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Purple Loosestrife (Lythrum salicaria, L. virgatum, and their hybrids)

An Attractive but Serious Threat to Wisconsin's Wetlands and Waterways

Purple Loosestrife. . . Picture of Purple Loosestrife

  • displaces native wetland vegetation
  • degrades wildlife habitat
  • displaces rare plants and animals
  • chokes waterways

See Also: Purple Loosestrife (Manual Excerpt)

Identification || Current Extent of Purple Loosestrife || Control Methods || WI Addendum to Brochure || Statewide Responsibility

Identification

Growth: Upright, semi-woody, hardy perennial with a dense bushy growth of one to 50 stems. The green to purple stems grow three to seven feet tall and die back each year.

Flowers: Purple to magenta, and numerous on long spikes. Individual flowers are 1/2 to 3/4 across, with five or six petals.

Leaves: Variable (usually opposite), linear shaped, smooth edged, and attached directly (without stalks) to four-sided stems. Sometimes alternate on the stem or bunched in whorls. Foliage may be hairy.

Roots: Woody taproot with numerous fibrous side shoots that form a dense mat.

Blooming Period: Early July into early September.

Habitat: Moist soil to shallow water sites, such as wet meadows and pastures, marshes, stream and river banks, lake shores and ditches. Established plants can tolerate dry conditions, and are sometimes planted in lawns and gardens.

Similar Species: Do not confuse with the less common, native loosestrife (Lythrum alatum) which is smaller in size and has fewer flowers.

Problem
Purple loosestrife is native to Europe and Asia. After being introduced to North America, it managed to escape the specialized insects and diseases that keep it in check in its native lands. Free from these natural controls, purple loosestrife gained a competitive edge over our native wetland plants. This advantage, along with prolific seed production and a large, hardy growth, has allowed purple loosestrife to invade many of our wetlands to the near total exclusion of most other vegetation; it literally shades everything else out. Loosestrife's numbers and rate-of-spread have increased rapidly in the Midwest during the past 15 to 20 years. As a result, not only is our diverse wetland vegetation (including rare and endangered plants) threatened, but also most wildlife that depend upon native vegetation for food and shelter. Once a wetland is dominated by purple loosestrife, traditional residents such as muskrat and waterfowl decline in numbers significantly. Others, such as marsh wrens and least bitterns, are displaced completely from the wetland. Purple loosestrife itself provides habitat and food for few wildlife species, and once it begins to invade, canopy closure is likely and nearly all native vegetation is lost.

Spread
Purple loosestrife spreads primarily by seed, but can also spread from broken-off stems that root themselves in moist soil. Purple loosestrife plants produce over 100,000 seeds a year. These tiny seeds can remain viable in the soil for many years. Although most fall within a few yards of the parent plant, water, animals, boats, construction equipment, and humans can transport seeds long distances. Use of purple loosestrife for gardens and bees probably caused its initial establishment in many areas of Wisconsin.

Any sunny wetland is susceptible to purple loosestrife invasion. But disturbances such as water drawdowns, damaged vegetation or exposed soil greatly accelerate the process by providing the substrate and sunlight exposure needed for germination. A loosestrife invasion usually begins with a few pioneering plants. These first plants may not spread for several years as they build up a large seed bank in the soil. When the right disturbance occurs, the loosestrife often spreads rapidly and can eventually takeover the entire wetland.

Distribution
Purple loosestrife can be found throughout Wisconsin, but it is still in low density in most areas. In fact, only about 5 percent of our total wetland acreage currently has purple loosestrife. A survey in 1988 found that half the colonies in the state had fewer than 20 plants. Nearly 80 percent had fewer than 100 plants. There is still time to act.

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Current Extent of Purple Loosestrife

Preventing Further Spread
Prevention is the best way to stop the purple loosestrife invasion. The Department of Natural Resources recommends the following steps to prevent its further spread:

  1. Be on the lookout for pioneering plants or isolated small colonies, especially in areas otherwise free of purple loosestrife. Remove pioneering plants immediately.
  2. Rinse off equipment, boats and trailers, clothing, and footwear used in infested areas before moving into uninfested areas.
  3. Remove and destroy purple loosestrife planted in lawns and gardens. It is illegal to cultivate purple loosestrife in Wisconsin.

Although purple loosestrife in gardens may seem harmless, its seeds eventually will spread to favorable moist soil. Some plant producers claim to have sterile varieties of purple loosestrife. Research has shown, however, that all cultivars are capable of producing seeds if they cross pollinate with another loosestrife plant. And, plants don't have to be near each other for pollination to occur. Honeybees, the main pollinators of loosestrife, commonly travel one to two miles during their foraging.

As of 1987, state law bans the sale, offering for sale, distribution, planting, or cultivation of purple loosestrife. This ban covers both Lythrum salicaria and L. virgatum. There are no exceptions for cultivars, hybrids, or so-called sterile varieties. Sale violations of purple loosestrife should be reported to the Department of Agriculture, Trade and Consumer Protection at Bureau of Plant Industry, P.O. Box 8911, Madison, WI 53708-8911 or phone at (608)-224-4571. Cultivation violations should be reported to county or municipal law enforcement officials. Each violation is subject to a $100 fine [sec. 66.955(2)(5)].

Illegal Lythrum plants have many horticultural names, some of which are:
Rose Queen
The Rocket
Morden Pink or Rose
Morden's Gleam
Dropmore Purple
Columbia Pink
The Beacon
Fire Candle		 Atropurpureum
Happy
Roseum Superbum
Purple Spire
Mr. Robert's or Robert's
Lady Sackville

There are many native plants that have similar color, structure, height and blooming that can be used as alternatives to purple loosestrife in the garden. Some examples are:
Gayfeather
Obedient Plant
Joe-Pye Weed
Marsh Milkweed
Blue Vervain
Fireweed
Lupine		 Ironweed
Bergamot
Great Blue Lobelia
Wild Blue Phlox
Beardstongue
Blazing Star

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Control Methods

Any control method you select should be repeated for several years to catch missed plants and those reestablishing from seed.

Hand Pulling/Digging
  • Small young plants can be hand pulled, especially those rooted in loose soil. Be sure to get the entire root. Roots left behind will resprout.
  • Older plants are generally too big for pulling and are difficult to dig up.
  • Avoid excessive soil disturbance. If this is unavoidable, consider chemical methods.
  • Handle plants prior to the onset of seeds (which begins in early August), or cut and bag the seed heads to avoid spreading seeds.
  • Removed plant parts should be dried and disposed properly. Don't throw them in your compost pile.
Mowing Mowing has not been found to be an effective way to destroy purple loosestrife unless the plants are cut below a level that water will cover for approximately 12 months.

Herbicide Application
  • Careful use of herbicide is the most effective, efficient and least destructive means of removing large purple loosestrife plants.
  • Currently glyphosate is the most effective active ingredient for killing loosestrife. It comes in two forms: (1) "Roundup" for use on dry sites, and (2) "Rodeo" for use on wet or standing water sites. Glyphosate must be applied in late July or August to be most effective. It can be sprayed onto loosestrife foliage in a 1 percent solution. Glyphosate is a nonselective herbicide, however, and will kill any green foliage that it comes in contact with. Great care must be taken to avoid treating non target plants. A more selective method of applying glyphosate is to cut the loosestrife stems near the ground and paint or dab the fresh cut surface with a 50 percent solution of the herbicide. The cut stems should be removed from the wetland or they may take root.
  • "Garlon 3A" is a broadleaf (dicot) specific herbicide recently found to be effective on loosestrife. Its advantage over glyphosate is that is doesn't harm monocots such as grasses, sedges, cattails, rushes, reeds, etc. Garlon 3A currently can be used on dry sites only and is not yet approved for over-water use. Approval for wet site use is expected by 1997. Check with the Aquatic Plant Coordinator at your local DNR office for the current status of this herbicide.
  • Follow all label instructions when using herbicides.
  • A permit is needed from the DNR when applying herbicides to Wisconsin waters. The permit is an important safeguard against possible harm to people and the environment. Applicants for permits to treat purple loosestrife are exempt from all permit fees. To determine whether you need a permit, contact your local DNR office.
Biological Control This may be the most viable long-term solution, promising to greatly reduce the need for other more costly and disruptive control methods. The DNR, in cooperation with the U.S. Fish and Wildlife Service, is introducing natural insect enemies of purple loosestrife from Europe. Careful research has shown that these insects are dependent on purple loosestrife and are not a threat to other plants. Insect releases in Wisconsin and elsewhere are being monitored to see if these insects can decrease purple loosestrife's vigor. This could let native plants reduce its numbers naturally through competition.

A suite of six different insect species has gained U.S. and Canadian approval for release as biological control of purple loosestrife in North America. Leaf beetles, that feed primarily on shoots and leaves, were the first control insects to be released in Wisconsin, at two DNR properties beginning in 1994. Release of root-boring weevils began in 1995 at four DNR sites in southern Wisconsin, and flower and seed-eating weevil releases are expected to start in 1996. A propagation program is being started for use of these insects on DNR lands, and, if successful, production may be expanded for use in other areas.

Estimates of the length of time required for effective biological control of purple loosestrife range as high as several decades. Traditional control methods need to be used in the interim where control insects have not yet been established. This will continue to minimize the damaging effects of purple loosestrife on wetland ecosystems and shorten the time necessary to achieve natural control.

Though purple loosestrife will probably never disappear completely from Wisconsin, we may be able to restore health to our wetland ecosystems efficiently by simply restoring some of the natural checks and balances necessary in maintaining a diverse, healthy environment.

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WISCONSIN ADDENDUM TO: Purple Loosestrife: What You Should Know, What You Should Do (brochure)

The Brochure was written for both Canada and the US. Read it carefully since it is very applicable to Wisconsin. However, please take a moment to read this additional information specific to our state.

Wisconsin Law bans the sale, distribution, planting, or cultivation of Purple Loosestrife (PL), including both Lythrum salicaria, L. virgatum and ALL cultivars, hybrids, and so-called sterile varieties. Try to get landowners to stop growing it first, then report continuing cultivation violations to local law enforcement. (If no local weed policy exists, work with local government to establish one.) There is a $100 fine per violation: [Statutes, sec. 23.235(4)]. Report any sales of the plant to WDATCP at 608-224-4571.

Home Gardens With PL Are Still A Big Problem. Seeds wash into nearby waters and wetlands with many home gardeners apparently unaware that this is a problem.Help us spread the word to eliminate PL in gardens! Some similar native Wisconsin plants that can replace it in moist gardens include ironweed, bergamot, joe-pye weed, marsh milkweed, great lobelia, wild blue phlox, and beardtongue.

Help us up-date our Wisconsin PL Distribution Map by reporting PL locations (latitude and longitude points from a GPS unit are best), area, size, and land owner s name and phone number to "PL Bio-Control" at WDNR Research Center, 1350 Femrite Dr., Monona, WI. (Do not call local WDNR.)

The Most Effective Immediate Control of PL is selective use of herbicides (treat only PL foliage or cut stumps; no broadcast spraying). This method usually requires follow-up spraying, is expensive on large sites and can disturb wetlands. Consequently, it is most often used on small sites, but must also be used wherever the PL can spread seeds to new areas. Dig or pull plants if they are small.

Biocontrol (BC) Is Likely The BestT Long Term Control, especially on large sites since it is inexpensive, also natural, self spreading and permanent. Wisconsin DNR started BC for PL in 1994 and the process is very promising. DNR and its cooperators have released insects on over 300 sites statewide, though still less than 1% of all infested Wisconsin wetland.Monitoring shows the insects live exclusively on PL, are very hardy, and can cut PL seed production and growth on small sites within just a few years.Large sites take longer. Some may never be controlled. The insects rarely kill PL, but cut seed production, plant vigor and competitiveness, allowing PL to be replaced by native plants.

The Key To Effective, Long-Term PL Control seems to be an integration of chemical, machanical and Bio-Controls. Eliminate PL use in gardens and dispersal by humans.Start small beetle insectaries. BC might eventually replace most traditional methods if control insects become ubiquitous. This is where you can help. Start BC in your area!

The Control Chart In The Canadian Brochure is based on infestation size and density and can help you choose the best control for your site. It is a suggestion only, so plan wisely based on your unique site needs and resources. Use traditional methods where they will be effective and BC everywhere else! Save some small, isolated infestations for beetle insectaries. Purple loosestrife is now a huge problem and many citizens must help rear control insects and release them into their local wetlands.

To Learn More About Helping, Please Call or Write the following Wisconsin DNR Regional Aquatic Plant Management Coordinators (APM-C) about:

Last Revised: April 4, 2004