Controlling Purple Loosestrife

Prevention || Mechanical Control || Chemical Control || Biological Control || Choosing Among Traditional and Biological Methods

Prevention

This is the easiest control method and the best way to stop the spread of purple loosestrife. Monitor your wetlands annually and remove any new young plants. Find pioneering plants or isolated small colonies, especially in areas otherwise free of loosestrife. Check areas near moving water, wetland/upland edges, storm sewer outlets or gardens that may have contained PL. Eliminate local PL seed sources, such as gardens (it is illegal to cultivate PL.) Plants are most easily located when flowering.One mature PL plant produces over 2 million seeds a season, so learn to recognize pre-flowering plants or search for them when they just start to bloom. Destroy plants before they mature, flower and drop seeds, which can start before flowering ends.It is believed that once flowering has begun, viable seed could be produced.When removing plants, take care not to leave stems or cuttings that can resprout or disperse viable seed.

Dispose of plants/seeds in a capped landfill, or dry and burn them. Composting will not kill the seeds. Keep clothing and equipment seed-free to prevent its spread. Rinse all equipment used in infested areas before moving into uninfested areas, including boats, trailers, clothing, and footwear. Take responsibility to remove new plants wherever you see them.(Get landowner permission first.)Again, it is illegal to cultivate purple loosestrife in Wisconsin.Follow-up is critical for all loosestrife control methods.

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Mechanical Control

This method includes cutting, pulling, digging and drowning. Cutting is best done just before plants begin flowering. Cutting too early encourages more flower stems to grow than before. If done too late, seed may have already fallen. Since lower pods can drop seed while upper flowers are still blooming, check for seed. If none, simply bag all cuttings (to prevent them from rooting). If there is seed, cut off each top while carefully holding it upright, then bend it over into a bag to catch any dropping seeds. Watch for holes in your bags so you don't spread seed where you drag the bags. Dispose of plant parts as above.

Pulling and digging can be effective, but can also be disruptive by creating disturbed bare spots, which are good sites for PL seeds to germinate, or leaving behind root fragments that grow into new plants. Use these methods primarily with small plants in loose soils, since they do not usually leave behind large gaps, nor root tips. Large plants with multiple stems and brittle roots often do. Dispose of plants as above. Drowning young PL is effective if plants are completely submerged for a year (often after being cut to decrease height.)

Mowing has not been effective with loosestrife unless the plants can be mowed to a height where the remaining stems will be covered with water for a full 12 months. Burning has also proven largely ineffective. Mowing and flooding are not encouraged because they can contribute to further dispersal of the species by disseminating seeds and stems. The U.S. Fish and Wildlife Service (FWS) (exit DNR) has found water level manipulation to be effective: reduce water level until loosestrife has sprouted, then increase the level to drown the stems.

Follow-up treatments are recommended for at least three years after removal.

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Chemical Control

This is usually the best way to eliminate PL quickly, especially with mature plants. Chemicals used, have a short soil life. Timing is important: Treat in late July or August, but before flowering to prevent seed set. Always back away from sprayed areas as you go, to prevent getting herbicide on your clothes. Generally, the formula designed for use on wet or standing sites should be used.The best method is to cut stems and paint the stump tops with herbicide. Cut low on the stem (about knee level) with one hand and apply the herbicide with a second hand, while carefully stuffing the plant top into a plastic bag with the third and fourth hands (a two person crew works well for this). The herbicide can be applied with a small drip bottle or spray bottle, which can be adjusted to release only a small amount. Try to cover the entire cut portion of the stem, but not let the herbicide drip onto other plants since it is non-selective and can kill any plant it touches.

Glyphosate herbicides: Roundup and Glyfos are typically used, but if there is any open water in the area use Rodeo, a glyphosate formulated and listed for use over water. Currently, glyphosate is the most commonly used chemical for killing loosestrife. Glyphosate must be applied in late July or August to bemost effective. It should be applied to loosestrife foliage in a 1% active ingredient (a.i.) solution--only 25% of the foliage of each plant need be covered. Glyphosate mixed to 3-10% a.i. can also be used on freshly-cut stems; this is especially effectiveon larger plants in areas of low loosestrife densities. Stem applied herbicides should be mixed to 20 to 40% active ingredient. Since you must treat at least some stems of each plant and they often grow together in a clump, all stems in the clump should be treated to be sure all plants are treated.

Another method is using very carefully targeted foliar applications of herbicide (NOT broadcast spraying). This may reduce costs for sites with very high densities of PL, since the work should be easier and there will be few other plant species to hit accidentally. Use a glyphosate formulated for use over water. A weak solution of around 1% active ingredient can be used and it is generally necessary to wet only 25% of the foliage to kill the plant.

Triclopyr (Garlon 3A) is another herbicide that can be used as a foliar spray. It is formulated for use around water and does not harm grasses or sedges. Unfortunately, it has not received final approval from the EPA for wetlands. Wet most of the foliage if using this chemical. (Broadcast spraying with a very weak solution of 0.5% active ingredient has been used as a broadcast spray and may reduce PL, but be less harmful for other plant species. Bog plants and alders are seriously harmed. This is not yet recommended, since quantitative data on its effectiveness and safety are lacking.) Triclopyr formulated for water dilution may be the most effective herbicide for loosestrife. Triclopyr is broadleaf-specific--it does not harm sedges or monocots. Foliar application must cover nearly all of the foliage.

You must obtain a permit from WDNR before applying any herbicide over water. The process has been streamlined for control of purple loosestrife and there is no cost. The appropriate person to contact is your regional Aquatic Plant Management Coordinator (listed in the brochure Addendum.) S/he will want to know about your site, may make control suggestions and will issue the permit.

Chemicals - Tools and Costs: Chemicals and tools to apply them are often available from local farm cooperative stores and garden shops. Your APM Coordinator may be able to help you locate a supplier near-by. If you cannot get the herbicides locally, UAP, a business in DeForest, Wisconsin, will ship them. Contact them by calling 800/362-8049. Roundup is for drier sites and costs about $90/gallon. Buying in volume will save you money: 2.5 gallons of Roundup costs about $110! It usually requires no additives for stump application, but may perform better with a surfactant and ammonium sulfate in certain situations. Rodeo is for very wet sites and costs around $120/gallon. It comes mixed with a nonionic surfactant. Monsanto (800/332-3111) manufactures both of these glyphosates. Generic versions of Rodeo and Roundup (such as Glyfos made by Cheminova (800/548-6113)) are available; talk to your local supplier or the manufacturer for details, especially reliability. Concentrations discussed above are approximate since requirements are variable on different populations of plants in different situations. Test yours to be sure you use an appropriate amount of active ingredient. Call manufacturers or your APM Coordinator if you have additional questions.

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Biological Control

Photo by UW-Madison Plant Image Collection Courtesy of Wisconsin Vascular Plants Website (exit DNR)

Conventional control methods like hand pulling, cutting, flooding, herbicides, and plant competition have only been moderately effective in controlling purple loosestrife; biocontrol is now considered the most viable option for more complete control for heavy infestations. The DNR, in cooperation with the U.S. Fish and Wildlife Service, is introducing several natural insect enemies of purple loosestrife from Europe. A species of weevil (exit DNR) (Hylobius transversovittatus) has been identified that lays eggs in the stem and upper root system of the plant; as larvae develop, they feed on root tissue. In addition, two species of leaf-eating beetles (exit DNR) (Galerucella calmariensis and G. pusilla) are being raised and released in the state, and another weevil that feeds on flowers (Nanophyes marmoratus) is being used to stress the plant in multiple ways. Research has shown that most of these insects are almost exclusively dependent upon Lythrum salicaria and do not threaten native plants, although one species showed some cross-over to native loosestrife. These insects will not eradicate loosestrife, but may significantly reduce the population so cohabitation with native species becomes a possibility. In a test area, 90% of the treated area saw a 90% reduction in loosestrife. Three additional species are also known to feed on purple loosestrife. All of these species remain experimental at the time of printing, but the DNR is seeking cooperators who will release and monitor the insects.

Biological control must be started in all PL infestations where there is no other effective control, no matter what size. Often this is on larger or higher density sites. Some sites may seem unlikely to benefit if they are ill suited to insect success (such as with summer flooding), but trying BC is cheap and it's probably worth trying. Some small infestations should receive beetles to serve as local insectaries--growing insects for propagation stock--as long as PL seed from these sites will not infest other nearby wetlands. Bio-control uses plant predator insects to control PL. It is inexpensive and natural and can become self-sustaining and self-disseminating (even to hard-to-reach places) with minimal disturbance to wetlands. However, set-up time is longer and its outcome less certain. An insect population must grow commensurately large to affect the local PL infestation, a process that usually takes at least several years, may require several infusions of propagated insects and may not work on some sites, especially if flooding can occur in mid summer. Many bio-control insect releases in Wisconsin have been quite successful, showing good control in only 2-3 years on smaller sites. Larger sites will take longer with the duration determined by many, hard-to-quantify factors. Contact Brock Woods (608/221-6349) at the DNR for further details.

Choosing Among Traditional and Biological Control Methods

Use traditional chemical and mechanical methods for quick initial control on all sites for which you have adequate resources and can be successful. These measures are labor intensive, and expensive on large sites, so small or low-density sites (usually 50-100 plants) are most often controlled this way. These methods should always be tried when quick action is necessary to prevent PL seed from dispersing to other new areas. Up to 95% control can be achieved with careful, additional annual vigilance. Even if all PL is accessible, some plants will be missed and a soil seed bank ensures germination of new PL plants for up to a decade. Thus, you must annually treat any missed or new plants. You also need to eliminate any PL in surrounding wetlands to stop seed dissemination to your site or hand control may not be worth the effort. All of these methods can be very disruptive to wetlands and, in addition to cost and chemical use, often suggest a serious consideration of alternative biological control.

Integration of traditional controls and biocontrol may turn out to be the best plan for many sites, since some immediate PL control can be maintained while biocontrol insects become common enough to exert meaningful control. As long as insects have foliage to eat and are not exposed to pesticides, various methods are compatible. One strategy is to cut inflorescences off to prevent seed production, but leave lower foliage to feed the developing beetle population. This eliminates the current year's seed production while encouraging beetles. Another is to start beetles in the center of a large PL infestation, but use traditional controls to minimize the spread of plants at the periphery. More research is needed to see if other combinations of methods may prove even more effective, especially at shortening the overall control process while keeping costs and chemical use to a minimum.

For more information, contact your regional WDNR Aquatic Plant Management Coordinator or the Purple Loosestrife Bio-Control Program at Brock.Woods@wisconsin.gov or brock.woods@ces.uwex.edu or WDNR Research Center, 1350 Femrite Drive, Monona, WI 53716 (608/221-6349.)

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