Class B Noxious Weed
- Native to Europe and Asia.
- May be confused with native plant species including Douglas spirea, and fireweed.
- Purple loosestrife is a pioneer species that can colonize and potentially invade any wet, disturbed area in North America.
- The sale of purple loosestrife, including all hybrids and cultivars, is prohibited in the state of Washington.
- Perennial emergent aquatic plant that can reach up to 10 feet tall and 5 feet wide, and has a persistent tap root and spreading rootstock.
- Annual herbaceous square-like stems, usually 4-sided, sometimes 6-sided.
- Leaves range from opposite, alternate or sometimes in whorls of three. Leaves are linear to lance shaped with smooth edges and fine hairs. Leaf shape is variable and influenced by light levels.
- Flowers are magenta or purple with 5-7 petals in spiked clusters; appear from July to October.
- Occurs in freshwater and brackish wetlands, along river and streams, lake and pond shorelines, marshes and other habitats with moist ground or standing water.
- Requires full or partial sunlight
Reproduction, Spread & Impact
- Alters wetland ecosystems by forming monotypic stands that replace native and beneficial plant communities.
- Spreads mainly by seed but also vegetatively when broken off stem and root fragments are left or buried in moist soil.
- Dispersal is mainly by water but seed can be transported by wildlife, humans, vehicles or boats.
- If plants are in flower or seed, cut and bag all flower stalks and seed heads.
- Hand pulling or digging up small infestations can be effective, making sure to remove the whole root system.
- Cutting will not kill the plant, but may prevent seeding. Do not leave cut plant parts on site, as they can resprout.
- Mowing is not recommended, since plant fragments can produce new shoots, which could facilitate spread rather than control it.
- Do not compost or place in yard waste. Burning is an acceptable disposal method but do not burn flowering stems or seed heads.
- Never apply non-aquatic herbicides near or directly to water since many include ingredients toxic to aquatic organisms. Only a licensed applicator with an aquatic endorsement is legally allowed to apply herbicide near or adjacent to waterbodies.
- Aquatically approved herbicides, with the following active ingredients are effective for controlling purple loosestrife:
- Glyphosate: Apply to actively growing plants; including early flowering stage, pre-flowering plants or seedlings.
- Imazapyr: Apply to actively growing plants
- Triclopyr: Apply when plants are in the mid to full-bloom stage; it may also be effective to apply to pre-flowering plants or seedlings. Triclopyr is a selective herbicide and will not kill monocots such as cattail, grass, sedges, and rushes.
- When large areas of weeds are removed, it’s recommended to replant with native plants to fill the void and prevent future infestation of noxious weeds.