Purple Loosestrife (Lythrum salicaria L.)
DESCRIPTION
Purple loosestrife is a stout, erect perennial herb with
a strongly developed taproot. It may grow from 3 to 10 feet tall, with
an average height of 5 feet. Its most notable characteristic is its showy
spike of rose-purple flowers that are present in mid-to-late summer (early
June thru early September). There are , however, several native species
that also produce purple spikes of flowers that superficially resemble
those of purple loosestrife. Purple loosestrife can be differentiated from
these species by a combination of other characteristics. Purple loose-strife
has flowers with 5 to 7 purple petals (petals are occasionally pink or
white also); the leaves are opposite or in whorls of 3, all lacking teeth;
and it has a stiff 4-sided stem that may appear woody at the base of large
plants.
SIMILAR SPECIES
Purple loosestrife may be distinguished from the native winged loosestrife (Lythrum alatum), which it most closely resembles, by its generally larger size (native loosestrife has an average height of only 2 feet) and its opposite leaves (native loosestrife's upper leaves are usually alternate). The flowers of native loosestrife are also more widely spaced than those of purple loosestrife. Purple loosestrife should be accurately identified before attempting any control measures. If identification of the species is in doubt, the plant's identity should be confirmed by a knowledgeable individual and/or by consulting appropriate books.
DISTRIBUTION
Purple loosestrife is a perennial herb native to Europe and Asia. It was introduced to North America in the 1800's for use as an ornamental and by beekeepers. For nearly a century it occurred as a pioneering species on the northeastern seaboard, then, about 60 years ago the range of purple loosestrife began rapidly expanding, reaching the upper-Midwest in the 1930's. The species appears to go through a period of acclimation after each range expansion during which the invasive character of the species is not expressed. Heaviest U.S. concentrations are in the glaciated wetlands of the Northeast, but it occurs in nearly all sections of the country. Botanists have been aware of purple loosestrife populations in Missouri since the early 1950's.
HABITAT
Purple loosestrife occurs widely in wet habitats, such as freshwater marshes, fens, sedge meadows, and wet prairies, but it also occurs in roadside ditches, on river and streambanks, and at the edges of lakes and reservoirs. It thrives best in moist soil conditions in full sun, but it can survive in as much as 50% shade.
LIFE HISTORY
The reproductive capacity of purple loosestrife is one of the most significant and relevant life history characteristics of this herbaceous perennial plant. A single stalk can produce 300,000 seeds, and densities as high as 80,000 stalks per acre have been recorded, with the potential of producing as many as 24 billion seeds per acre. The seeds can remain viable even after 20 months of submergence in water. Seed set begins in mid-to-late July and continues through late summer. Seeds may be dispersed by water, wind and in mud attached to animals. Purple loosestrife also spreads vegetatively. Root or stem segments can form new flowering stems. Muskrat cuttings and mechanical clipping can also contribute to rapid spread by floating in riverine and lacustrine systems. Purple loosestrife lacks natural enemies in the United States.
EFFECTS UPON NATURAL AREAS
Purple loosestrife quickly crowds out most native vegetation in marsh, fen, sedge meadow, and wet prairie communities, creating a monoculture that provides little food or shelter for native wildlife. Once established, it can destroy marshes and wet prairies and choke waterways.
CURRENT STATUS
Purple loosestrife was declared a noxious weed by the Missouri Legislature in 1989. As such, the sale in Missouri of purple loosestrife or any of its hybrids is prohibited. It is also unlawful to distribute or plant seeds, plants or plant parts in the state.
Control Recommendations
Current mechanical and chemical control methods have only limited success. Smaller infestations are more likely to be controllable, therefore early diagnosis is critical.
Biological control of purple loosestrife is being studied by Dr. Richard Malecki of the New York Cooperative Wildlife Research Unit at Cornell University. Three beetles, two leaf-eaters and a root miner, that are natural predators of purple loosestrife in Europe have been approved for release in the U.S. after extensive testing. Initial releases will probably take place in Minnesota and Washington. These insects were selected for their specificity to purple loosestrife as a host plant. In Missouri, other species that may be affected are Lythrum alatum (winged loosestrife) and Decodon verticillatus (swamp loosestrife).
RECOMMENDED PRACTICES IN NATURAL COMMUNITIES WITH NO KNOWN INVASION
Potential loosestrife habitat should be searched annually during late July and August for the plant. Early detection is the best approach!
RECOMMENDED PRACTICES IN NATURAL COMMUNITIES WITH INVASIONS
Effort in areas with individual plants and clusters of up to 100 plants
Younger plants (1-2 years old) can be hand-pulled. Do not pull after flowering because this will scatter seed. Older plants, especially those in fens or in deep organic soils, can be dug out. Roots of older plants can be "teased" loose with a hand cultivator. Bag and remove the plants from the site. Failure to place the removed plants in a bag could result in spreading the plant along your exit route because fragments may be dropped. Dispose of the plant by burning (preferable) or in an approved landfill. Follow-up treatments are recommended for three years after the plants are removed. Clothing, equipment and personnel should be cleaned to insure no seeds are spread on them, if seeds were present on plants.
If the above control method is not feasible in areas with relatively small infestations, spot application of glyphosate herbicides can be used as described below.
Effort in areas with clusters in excess of 100 plants (up to 4 acres in size)
Spot application of a glyphosate herbicide to individual purple loosestrife plants is the recommended treatment where hand pulling is not feasible. Glyphosate is available under the trade names Roundup and Rodeo, products manufactured by Monsanto. Only Rodeo is registered for use over open water. By law herbicides may only be applied according to label directions.
Glyphosate is non-selective so care should be taken not to let it come in contact with non-target species. Glyphosate application is most effective when plants have just begun flowering. Timing is crucial, because seed set can occur if plants are in mid-late flower. Where feasible, the flower heads should be cut, bagged, and removed from the site before application to prevent seed set. Roundup should be applied by hand sprayer as a one and one-half% solution (2 oz. Roundup per gallon of clean water). Rodeo should also be applied as a one and one-half% solution (2 oz. Rodeo per gallon clean water) with the addition of a wetting agent, as specified on the Rodeo label.
Another option is to apply glyphosate twice during the growing season. Foliage should be sprayed as described above, once when flowering has just started and a second time 2-3 weeks later. With this procedure control is likely more effective, because plants are not allowed to set seed and those missed because they were not flowering the first time are treated the second time.
Excessive application of herbicide (causing dripping from the plant) can kill desirable plants under the loosestrife. These plants, left unharmed, will be important in recolonizing the site after the loosestrife has been controlled. If the desirable plants are killed, the vigorously resprouting and growing purple loosestrife seeds present in the soil will fill the void. Since purple loosestrife is usually taller than the surrounding vegetation, application to the tops of plants alone can be very effective and limit exposure of non-target species. Complete coverage is not required to affect control.
The herbicide should be applied while backing away from treated areas to avoid walking through the wet herbicide. Equipment, clothing and personnel should be cleaned completely before entering other uninfested sensitive areas, if seeds were present in the treated area. It will be necessary to treat the same area again annually until missed plants and plants originating from the seed bank are eliminated. Relatively young populations seem to be almost eliminated in 2-3 years of consecutive treatment while older stands will require more treatment. Cutting purple loosestrife and subsequently flooding the area so that cut plant stalks are completely immersed has controlled purple loosestrife in at least one case. However, flooding may encourage the spread of purple loosestrife if seed are present in the soil. Artificial flooding should not be used in high-quality natural communities with an intact natural flooding regime.
Effort in areas with large monocultures (greater than 4 acres in size)
For large purple loosestrife populations, an assessment should be made to determine if the loosestrife can be eradicated with available resources. If it can not be controlled, then efforts should be placed on keeping the loosestrife out of the highest quality areas. Applying glyphosate from a vehicle mounted sprayer is usually necessary in areas with extensive stands of purple loosestrife. The most effective control can be achieved by beginning treatment at the periphery of large patches and working towards the center in successive years. This allows peripheral native vegetation to reinvade the treated area as the loosestrife is eliminated.
Failed or Ineffective Practices
Mowing, burning, and flooding have proven largely ineffective. A single known exception is cutting followed by flooding as described above. Mowing and flooding can actually contribute to further spread of the species by disseminating seed and cut plant stems. Do not mow because cut parts may re-root.
Efforts to eliminate Purple Loosestrife in Missouri
Botanists have been aware of purple loosestrife populations in parts of Missouri since the early 1950s. An active eradication program began in 1990 when herbicide was distributed, free of charge, to landowners with purple loosestrife populations. Successful control was limited, probably due to poor application techniques or poor timing of treatment.
Since 1991, a more aggressive approach has been used in which the Missouri Department of Conservation (MDC) personnel conduct the chemical treatment. MDC treatment crews use Rodeo at a 1% solution with a surfactant. The age of purple loosestrife infestations seems to be a major factor in determining the effectiveness of control. Early treatment, excellent landowner cooperation and dedicated treatment crews are in large part responsible for good progress toward control of purple loosestrife in Missouri.
Landowners or managers who know of sites of purple loosestrife infestations in Missouri should contact the Missouri Department of Conservation so that treatment can be initiated.
Adapted from material written for the Illinois Nature Preserves Commission by: Randy Heidorn Illinois Department of Conservation Region II Office 110 James Road Spring Grove, Illinois 60081
