Noxious Weed IVM Guide Contents, IVM for Noxious Weeds
Broom, Gorse, Knapweeds, Leafy Spurge, Purple Starthistle,
Smooth Cordgrass, Tansy Ragwort, Yellow Starthistle
IPM Access Key Documents, Home Page
IVM Technical Bulletin
Purple Loosestrife
Gather Background Information

The first step in an IVM program is to gather information on the life cycle and habits of the noxious weed.

Purple loosestrife, Lythrum salicaria, is a wetland perennial herb that can grow up to 9 ft tall. Each spring, some 30 to 50 stems arise from a perennial, woody rootstalk. In flooded areas, purple loosestrife can form dense, fibrous rootmats. In autumn, stems die back and purple loosestrife stays dormant through the winter. Stems have short, slender branches and evenly spaced nodes. Leaves are elongate, smooth-edged, rounded or heart-shaped at the base, and attached close to the stems. Showy magenta or purple flowers grow on long spikes and occur in pairs or clusters.

Purple loosestrife can easily be confused with other native wetland plants such as fireweed, Epilobium angustifolium. Look for purple loosestrife's squarish stems, opposite leaves, and flowers with 5 to 7 narrow petals. Fireweed has a rounded stem, alternate leaves and flowers with 4 broad petals (Mal et al. 1992).

Purple loosestrife is native to Eurasia and was first cited in the United States in 1843 and has since spread throughout the United States and Canada. It was accidentally introduced from European ship ballasts or on infested sheep and raw wool. Purple loosestrife may also have been intentionally introduced by horticulturists as ornamentals and for treatment of diarrhea, dysentery, and bleeding (Mullin et al. 1993; Cornell 1997).

Purple loosestrife is invasive and forms dense monocultures, crowding out native vegetation and decreasing species diversity in many infested areas. Dense infestations choke off wildlife habitat, waterfowl nesting areas, and wildlife access to water.

In areas where purple loosestrife has migrated from wetlands to lowland pastures, it can reduce land values and crop yields. It can impede water flow in drainage and irrigation ditches and its rapid leaf decomposition rates can potentially alter nutrient cycling. Emery and Perry (1996) hypothesize that, "a wetland converted from cattails to purple loosestrife may function less effectively as a nutrient bio-filter and may accelerate eutrofication in downstream water bodies" (Mal et al. 1992).

In the United States, purple loosestrife has spread to all states, and in Canada to all provinces except the Yukon and Northwest Territory (Cornell 1997; Harper-Lore 1997).

Purple loosestrife commonly grows in moist to wet areas such as marshes, wetlands, riparian meadows, and pastures. It can also invade drier areas, causing increasing concerns on agricultural lands and pastures (OFAH 1996).

Life Cycle
Purple loosestrife is a long-lived perennial that can still be vigorous after 10 years. Purple loosestrife reproduces from plant fragments or by seed. It generally flowers from late June through August and can continue into October. Once flower petals start to drop from the bottom of the floral spike, purple loosestrife begins to produce seed, near early August. This event should be noted since most control measures should occur before seed set (Mal et al. 1992; OFAH 1996).

A single mature plant can produce 2.7 million seeds each year, each seed about the size of a grain of sand. Seeds can be dispersed far distances by wind, water, ducks, or other wildlife and remain dormant in the soil for many years (McEachen 1997; OFAH 1996).

Purple loosestrife grows best in full sunlight. A decrease to 40% full sunlight results in a significant reduction in seed. In deeply shaded environments the plant often does not produce flowers (Mal et al. 1992; Fournier 1997).

Special Challenges to Management
Purple loosestrife can grow in poor soil and tolerate a wide range of soil textures, such as sand, clay, gravel, organic soils, and crushed rock. Its success may be traced to disturbances. Shamsi (1976) reported that purple loosestrife survival and growth was improved with fertilizer treatment and greater spacing between plants. In the Great Lakes, and possibly other areas, purple loosestrife may have been aided by fertilizer runoff from surrounding farmlands.

In addition, purple loosestrife and its cultivars are still sold in many areas because of its beautiful flowers. Cultivars such as 'Morden Pink', 'Morden Gleam', and 'Morden Rose' are sterile; however, they can outcross with wild populations of purple loosestrife and produce long-lasting viable seed (Mal et al. 1992).

Site-Specific Questions

Some questions, such as those below, can only be answered on site.

Set Management Objectives

Set Realistic Goals for Your IVM Program
The answers to the following questions can help you set realistic objectives and goals.

Levels of Control
Containment - keeping an established population of the weed from spreading to non-infested areas. This strategy is especially useful when time and money are in short supply or when the infestation is very large. For example, a barrier strip between infested and non-infested areas can be maintained and monitored so that adjacent lands remain weed free. In addition, measures that stop seed production will prevent further spread of the weed.

Reduction - reducing the area covered by purple loosestrife, or reducing its dominance. This strategy can also be used against new or established weeds, but it requires more resources and more time than containment.

The "Bradley Method" (see Appendix 2), developed in Australia, is a simple yet innovative strategy for natural areas that combines containment and reduction.

Eradication - completely eliminating the weed from the management area.This strategy usually consumes the greatest amount of time and resources and is applicable mainly to newly-invading weeds that are confined to a limited number of small areas.

Establish Monitoring Programs

When planning a monitoring program, keep in mind the context of your target weed: is it invading or has it already invaded?

Locate and record purple loosestrife infestations on a map. (Chapter 2 of the University of Northern Iowa IVRM Technical Manual contains a detailed discussion on how to map and inventory vegetation - see Bibliography). Note particularly sensitive areas on the map, such as critical habitat for threatened or endangered species, wetland or riparian areas, or areas subject to frequent disturbance and thus prone to invasion. Update maps at regular intervals.

Focus monitoring efforts on sites where purple loosestrife problems are most likely to occur (see Distribution). Encourage public sighting and reporting through an education or incentive program (see Educate Vegetation Management Personnel and the Public).

Prioritize the sites you will work on. Make a realistic assessment of your weed management resources, keeping in mind the goals of your project and the cost of a follow-up program after any treatments. Without follow-up, your control efforts will be wasted. It is better to thoroughly control a weed at one or two sites than to use up resources to incompletely control the weed at many sites. If the weed is very widespread, try to determine where it poses the most serious economic, social, or environmental problem and concentrate on those areas.

Plan monitoring and treatment efforts to coincide with critical life stages of the weed. To use your resources efficiently, try to include monitoring with other planned activities in the area.

Maintain records of your monitoring activities. Creating standardized forms will make data collection easier and help remind you to gather all the information you need. Forms work best if they include labeled blanks for all pertinent information and allow the user to check or circle rather than having to write words or numbers (See Appendix 3 for some examples of forms).

Include information such as the name(s) of the person(s) collecting the data, the location, and date of monitoring; a qualitative description of the vegetation, such as the names of the plants or types of plants (native vegetation, annual/perennial weeds, trees, etc.) and stage of growth (germinating, flowering, setting seed, etc.); a quantitative description, such as percent cover, plant density, size of the patch, or if possible, the number of plants.

Note special conditions such as unusual weather events and record treatment history, including information on treatment applications (who, when, where, how, cost, difficulties, and successes). This will allow you to evaluate and fine-tune treatments.

Set Treatment Thresholds

Setting treatment thresholds includes prioritizing and balancing treatments with resources. Weeds are treated when populations increase beyond a predetermined level. This level will largely depend on the characteristics of the site and weed. In some cases the level may be no weeds at all, and in other cases the number of weeds you can tolerate may be much greater.

Considerations for Setting Priorities
What is the size of the weed population? The opportunity for control is related to the infested area. Small patches can be more easily controlled than large infestations.

What is the level of the threat? Is the purple loosestrife population changing? Is it in an area where soils are frequently disturbed? Is it threatening wildlife habitat, or agricultural areas? Is it encroaching on critical habitat for a rare, threatened, or endangered species? Is it displacing the best examples of native communities?

What resources are available? Do you have the resources required for carrying out your goal?


With the advent of herbicides, prevention as a weed management technique has often been neglected; however, it is a practical, cost-effective, and extremely important part of noxious weed control.

General Weed Prevention Measures

Revegetation - Follow-up Weed Prevention
After treatments are applied, it is very important to establish dense, competitive vegetation that will permanently replace weeds. Not only is revegetation critical after treatments open the canopy or leave bare soil, but it is also crucial in preventing weed infestations in areas where the soil has been disturbed or the vegetation removed. In some situations you may be able to encourage desirable vegetation that is already in place; but because of the aggressive nature of purple loosestrife, it is more likely that you will need to thickly sow seeds of desirable, competitive plants. Japanese millet (Echinochloa frumentacia), grows rapidly, can tolerate periodic flooding, and can outcompete purple loose- strife in some cases. In addition, Japanese millet can provide food for waterfowl. Malecki and Rawinski (1985) recommend planting immediately after soils are exposed and when purple loosestrife seedlings have not yet sprouted.

Purple loosestrife control can be enhanced by a combination of planting competitive grasses and selective herbicide. After purple loosestrife is removed and competitive grasses are seeded, it is likely to see a flush of purple loosestrife seedlings. These seedlings are vulnerable and can be targeted with a selective broadleaf herbicide that does not affect grasses. Once purple loosestrife seedlings are controlled, other broadleaf plants and shrubs can be planted.

Apply Management Methods

No individual method will control purple loosestrife in a single treatment; diligence and persistence will be required over a number of years to subdue this weed. The treatment methods described in this section will help you to design an integrated program that will suit the circumstances of your particular situation.

Biological Controls
Biological control does not aim to eradicate weeds, but to keep them at low, manageable levels. After their introduction, biocontrol agents can take 5 to 10 years to become established and increase to numbers large enough to reduce the density of the target weed. Once established, effective biological controls provide an inexpensive, long-term, and non-toxic means to control weed populations. Since insects have specific requirements for growing and thriving, it is important to match the insect to the weed management site. Understanding these requirements will help you integrate the insects into other weed control efforts. When you release biocontrols, continue using other control methods on the perimeter of the release site, but avoid using them where they might adversely impact the insect population.

The information provided below is only a summary. For more information consult Biological Control of Weeds in the West (see Bibliography) or contact commercial weed biocontrol insectaries (see Insectaries).


Insect release protocols for Hylobius transversovittatus, Galerucella calmariensis, G. pusilla, and Nanophyes mamoratus are available through the World Wide Web site: ( or contact your County Weed Extension Agent.

Septoria lythrina, Alternaria alternata, Botrytis cinerea, and Colletotrichum tuncatum can weaken plants and make purple loosestrife more susceptible to control methods. More studies of their efficacy as biocontrol agents of purple loosestrife are under way (Nyvall 1997).

Adult purple loosestrife is unpalatable and trampling by animals in soft riparian areas can lead to disturbances that would further favor purple loosestrife. There are no published studies on controlling purple loosestrife by grazing (Brookreson et al 1993).

Physical Controls

Manual Removal
When beginning a hand removal project, flag the treated areas so they can be identified for follow-up in subsequent seasons. It is easiest to work in relatively small areas of infestation.

When faced with dense or extensive stands of purple loosestrife, it is best to divide them into grids (with flags, stakes, etc.) so that workers can thoroughly weed smaller areas before moving onto the next grid. The grid system also facilitates dividing work activities between those pulling and those removing the debris.

Mechanical Removal Controlled Burns
Roots are largely unaffected by controlled burns and the plant can later resprout from the rootstalk. Small areas of adult purple loosestrife can be burned to facilitate decomposition. In areas blackened by fire, increased soil temperatures produce a flush of purple loosestrife seedlings which can be kept in check by hand pulling or selective herbicide. Surrounding grasses can then move in (McEachen 1997).

Covering dense populations of plants with black plastic can block sunlight and kill plants. This technique works best for pure stands of seedlings. Plants should be cut or mowed and then covered with black plastic for at least 5 consecutive months, beginning in early spring (April or May). To be effective, the treatment may have to be repeated the following year.

Although young plants will be killed, older plants and seeds may still survive. Because it is labor intensive, smothering is more effective on a small scale. Constant monitoring is necessary since weeds can grow through small holes and cracks in the plastic (Brookreson et al. 1993; McEachen 1997).

Water Level Manipulation
Although purple loosestrife can withstand short periods of flooding, long periods of deep flooding using storm water retention cells can help control loosestrife populations. Mature plants are severely impacted by flooding for one to two months at a water level of 1 to 3.5 ft. The effectiveness of this technique is increased when cutting precedes flooding.

Seedlings die after 8 weeks of flooding and are affected more by the duration rather than the depth of flooding. On the other hand, seeds can remain viable after submergence for 20 months. Shallow flooding (less than 12 inches) did not significantly affect seedlings (Haworth-Brockman et al. 1993; Comas et al. 1992; Malecki and Rawinski 1985; Heidorn and Anderson 1991).

Although purple loosestrife populations have been observed to decrease in size and vigor following a natural flood, it is quick to regain its former dominance in the following 2 years. Using storm water retention cells is impractical in certain areas and flooding can facilitate seed dispersal to unvegetated sand bars or to downstream areas. Since flooding can also be detrimental to wildlife and native vegetation, it may not be suitable for some environments (Kincaid 1997).

Chemical Controls
In IVM programs, herbicides are considered transition tools that enable the manager to suppress weeds and replace them with desirable, competitive vegetation. Thus, it is important to select the least-toxic, low-residual herbicide that is effective against the target weed, and to apply them in a judicious manner.

Currently, the effects of herbicide on purple loosestrife biocontrol agents are not very clear. As a precaution, herbicide use is not recommended in insect release areas. Mosquito control fogging treatments may pose a threat to natural enemy populations. Additional studies continue to clarify the effects of mosquito control programs on purple loosestrife biocontrols (Blossey 1998).

Proper Timing
Applying herbicide to plants when purple loosestrife is most susceptible (preferably before seeds are produced) is crucial to the effectiveness of the treatment.

Educate Vegetation Management Personnel
and the Public

Evaluate the Vegetation Management Program


Anderson, M.G. Interactions between Lythrum salicaria and native organisms: a critical review. Environmental Mangmt. 19(2): 225-231.

Anonymous. 1995. A practical approach to weed management- reason, reality, reponsibility. 1995. Proc. of the 47th Annual California Weed Science Society: 87-92.

Blossey, B. et al. 1994. Host specificity and environmental impact of the weevil Hylobius transversovittatus, a biological control agent of purple loosestrife (Lythrum salicaria). Weed Science 42: 128-133.

Blossey, B. and M. Schat. 1997. Performance of Galerucella calmariensis (Coleoptera: Chrysomelidae) on different North American populations of purple loosestrife. Environmental Entomology 26 (2): 439-445.

Blossey, B. 1993. Herbivory below ground and biological weed control: life history of a root-boring weevil on purple loosestrife. Oecologia 94: 380-387.

Blossey, B. "Purple loosestrife." Personal E-mail (9 Sept. 1997).

Blossey, B. Personal E-mail (23 Feb. 1998).

Brookreson,W.E. et al. 1993. Noxious Emergent Plant Management: Env. Impact Statement. Washington State Dept. of Ag. 94pp.

Comas, L., K. Edwards, and B. Lynch. 1992. Control of purple loosestrife (Lythrum salicaria L.) at Indiana Dunes National Lakeshore by cutting followed by overwinter flooding. National Park Serv. National Biological Survey. Wisconsin Coop. Res. Unit 12pp.

Cornell. Purple Loosestrife Website. (visited Sept. 1997).

Dewey, S.A. and J.M. Torell. 1991 What is a noxious weed? In: James et al., Noxious Range Weeds. Westview Press, Boulder, CO.

Dundas, H. 1997. Pers. Comm. Project Manageer. Purple Loosestrife Eradication Project. 115 Perimeter Rd. Saskatoon, SK S7N OX4.

Emery, S.L. and J.A. Perry. 1996. Decomposition rates and phosphorus concentrations of purple loosestrife (Lythrum salicaria) and cattail (Typha spp.) in fourteen Minnesota wetlands. Hydrobiologia 323:129-138.

Fay, P.K., T.D. Whitson, S.A. Dewey, and R. Sheley, eds. 1995. 1995-1996 Montana-Utah-Wyoming Weed Management Handbook. Coop. Ext. Serv., Montana State Univ., Bozeman, MT. 245 pp.

Fournier, B. Pers. Comm. 1997. Purple loosestrife control. c/o Kitsap County Parks, 1200 Fairgrounds Rd. Bremerton, WA 98311.

Fuller, T.C., and G.D. Barbe. 1985. The Bradley method of eliminating exotic plants from natural reserves. Fremontia 13(2): 24-25.

Harper-Lore, B. 1997. presentation. "Update from the Federal Interagency Committee on Management of Noxious and Exotic Weeds (FICMNEW). Federal Highway Administration, US Dept. of Transportation. Proceedings Cal EPPC Symposium ’97.

Haworth-Brockman, M.J., H.R. Murkin, R.T. Clay. 1993. Effects of shallow flooding on newly established purple loosestrife seedlings. Wetlands 13(3): 224-227.

Heidorn R., and B. Anderson. 1991. Vegetation management guideline: purple loosestrife (Lythrum salicaria L.). Nautral Areas Journal 11(3): 172-173.

Hight, S.D., B. Blossey, J. Laing, and R. Declerck-Floate. 1995. Establishment of insect biological control agents from Europe against Lythrum salicaria in North America. Env. Ent. 24(4): 967-977.

International Institute of Biological Control Annual Report 1996 (IIBC). 1997. CAB International. Oxon, UK. 132pp.

Jacobs, J.S., R.L. Sheley, and B.D. Maxwell. 1997. Yellow starthistle population dynamics model. Colorado Weed Management Association 1997 Annual Conference Proceedings. Granby, CO.

Kincaid, R. 1997. Pers. Comm. Nebraska Purple Loosestrife Awareness Committee. 1303 East 22nd St. Kearney, NE 68847.

Kok, L.T., T.J. McAvoy, R.A. Malecki, et al. 1992. Host specificity tests of Galerucella calmariensis (L.) and G. pusilla (Duft.) (Coleoptera: Chrysomelidae), potential biological control agents of purple loosestrife, Lythrum salicaria L (Lythraceae). Biological Control 2: 282-290.

Lacey, C.A., et al. 1988. Bounty programs—an effective weed management tool. Weed Technology 2: 196-197.

Mal, T.K., J. Lovett-Doust, L. Lovett- Doust, and F.A. Mulligan. 1992. The biology of Canadian weeds. 100. Lythrum salicaria. Canadian Journal of Plant Science 72: 1305-1330.

Malecki, R.A., and T.J. Rawinski. 1985. New methods for controlling purple loosestrife. New York Fish and Game Journal 32(1): 9-19.

Malecki, R.A., B. Blossey, S.D. Hight, et al. 1993. Biological control of purple loosestrife. Bioscience 43(10): 680-686.

Maxwell, B. " YST software." Personal e-mail (9 Feb 1998).

McEachen, H. 1997. Pers. Comm. Agronomist. PO Box 96. Mesa, WA 99343.

Mullin, B.H., D. Zamora and P.K. Fay. 1993. Purple loosestrife: a new weed threat to wetlands in Montana. Montana State University Extension Service EB 70. 9pp.

Nechols, F.R., H.H. Obrychi, C.A. Tauber, M.J. Tauber. 1996. Potential impact of the native natural enemies on Galerucella spp. (Coleoptera: Chrysomelidae) imported for biological control of purple loosestrife: a field evaluation. Biological Control 7: 60-66.

Nyvall, R.F. and A. Hu. 1997. Laboratory evaluation of indigenous North American fungi for biological control of purple loosestrife. Biological Control 8: 37-42.

Ontario Federation of Anglers and Hunters (OFAH). 1996 "Purple Loosestrife: what you should know, what you can do." PO Box 2800. Peterborough, Ontario. K9J 8L5. 7pp.

Pearson, W. 1998. Pers. Comm. County Weed Extension Agent. Stillwater County, Columbus, Montana.

Piper, G.L. 1996. Biological control of the wetlands weed purple loosesetrife (Lythrum salicaria) in the Pacific northwestern United States. Hydrobiologia 340: 291-294.

Randall, J.M. and J. Marinelli eds. 1996. Invasive Plants: Weeds of the Global Garden. Brooklyn Botanic Garden , Inc. Brooklyn, NY. 108pp.

Rees, N.E., P.C. Quimby, Jr., G.L. Piper, E.M. Coombs, C.E. Turner, N.R. Spencer, and L.V. Knutson, eds. 1996. Biological Control of Weeds in the West. Western Society of Weed Science, USDA/ARS, Montana Dept. Agric., Montana State University, Bozeman, MT.

Shamsi, S.R.A. 1976. Some effects of density and fertilizer on the growth and competition of Epilobium hirsutum and Lythrum salicaria. Pak. J. Bot. 8(2): 213-220. In, Mal et al. 1992. The biology of Canadian weeds. 100. Lythrum salicaria. Canadian Journal of Plant Science 72: 1305-1330.

University of Northern Iowa. 1993. Integrated Roadside Vegetation Management Technical Manual. Produced by the Roadside Management Program. To obtain, call Kirk Henderson at 319-273-2813.

Voegtlin, D.J. 1995. Potential of Myzus lythri (Homopthera: Aphididae) to influence growth and development of Lythrum salicaria (Myrtiflorae: Lythraceae). Environmental Entomology 24(3): 724-729.

Welling, C.H. and R.L. Becker. 1993. Reduction of purple loosestrife establishment in Minnesota wetlands. Wildlife Soc. Bull. 21(1): 56-64.

Wilcox, D.A. 1989. Migration and control of purple loosestrife (Lythrum salicaria L.) along highway corridors. Environmental Management 13 (3): 365-370.

"Invasive Exotic Plants of Canada Fact Sheet No. 4." (visited Sept. 1997).

Further Information

(Below are web pages specifically regarding purple loosestrife; see link above for other websites.)

Cornell. Purple Loosestrife Website.

CAPS Purple loosestrife fact sheet 42. Purple loosestrife: public enemy #1 on federal lands: http//www.ceris.purdue.ude/napis/pests/pls/factspls/

Ducks Unlimited Canada.

"Invasive Exotic Plants of Canada Fact Sheet No. 4." (visited Sept. 1997)

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