Weed of the Week: Purple Loosestrife

“Vibrant stands of purple loosestrife(Lythrum salicaria) that border many lakes, rivers, and wetland areas of BC are often selected by gardeners for their dramatic display of colour.” Unfortunately, this beautiful showpiece is actually an aggressive invasive plant that severely disrupts water flow in rivers and canals, and causes a sharp decline in biodiversity.

Dense stands of purple loosestrife aggressively crowd out native vegetation, thereby threatening the life cycle of native waterfowl, amphibians, and other wetland species in BC. Habitat where fish and wildlife feed, seek shelter, reproduce and rear young, quickly becomes choked under a sea of purple flowers.

Purple loosestrife is one of the most “unwanted” invasive plants impacting BC’s environment, economy, and society. In fact, invasive plants are the second greatest threat to biodiversity after habitat loss, according to the International Union for Conservation of Nature (IUCN).

Purple loosestrife is a wetland perennial, considered regionally noxious under the BC Weed Control Act. Purple loosestrife is found in wet areas at low- to mid-elevations, growing in ditches, irrigation canals, marshes, stream and lake shorelines and shallow ponds. It is common in the lower Fraser Valley and frequent on southern Vancouver Island, and in the Okanagan. There are also localized patches in the Kootenay and Omineca regions.

Often confused with fireweed (Epilobium angustifolium), purple loosestrife is an escaped ornamental that tolerates a wide range of weather conditions and will grow in standing water. Shrub-like in appearance, purple loosestrife has stiff, four-sided stems ending in dense spikes of striking purple flowers. Plants have narrow, stalkless leaves and can grow up to three metres in height at maturity.

Purple loosestrife readily forms dense, impenetrable stands that are unsuitable as cover, food, or nesting sites for a wide range of native wetland animals. Wetlands lose 50-100% of their native biomass due to purple loosestrife invasions. In the United States, it is estimated that 200,000 hectares of wetlands are lost annually due to invasion by this species.

Purple loosestrife is highly competitive due to ample seed production, giving it an advantage over native plant species. Seeds distribute easily through wind, water, people, and animals. A single plant produces over 2.5 million seeds that drop in early fall when temperatures cool. This plant is also able to re-sprout from roots and broken stems that fall to the ground or into the water.

Purple loosestrife is among the 75% of BC’s invasive plants that were intentionally introduced as ornamentals since the early 1800s. It may also have been accidentally introduced with soil transferred in ship ballasts. This invasive plant now occurs in all Canadian provinces and every U.S. state except Florida.

Help your community protect local resources by preventing the establishment of purple loosestrife and managing any existing infestations. There are hundreds of native riparian and aquatic plants that are critically important to parts of a lake or river ecosystem, and alternatives are available to replace this attractive invasive in your backyard. A Grow Me Instead booklet that identifies alternatives to the most ‘unwanted’ invasive plants in BC is available from Resources.

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