Working together to strengthen and support noxious weed management efforts in Montana.

Weed of the Week – Purple Loosestrife

Thursday, October 14th, 2010

This beautiful riparian plant was introduced as an ornamental from Europe in the 1800’s. It was first reported to be a problem weed in the 1940’s.  Purple loosestrife is established in nearly every state in the U.S.  Purple loosestrife lives in moist, marshy sites.  It will inhibit streams, rivers banks, bogs, irrigation canals, lakesides, and drainage ditches.

Concerns Though this plant is very beautiful it is very problematic for aquatic habitats.  Purple loosestrife chokes out native vegetation and creates narrow waterways.  The narrowing of waterways increases stream sedimentation and this can negatively affect fish habitat.  This weed also negatively impacts waterfowl habitat by degrading nesting sites.  Purple loosestrife can easily spread by seed dispersal.  A single purple loosestrife plant can produce up to 2.5 million seeds annually.

Identification Purple loosestrife is 4-7 feet tall and can produce many stems per plant.  The flowers are a rose-purple color and are clustered along the stem.  The flowering stem is spike-like and resembles a lupine panicle.  The stems are square to octagonal.  The long, narrow leaves come off the stem oppositely.  This plant is sometimes confused with native fireweed.

What can you do? Prevention is the most important tool in the fight against the spread of purple loosestrife.  Prevent disturbances in wetland areas because this is where purple loosestrife thrives.  It is important that purple loosestrife plants be detected early before they are able to spread out of control.  Cutting is relatively ineffective, it will decrease the spread of seeds but will not kill the plant.  Hand pulling can be done on small patches and is mildly successful.  Reseeding should occur after a treatment.  A biological control does exist for large infestations.  There are leaf-eating beetles and root-mining larvae that are known to help reduce the outbreak of purple loosestrife.  The most efficient method is herbicide application.  Herbicide application on this weed should be used with extreme caution due to the plants proximity to water sources.  For more information and guidance on your weed issues call your local weed district.

Visit the MWCA Weed ID pages for additional information and pictures of  purple loosestrife.

This series of articles was developed by Ravalli County.  If you would like to use these articles please contact Ravalli County Weed District Weed Coordinator at (406) 777-5842.

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