Purple Loosestrife – July’s Weed of the Month

Crossposted from Noxious Weeds Blog


Purple loosestrife flowers are quite lovely. In July, the flowers start to open on the lower parts of the spikes first, followed by the upper flowers.

Although it is perhaps one of the prettiest noxious weeds, purple loosestrife (Lythrum salicaria) is widely recognized throughout North America as a highly invasive and damaging weed in wetlands and along shorelines. Vigorous perennial roots and rhizomes, combined with incredibly prolific seed production, result in one of the most successful invasive plants we have on this continent.

Unfortunately, dense populations of purple loosestrife significantly reduce the habitat quality of wetlands and shorelines for waterfowl, amphibians, fish and other critters. Purple loosestrife does not play well with other plants and can completely dominate the areas it invades. It can also clog up waterways and increase flooding.

Purple loosestrife is a tall, multi-stemmed perennial with narrow spikes densely packed with small magenta or purple flowers. The stems are 4-6 sided, especially on the newer growth, and the leaves are long and narrow with untoothed edges. The leaves attach directly to the main stem and are arranged opposite or whorled on the stem. Purple loosestrife grows mostly in wet areas such as shorelines or marshes, but it can survive in gardens or on roadsides as well.

Identifying purple loosestrife is sometimes challenging because of several similar species that flower at the same time. The plants that are most often confused with purple loosestrife that are native to Washington include Douglas spirea (Spiraea douglasii), fireweed (Chamaenerion angustifolium), and Watson’s willow-herb (Epilobium ciliatum ssp. watsonii). None of the look-alikes have the combination of square stems, untoothed leaf edges, opposite leaves, perennial roots, and tightly packed flower spikes that are found on purple loosestrife plants.


Purple loosestrife flowers have 5 to 7 narrow petals and are attached right to the stem. Photo by Jo Wilhelm.

In King County, purple loosestrife is our most abundant regulated aquatic noxious weed. It is found on about 30 lakes in the county, including all three of the large lakes, four of the large rivers, many creeks, ponds, wet ditches, and numerous other sites. Over 1,000 sites of purple loosestrife were surveyed in 2017, but many of those are small. If all the patches were put together, they would only cover about 15 acres.

We are making progress in spite of the challenges. Although the populations are very persistent, purple loosestrife is no longer found at 28 percent of the sites we’ve found since 1997, and we continue to try to eliminate more populations. Although we do find new sites each year, we are finding fewer new sites now, and more and more sites get controlled every year. Each year we survey more of the county’s lakes and wetlands, trying to find where this plant is hiding out. Our goal is to continue hammering away at purple loosestrife with a combination of tools, focusing our efforts where they can do the most good and working to reduce the overall impact of this weed on our county’s lakes and wetlands.

Even individual plants are very difficult to control due to purple loosestrife’s ability to regenerate from very small fragments of roots or stems left on moist soil. And, if you are able to control the existing plants, you will continue to fight the plants coming from the seed bank for many years. Combine this with the fact that purple loosestrife invades highly sensitive and often inaccessible wetlands and you can see how challenging this plant is to keep in check.


In wetlands, purple loosestrife can grow so densely that it excludes native plants and fills in open water habitat.

In addition to the traditional weed control methods of hand removal and aquatic herbicide treatment, we have several biocontrol insects that are very helpful for reducing the impacts of purple loosestrife where we can’t use other methods. The most visible biocontrol agent is the loosestrife beetle Galerucella. The other two insects are the loosestrife root weevil Hylobius, and the seed weevil Nanophyes. All three insects have been tested extensively and only have impacts on non-native purple loosestrife in Washington. They will never eradicate their host plant, but they do help reduce the dominance of purple loosestrife in areas where we can’t control it with other methods due to access problems or for very large infestations.

If you see purple loosestrife in King County, especially somewhere we might not see it, or if you are worried that no one is doing anything, please report the location or contact us by email at noxious.weeds@kingcounty.gov or by phone at 206-477-9333.

For more information about purple loosestrife, please visit our website: https://www.kingcounty.gov/services/environment/animals-and-plants/noxious-weeds/weed-identification/purple-loosestrife.aspx.

For more great photos with information, visit the original post on Noxious Weeds Blog.