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Weed of the Week: Purple loosestrife beautiful but problematic for aquatic habitats
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Weed of the Week: Purple loosestrife beautiful but problematic for aquatic habitats

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Purple loosestrife (Lythrum salicaria) is a beautiful riparian plant that was introduced as an ornamental from Europe in the 1800s. It was first reported to be a problem weed in the 1940s.

Purple loosestrife is established in nearly every state in the U.S. It lives in moist, marshy sites and will inhabit streams, rivers banks, bogs, irrigation canals, lakesides and drainage ditches. It is a problem in Flathead County and Missoula County. The potential for this weed to spread in Ravalli County is very great as we have a limited number of plants.

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 is affecting waterfowl habitat by creating poor nesting sites. Purple loosestrife can easily spread by seed dispersal. A single purple loosestrife plant can produce up to 2.5 million ground pepper-like seeds annually.

Purple loosestrife is a 4-7 foot tall plant and one plant can produce many stems. 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 stout and square to octagonal. The leaves come off the stem oppositely and they are long and narrow. This plant is sometimes confused with native fireweed.

The most important thing a resident of Ravalli County can do is to help prevent the spread of purple loosestrife. Prevent disturbances in wetland areas because this is where purple loosestrife thrives. There has been only one reported case of this weed in Ravalli County and we would like to keep it that way. 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 be used after a treatment has been applied.

For large infestations, biological control does exist. 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 precaution due to the plant’s proximity to water sources. If you think you have purple loosestrife on your property please call, there is a program for you to get free treatment on it.

For more information and guidance on your weed issues call the Ravalli County Weed District at 777-5842 or check our website at


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