Southeast Exotic Pest Plant Council Invasive Plant Manual
Common Name: Purple Loosestrife
Scientific Name: Lythrum salicaria L.
Purple loosestrife (Lythrum salicaria L.) is a wetland perennial that forms large, monotypic stands throughout the temperate regions of the U.S. and Canada. This aggressive invader replaces native vegetation, degrades wildlife habitat, and obstructs natural waterways. Also known as Bouquet-violet, it belongs to the Lythraceae (Loosestrife) family.
The native winged loosestrife (Lythrum alatum Pursh) most closely resembles purple loosestrife. However, winged loosestrife has alternate leaves, more widely spaced flowers, and is smaller in size (an average of 0.6 m or 2 ft tall) than purple loosestrife. Other species that might easily be confused with purple loosestrife on first glance include fireweed (Epilobium angustifolium L.), blue vervain (Verbena hastata L.), and blazing star (Liatris spicata L. Willd.), although their preferred habitats are considerably drier.
Purple loosestrife is an aquatic to semiaquatic species occurring in a variety of different shallow water wetlands including marshes, bogs, wet meadows, stream and river banks, shores of lakes and reservoirs, wet pastures, roadside ditches, and disturbed wet soils. Plants thrive under moist soil conditions and in full sun; however, they can survive in up to 50% shade.
Hand Pulling: In areas that contain less than 100 plants, younger plants (1-2 years old) can be hand-pulled. Plants more than 2 years old should be dug out with special care to include the entire rootstock. Removal activities should take place before flowering to ensure that seeds are not dispersed during the disturbance. All plant parts should be carefully bagged, removed from the site, and placed in approved landfills or preferably burned. Any plant fragment that escapes proper disposal could spread purple loosestrife on your control site or along your travel route. In addition, all clothing, boots, and equipment should be properly cleaned to ensure that no seeds are transported. If feasible, native plants should be restored to the control area by seeding or planting. This reestablishment of vegetation will deter new loosestrife seedling development through competition. Do not cut or mow purple loosestrife. These methods will simply increase the spread of plants since they can sprout vegetatively.
Long-term studies on the effectiveness of biological controls are being conducted at the New York Cooperative Wildlife Research Unit at Cornell University. Several phytophagous insects which specifically feed on purple loosestrife in Europe have under-gone a series of intensive lab and field testing. Three beetles-two leaf eaters, and one root miner-have been approved for release in the U.S. Experimental importation of these insects has been made in the northeast. Success of these efforts could pave the way for the use of biological controls to manage purple loosestrife in a permanent, cost-effective, and environmentally sound way. There is significant concern about other native species of the genus Lythrum that may also be fed upon, although to a lesser degree, by these insects. Since other control measures may harm a variety of non-target plant species, this non-target feeding may occur at a level which is preferable to alternative control techniques.
Cut Stump Treatment: In areas that contain more than 100 plants, a spot application of a glyphosate herbicide (one that is approved for use in and near water) is recommended. Individual purple loosestrife plants should be cut about 15 cm (6 inches) above the ground. A 20-30% solution of glyphosate and water should be applied directly to the cut surface either by a wick or injection into the stem.
Foliar Spray Method: If purple loosestrife covers a large area, a foliar spray can be applied using a 2% glyphosate solution and water plus 0.5% non-ionic surfactant. To be most effective herbicide should be applied just when plants have begun flowering. Where feasible, flower heads should be cut, bagged, and removed from the site before application to prevent the production of seed. Glyphosate is a non-selective herbicide, and extreme care must be taken to avoid contact with non-target plant species. The restoration of sites depends on these non-target species as they recolonize the area after the purple loosestrife is eliminated.
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