When: Plan on spraying in mid-summer through early fall (July 1 - September 1) because the herbicides are most effective at this time and purple loosestrife plants are easily identified. Treat as soon as possible after loosestrife begins to flower. This will minimize seed production. If plants are already well-established at the site, there is probably already a high density of viable seeds in the soil. The disadvantage of treating early in the season is that loosestrife plants are difficult to locate because they are not in flower.
Permits: If purple loosestrife is located in or along a water course, lake basin or wetland, a permit is probably required for control work. An Aquatic Nuisance Control (ANC) permit is required for chemical control of purple loosestrife within the boundaries of the state's protected waters. When treating loosestrife, there is no fee for this permit. Contact your local DNR office if you aren't sure if the lake, stream or wetland is protected and whether a permit is needed or check the permit pages for more details.
Applications for an Aquatic Nuisance Control permit may be made by the riparian owner on that body of water or by a representative of a group of riparian owners, such as a lake association.
Sprayers: Use only a plastic or stainless steel sprayer. Use clean water, check your sprayer for leaks and adjust the nozzle to provide a spray of fine droplets. Do not adjust to a mist, since a fine mist is likely to drift and kill desirable vegetation.
Weather: Treat when rain is not expected for at least 8 hours, preferably 24 hours, and treat only during mid-morning to afternoon (wait until the dew is off and the plants are dry). Do not treat on windy days. Refer to herbicide labels for wind and temperature limitations.
Mixing: Follow the instructions on the label of the herbicide you purchase. For example - Rodeo and Pondmaster: Mix a 1% solution (1 1/3 ounce Rodeo per gallon of clean water) and .25% of Ortho X-77 Spreader (1/3 ounce per gallon).
Apply Herbicide: Wet about 1/4 to 1/2 of the leaf areas of each plant (a "clump" of loosestrife is an individual plant), taking care to avoid spraying other species. Often loosestrife is taller than the surrounding vegetation, so you can spray the top of the plant. Many plants require only one brief squirt of herbicide
Planning: Work through the colony starting at one side and backing away from the area you have sprayed to avoid walking through the wet herbicide. For larger patches, tie some bright colored flags to tall plants to mark the boundaries of the areas that have been treated.
Signs: If the treatment is carried out on Minnesota public waters or wetlands, you need to post "Loosestrife Control Site" signs in the treated area to serve notice to anyone who may use the area for water recreation. Use restrictions vary depending on the herbicides and how they are used. Signs are provided by the DNR when the Aquatic Nuisance Control (ANC) permit is issued.
Revisit: Revisit the wetland each year to kill any surviving plants and new seedlings to prevent invasion. Also watch any areas where soil disturbances or exposed mud flats may provide a site for seedling establishment.
Aquatic Herbicides
Glyphosate herbicides are very effective for killing purple loosestrife. Glyphosate is available under multiple trade names. Only aquatic formulations of glyphosate (such as Rodeo, Pondmaster and Eagre) may be used to control purple loosestrife at aquatic sites. Some glyphosate formulations may be used on upland areas for vegetation control. Always read and follow herbicide labels. Glyphosate is nonselective, meaning it will kill all types of plants. However, selective application techniques allow it to be used effectively with minimum damage to desirable plants. It is taken up through the leaves or young stems and will kill any plant that it is applied to. Therefore, treat only the loosestrife plants and avoid contact with valuable wetland plants such as cattails. Glyphosate is biodegradable, very short-lived and becomes quickly inactivated when it contacts moist soil.
Aquatic formulations of glyphosate may be mixed with Ortho X-77 Spreader, or another approved wetting agent, to improve control. The glyphosate herbicides readily available at most feed and garden stores are generally formulations only approved for upland (non-aquatic) sites. Aquatic formulations of glyphosate (such as Rodeo) are often only sold in large quantities and by very few dealers. Pondmaster is available in smaller quantities.
The overall objective when controlling purple loosestrife with glyphosate herbicide is to spray very carefully so the loosestrife plants are selectively removed but the surrounding desirable vegetation are not harmed. If this can be accomplished most of the loosestrife will be controlled. The relatively small "holes" in the vegetation will be quickly filled by other plants precluding the establishment of loosestrife seedlings. Follow-up treatment is needed each growing season since some plants will be missed, new seedlings will sprout, and a few plants will survive the initial treatment. Improper mixtures and careless application, however, inevitably kills more surrounding vegetation and leads to the establishment of more loosestrife seedlings.
Triclopyr, a broadleaf herbicide, can be effective on loosestrife, when used from late May through September. This herbicide is more selective and will not harm monocot species such as cattails. Renovate is the aquatic formulation of triclopyr that can be used to control loosestrife in Minnesota. This product however, is sold by very few dealers and in large quantities.