasive Species Council of BC, Purple Loosestrife TIPS Factsheet, https://bcinvasives.ca/wp-content/uploads/2021/01/Purple-Loosestrife.pdf
- Invasive Species Council of Metro Vancouver, Purple Loosestrife, https://iscmv.ca/invasives/detail/purple-loosestrife
- Metro Vancouver, Best Management Practices for Purple Loosestrife, http://www.metrovancouver.org/services/regional-planning/PlanningPublications/PurpleLoosestrifeBMP.pdf
- Metro Vancouver, Tackling Purple Loosestrife, http://www.metrovancouver.org/services/regional-planning/PlanningPublications/PurpleLoosestrifeFactSheet.pdf
- Ontario Invasive Plant Council and Environment and Climate Change Canada, Purple Loosestrife: Best Management Practices in Ontario, http://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/07/Purple-Loosestrife-BMP-April-2016-final.pdf
- Ontario’s Invading Species Awareness Program, Purple Loosestrife, http://www.invadingspecies.com/purple-loosestrife/
- Purple Loosestrife Manitoba, Purple Loosestrife, http://purpleloosestrife.org/
- Regional District Okanagan-Similkameen, Purple Loosestrife Factsheet, https://www.rdos.bc.ca/assets/PUBLICWORKS/Weed-Control/FACTSHEET-purple-loosestrife.pdf