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Purple Loosestrife on the Loose

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Published on: Aug. 2, 1998

Last revision: Nov. 1, 2010

shortcoming of herbicide treatments is that they must be repeated annually. If any residual seeds persist in the soil, they will continue to germinate in successive years until the seed bank is exhausted.

Treatment of sites on private land requires permission of landowners. Most private landowners are cooperative and allow Conservation Department employees to apply herbicides to loosestrife colonies on their land. However, some landowners like the plant for its beauty and don't consider it a problem because it occupies unused habitats, such as pond banks, ditches, and wet areas. The danger in allowing populations to persist, however, is that purple loosestrife eventually will find its way into natural wetlands and greatly diminish the variety of plants and animals that live there.

Because herbicide treatment is not practical in regions with many large infestations, several states are trying a biological control method. Biological control makes use of other organisms to combat the problem species, often reuniting the problem species with its natural enemies from its native range.

Researchers studied the organisms that keep purple loosestrife under control in Europe and Asia and evaluated them for use in North America. They selected a trio of insects to do battle: a leaf-eating beetle, a root-mining weevil and a seed-eating weevil. Years of tests minimized the chance that these exotic insects would harm non-target species in North America.

Biological control is not risk-free because it involves introducing additional non-native species to try to control an earlier non native introduction. Thus far, none of these insects have been released in Missouri, but it may be necessary and practical to use them in the future. For example, one Missouri purple loosestrife population exists around the margin of a lake that is a municipal water supply, so using herbicides is prohibited. Biological control may be the only practical treatment for purple loosestrife at such a site. Other midwestern states with more and larger infestations have begun introducing insects for biological control with encouraging preliminary results.

Conservation Department biologists will continue to track populations of purple loosestrife in Missouri and attempt eradication wherever possible. You can help by reporting any wild populations. We are not interested in locations of ornamental plantings where plants are not spreading from the planting site. For a descriptive color poster, write Purple Loosestrife Poster, Natural History Section, Missouri Department of Conservation, P.O. Box 180, Jefferson City, MO 65102-0180.

Purple loosestrife regulation

Two species of plants, numerous hybrids and selections of these are called purple loosestrife. One of the species, Lythrum salicaria, was declared a noxious weed by the Missouri Legislature in 1989. This prohibits the sale of that species or any of its hybrids in Missouri. It is also unlawful to distribute or plant its seeds or any part of the plant in the state.

The other species, Lythrum virgatum, and its hybrids do not usually produce viable seeds. Therefore they do not spread and are still legal to sell as potted plants in Missouri. Some states have prohibited the sale of all purple loosestrife because of the difficulty in determining which plants are aggressive spreaders and because cross-pollination of non-spreading plants can produce ones that spread. The sale of any purple loosestrife seeds is illegal in Missouri.

Success Story

Joyce and Gary Burton are pleased to report that only a few plants of purple loosestrife remain around the Burton pond in Randolph County. Annual application of herbicide has prevented the plants from taking over the banks and shallow water areas of the pond and a wet area below. The Burtons' experience with purple loosestrife began in 1985 when they recognized the purple-flowered plants on their property as the same ones pictured in a Missouri Conservationist article that warned of the plant.

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