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Management Of Purple Loosestrife

Purple loosestrife

Purple loosestrife (Lythrum salicaria L.) is an erect, herbaceous perennial of Eurasian origin that became established in the estuaries of northeastern North America by the early 1800s. By the late 1800s it had spread throughout the northeastern United States and southeastern Canada, reaching as far north and west as Manitoba. L. salicaria caused few problems until the 1930s when it became aggressive in the floodplain pastures of the St. Lawrence River. Since then, it has steadily expanded its local distribution and now poses a serious threat to native emergent vegetation in shallow water marshes throughout the northeastern and northcentral regions. Recent records indicate that purple loosestrife is also tolerant of soils and climates beyond these regions and threatens to become a serious problem in wetlands and irrigation systems in the Great Plains and the Far West.

Mechanical control

The most important goal of controlling this weed is to prevent seed production. By late August, seed heads must be cut, bagged, and properly disposed of. It is more important to prevent seed drop than to kill the plant. If you plan to pull or dig plants, you must get all of the root or the plant will grow back since it is a perennial.

Most seeds do not begin to fall from the seed pods until September, but some have been observed falling as early as mid-August.

Chemical control

In general, effective chemical control requires multiple applications. Timing and application rate are crucial for successful control.

Purple loosestrife can be controlled with an aquatic formulation of glyphosate such as Rodeo. Rodeo should be applied at 4 pints per acre as a broadcast spray or 1-2 ounces per gallon as a solution. Treat when plants are actively growing at or beyond the bloom stage of growth. Best results are achieved when application is made during summer or fall months. Fall treatments must be applied before a killing frost.

Biological control

Biological control is only recommended for large infestations.

Hylobius transversovittatus is a large long-lived weevil that helps control purple loosestrife plants. The adults overwinter in the soil close to the location of loosestrife plants. They are nocturnal and feed mainly on the leaves. The larvae do the most damage to the plant by feeding on the root cortex and then entering the root itself and packing the roots with light brown frass. This species is being reared at the Insectary and will be available for release when numbers increase.

The Insectary also received a culture of Galerucella calmariensis and Galerucella pusilla, which are foliage feeding beetles of purple loosestrife. These insects are being reared in the laboratory for general release. Galerucella are voracious feeders and can completely defoliate a plant, thus eliminating any seed production. It is available upon request from the Colorado Department of Agriculture's Insectary.