Purple Loosestrife

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Invasive Plants, Pacific Northwest

Purple Loosestrife, BC Coastal Region
Purple Loosestrife, Photo By Bud Logan

Purple loosestrife is a woody half shrub, wetland perennial, considered regionally noxious under the BC Weed Control Act. It 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 found all over Vancouver Island.

Shrub like in appearance, it has stiff, four sided stems ending in dense spikes of showy purple flowers. Plants have narrow stalkless leaves, growing up to 3 m in height at maturity.

The best way to identify purple loosestrife is by its square stems and opposite leaves, just roll the stem in your fingers to feel the what I mean by square stems. If you have purple loosestrife in your garden, remove it immediately. Pull or dig the plants out and ensure that all root fragments are removed to prevent re growth.

Purple Loosestrife, BC Coastal Region
Purple Loosestrife, Photo By Bud Logan

For large patches, there are relatively effective bio-control agents like the Galerucella and Calmariensis beetles, both are also called loosestrife beetles and the Pusilla beetle, also called the birch leaf miner beetle. These beetles feed on the plant stems in their larval stage and can help get a problem under control.

Often confused with fire weed, purple loosestrife is an escaped ornamental that tolerates a wide range of weather conditions and can grow in standing water. Seeds distribute through water, humans and animals, with a single plant producing over 2.5 million seeds that drop in early fall when temperatures cool.

Boaters are one of the main ways for these plants to move around, one tiny piece of plant stem can start a new colony, so when moving a boat from one water coarse to another, you should be sure to remove any debris on the boat bottom or motor to insure you do not transport and start a new growth.

Dense stands of purple loosestrife threaten plant and animal diversity in wetland ecosystems

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