Purple Loosestrife – July 2018 Weed of the Month

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.

In wetlands, purple loosestrife can grow so densely that it excludes native plants and fills in open water habitat.
purple loosestrife stems
Purple loosestrife seeds are carried by ducks and other aquatic animals to new wetlands and lakes.

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.

Purple loosestrife is a tall perennial usually with multiple flowering stems from a single woody base.
Purple loosestrife flowers have 5 to 7 narrow petals and are attached right to the stem. Photo by Jo Wilhelm.
purple loosestrife stem and leaves
Purple loosestrife stems have 4 to 6 sides and leaves are narrow with untoothed edges, arranged opposite or in whorls.
purple loosestrife flower stalks with honeybee
Purple loosestrife is popular with honey producers. You may want to notify beekeepers in the area before controlling purple loosestrife, or wait until the majority of the flowers have faded before controlling.

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.

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.

Purple loosestrife can be found on many small lakes in King County, including developed lakes such as Lake Burien. Photo by Mary Fee.
purple loosestrife on log in lake
Purple loosestrife can also be found in less developed lakes such as Lake Borst in Snoqualmie.

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.

Small infestations of purple loosestrife can be managed by hand removal if the soil is loose enough, but expect some regrowth from the roots and seeds in the soil. Photo by Ben Peterson.
It is sometimes possible to hand pull even large purple loosestrife plants if they are growing in loose mud, just make sure you are following State Fish and Wildlife rules. Photo by Ben Peterson.
When purple loosestrife is in bloom, cutting off the flowering stems can buy you time and prevent millions of seeds from being produced.
Near water and in wetlands, purple loosestrife control with herbicide can only be done with a state permit by licensed applicators using state-approved aquatic herbicides. Photo by Ben Peterson.

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.

releasing galerucella beetles on purple loosestrife
Biocontrol is a useful method for reducing the impact of purple loosestrife, using the plant’s natural enemies such as the Galerucella beetles.
Galerucella beetle on purple loosestrife leaf
The purple loosestrife beetle Galerucella is a helpful addition to an integrated pest management approach to controlling purple loosestrife.
Hylobius weevils are a biocontrol agent for purple loosestrife. Photo by Ben Peterson.
The purple loosestrife beetle Galerucella can have a significant impact on the plant’s growth, and can even kill entire plants.

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.