Purlple Loosestrife

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Purple Loosestrife

Vicki Renzulli

Purple loosestrife (Lythrum salicaria) is an exotic perennial herb that grows in sunny wetlands, ditches, around farm ponds and in other disturbed habitats. Purple loosestrife was accidentally introduced in North America by European immigrants in the early 1800’s. Many believe that the seeds were brought to North America in the ballast water of ships. Purple loosestrife has spread to the United States were it has become a major nuisance (http://www.anr.state.vt.us/dec/waterq/ans/plpage.htm). Scientists have discovered that there are no natural enemies in the United States, which has caused the plant to spread aggressively into the wetlands throughout the northeast and the upper Midwest.

L. salicaria is a showy, attractive plant with pink and purple flowers arranged on a spike, which grows from 0.5 meters to 2.0 meters in height (Bender 1987). Loosestrife has angled stems that emerge from a woody rootstalk, attached directly to the stem are the leaves, which often have smooth edges. Leaves tend to be found on opposite sides of the stalk, which seems to be related to the number of sides on each stem: four-sided stems have opposite leaves, five-sided stems have leaves in spiral arrangements, and six-sided stems have leaves in whorls. These various stem types can all be found on a single loosestrife plant (Borman 1997).

From a distance, purple loosestrife could be confused with a number of other purple-spiked flowers, these include Liatris pycnostachya (gayfeather), Verbena hastata (blue vervain), or Epilobium angustifolium (fireweed) (Borman 1997). Upon closer examination purple loosestrife is easily distinguished from these other magenta flowered plants. There are various characteristics for these flowers that can distinguish them from purple loosestrife. The gayfeather can be differentiated by the soil type gayfeathers prefer well drained, gravely or sandy soils. Blue vervian are common in areas were the water is fairly fresh, shores, stream banks, ditches and springs. Blue vervian characteristics are very similar to that of the purple loosestrife but the flower color is one distinct way that we can them apart. The color of the flower on the blue vervain is dark blue to purple. Firewood another plant that can be confused with purple loosestrife can be distinguished in terms of their habitat. Firewood grow in disturbed areas such as cut-over or burned forests and swamps, avalanche areas, recently deglaciated areas, and riverbanks.

Purple loosestrife begins to bloom in June and continues until September. After the flowers have bloomed, several different types of bees and butterflies pollinate them. Loosestrife produces a large quantity of seeds, allowing it to the spread rapidly. Plants sometime can have up to 300,000 seeds per plant (Cunningham 1994). The seeds are small weighing 0.6mg and are efficiently dispersed (Bender 1987). River water and floods are the primary ways that seeds and plants are transported to new areas but seeds can also be transported on the feet of waterfowl or other wetland animals; humans can also carry seeds on clothing or shoes. Throughout the winter stalks remain standing and seeds continue to drop out of the dried fruit capsules.

Soil moisture is the most important factor for seed germination. Seeds germinate best on moist open grounds and can survive in 50% sunlight. Temperature at the soil surface is a critical factor for germination. Seeds will germinate at temperatures ranging from 15 to 20oC (Bender 1987). Germination usually takes about eight to ten weeks.

Purple loosestrife is so aggressive that it crowds out native cattails and sedge, that are used for food and shelter by wildlife. Purple loosestrife has almost no value to wildlife, so where loosestrife invades, valuable wildlife habitats are destroyed. Cattails and pondweed are displaced or shaded out as L. salicaria expands across wetlands. The loosestrife invasion leads to a loss in plant and wildlife diversity.

There are several control methods that can be used to reduce the invasion of Purple loosestrife. The first method is to remove the plant by hand. The entire plant must be removed to minimize the chance of regrowth. Small infestations of up to 100 plants are best eliminated by hand pulling (http://www.ceris.purdue.edu/napis/pests/pls/plsintro.html). If the stem is broken off at the soil surface, the root will sprout new stems. Plants should be pulled early in the flowering season to avoid scattering seeds in the removal process (http://www.ceris.purdue.edu/napis/pests/pls/plsintro.html).

Large populations extending over three acres or more are best controlled by herbicides. The herbicides that are most commonly used to control L. salicaria are Roundup and Rodeo (Bender 1987). Roundup is used on land. Rodeo has been approved for use over water. When a diluted solution of Rodeo is sprayed directly on mature leaves, it disrupts protein synthesis and causes the plant to die within two to seven days (Bender 1987). Spraying should be done after the period of peak bloom, usually late August (Bender 1987). After the first spray, it is critical that the application be repeated. The reason to reapply for several years afterwards is that some plants may be missed, new seedlings may sprout, and a few plants will survive the previous treatments.

Once all plant matter has been removed it should be placed in a carton or a protected site so all matter can dry completely without the danger of being spread. Once the matter is totally dried, it can be burned, packaged for disposal or composted (http://www.ducks.ca/purple/InfoSite/broch2.htm). All matter needs to be completely destroyed, well packaged or completely dried to avoid further contamination.

A new method that has been recently introduced to aid in the fight against purple loosestrife is Biological control. Bio-control works by using a plant’s natural enemy, which feeds upon the plants. Five European insect species were approved as bio-control agents in North America, but approval followed five years of intense testing to make sure the insects did not feed on agricultural crops and wetland species other than purple loosestrife. The European insects were found through a federally funded search about a decade ago (Frenay 1997). There were six species selected as the most promising for biological control. These species included two leaf-eating beetles, Galerucella calmarirnsis and Galerucella pusilla which are capable of completely defoliating the plant, a rooting mining weevil, Hylobius tranversovittatus, which attacks the main storage tissue of loosestrife, two flower feeding beetles that were also introduced, Nanophyes marmoratus and Nanophyes brevis, which help reduce seed production, and the last insect was a gall midge, Bayeriola salicariae, which reduces seed production as well by attacking the flower buds (Malecki 1993). These insects will not completely destroy loosestrife, but they may significantly reduce the population of cohabitation with native species becomes a possibility. The details pertaining to numbers of insects to be released, location of release sites, time of releases, use of multiple-species verses single-species release, and so on, are still being developed (Malecki 1993). With this in mind we as humans need to help in reducing the spread of purple loosestrife.

Purple loosestrife has become a major problem in the United States. Scientists are still working on ways to reduce the spread of this exotic plant. The European insects were released to many sites in 1992, and since then there have been many more releases. These insects that have been released appear to be a successful way in reducing the spread of purple loosestrife.


Anonymous. Purple loosestrife. Online. Available: http://www.ducks.ca/purple/InfoSite/broch2.htm. September 19, 1999.

Anonymous. Introduction Purple loosestrife (PLS). Online. Available: http://www.ceris.purdue.edu/napis/pests/pls/plsintro.html. September 19, 1999.

Anonymous. Purple Loosestrife. Online. Available: http://www.anr.state.vt.us/dec/waterq/ans/plpage.htm. Last Updated March 6, 1998.

Bender, J., & J. Rendall. 1987. Lythrum salicaria: element stewardship abstract. The Nature Conservancy, Minneapolis Online. Available:


Borman, S., R. Korth, & J. Temte. 1997. Through the Looking Glass: A field guide to aquatic plants. Wisconsin Lakes Partnership. 80-81 p.

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Malecki, R.A., B. Blossey, S.D. Hight, D. Schroeder, L.T. Kok, & J.R. Coulson. 1993. Biological control of purple loosestrife. Bioscience. 43: (10) 680-686 p.