Herbicide

There are four chemicals that can be used to manage purple loosestrife on sites with standing or moving water typical of where it invades. Table 1 lists herbicides, rates, soil half-life, and eco-toxicity for chemicals commonly used to control purple loosestrife.  Triclopyr and glyphosate are used most commonly. However, 2,4-D, and imazapyr are also formulated for aquatic applications. The surfactants used in the formulation of herbicides or added to spray solutions to improve herbicide performance can be more toxic to non-target aquatic organisms than the active ingredient of the herbicide. Therefore it is important to only add surfactants with an aquatic label where the spray may come in contact with standing or moving water. In Montana, applicators need a 308 permit from the Montana Department of Environmental Quality before applying aquatic herbicides to water.

If you encounter any problems with the file provided on this page, please contact Technical Resources at 406-587-6822.

A printer-friendly version of the permit is available in Adobe Reader format.

Montana Department of Environmental Quality Application for Short-Term Exemption from Surface Water Quality Standards for Emergency Remediation/Pesticide Application - 75-5-308, MCA (PDF; 153 KB)

Experimental treatments using glyphosate or triclopyr eliminated all purple loosestrife stems the first year after treatment. However, two years after herbicide application, purple loosestrife stem densities were almost nine times higher where triclopyr was sprayed and 4.5 times higher where glyphosate was sprayed compared to pre-treatment levels. This indicates repeated applications, or herbicide applications combined with biological control insects, are needed for long-term population suppression.

Triclopyr formulated for use in wetlands (Garlon 3A®, Triclopyr 3 SL®, and Renovate3®) applied as a foliar broadcast application at six to eight quarts per acre is most effective when purple loosestrife is in the bud to mid-bloom stage. This is when plants are actively growing, the toxic chemical is moved into roots, and when roots reserves needed to regenerate from defoliation are at their lowest level. The second optimum timing is in September when starches in the roots are being converted to sucrose important for cold tolerance and surviving freezing temperatures over winter. A wetland non-ionic surfactant (Agri-Dex®, LI 700®, Cygnet Plus®, and others) is needed at 0.5 percent by volume (two quarts per 100 gallons solution) in the spray solution. A minimum of 50 gallons spray solution per acre is recommended for thorough wetting of the weed canopy and adequate control. A 1 percent to 1.5 percent solution (5 to 7.6 fluid ounces in four gallons water) can be used for backpack spraying. All purple loosestrife plants should be thoroughly wetted. Annual application may be needed to target plants re-growing from root stocks and the seed bank.

Glyphosate, a non-selective herbicide that affects both broadleaf and grassy plants, can be broadcast sprayed at two quarts per acre, as a 1 percent solution, or wick applied in a 33 percent solution. A formulation registered for use in and around water (Rodeo®) should be used, and a non-ionic surfactant labeled for use in water can be included in the spray solution at 0.5  percent volume (two quarts per 100 gallons solution). Glyphosate should be applied to actively growing plants and best results can be expected when it is applied during bloom. If applied at late bloom, plants may still produce seeds which can be clipped and bagged to prevent spread. One study found glyphosate most effective when applied in mid-August (late-bloom). Glyphosate is also effective when applied in September to actively growing plants. Selective spot treatment can be used to avoid injuring neighboring non-target plants that can fill in the areas opened by removing the purple loosestrife.

Imazapyr can be applied at one pint per acre (Habitat®) to actively growing plants. A non-ionic surfactant with an aquatic label is needed for imazapyr to be effective. Broadcast applications of 2,4-D with an aquatic label (DMA*4IVM®) at one to two quarts per acre or a 0.5 percent to 1 percent solution applied to the bud or early bloom stage will kill top growth. Repeated applications will be needed to target plants regenerating from roots. An aquatic surfactant can be used (0.5 percent by volume) to improve effectiveness.

^1/ Any mention of products in this publication does not constitute a recommendation by the NRCS. It is a violation of Federal law to use herbicides in a manner inconsistent with their labeling.

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Last Modified: 08/14/2008