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Madison, WI 53705-0274
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Purple loosestrife (Lythrum salicaria) is a nuisance exotic
weed that is extremely invasive in Wisconsins wetlands. Although
colorful, this plant is extremely undesirable because it prevents many
desirable native wetland plants from becoming established.
Purple loosestrife is a perennial herb 3-7 feet tall with a dense bushy
growth of 1-50 stems. Showy flowers vary from purple to magenta, possess
5-6 petals aggregated into numerous long spikes, and bloom from July to
September. Leaves are opposite, nearly linear, and attached to four-sided
stems without stalks.
Purple loosestrife was first introduced as a garden perennial from Europe
during the 1800's. The plant was first detected in Wisconsin in the early
1930's, but remained uncommon until the 1970's. It is now widely dispersed
in the state, and has been recorded in 70 of Wisconsin's 72 counties.
Low densities in most areas of the state suggest that the plant is still
in the pioneering stage of establishment. Areas of heaviest infestation
are sections of the Wisconsin River, the extreme southeastern part of
the state, and the Wolf and Fox River drainage systems.
This plant's optimal habitat includes marshes, stream margins, flood
plains, sedge meadows, and wet prairies. It is tolerant of moist soil
and shallow water sites such as pastures and meadows, although established
plants can tolerate drier conditions.
Purple loosestrife spreads mainly by seed, but it can also spread vegetatively
from root or stem segments. A single stalk can produce from 100,000 to
300,000 seeds per year. Seed survival is up to 60-70%, resulting in an
extensive seed bank. The absence of natural predators, like European species
of herbivorous beetles that feed on the plant's roots and leaves, also
contributes to its proliferation in North America.
The plant's ability to adjust to a wide range of environmental conditions
gives it a competitive advantage; coupled with its reproductive strategy,
purple loosestrife tends to create monotypic stands that reduce biotic
diversity. Purple loosestrife displaces native wetland vegetation and
degrades wildlife habitat. As native vegetation is displaced, rare plants
are often the first species to disappear. Eventually, purple loosestrife
can overrun wetlands thousands of acres in size, and almost entirely eliminate
the open water habitat. The plant can also be detrimental to recreation
by choking waterways.
By law, purple loosestrife is a nuisance species in Wisconsin. It is
illegal to sell, distribute, or cultivate the plants or seeds, including
any of its cultivars.
The primary control season is mid-July through mid-August.
Plants are treated with the herbicide Rodeo, with a concentration of
active ingredient of 10% in water. (Rodeo is a form of the herbicide glyphosate
that has been formulated for use in and around water.)
The method preferred by the Wisconsin
Department of Natural Resources is to traverse the wetland on foot.
Old shoes and pants should be worn. In cases of extreme muck these articles
of clothing may need to be discarded afterwards.
Once the target (Purple Loosestrife) is located, tear or cut off the
inflorescence (flower cluster). If early enough in July, and the plants
are not beginning to make seed, the inflorescence can be discarded on
the ground. If seed production has already begun, the inflorescence must
be bagged and removed from the site.
A wooden-handled paintbrush is affixed with a screw inside a wide-mouth plastic water bottle, such as a Nalgene (available at an outdoor store). Cut the top of the paintbrush square and to a length so that it just reaches the bottom of the bottle. (See photo)
It is important to seal around the screw and washer with glue so that the herbicide cannot leak out.
In use, hold the paintbrush containing herbicide by the plastic top and wipe off the excess liquid before painting the loosestrife stems.
With the paintbrush herbicide applicator, paint each stem of the loosestrife
plant from the top down about 3 feet. Be careful to get as many of the
stalks painted as possible. Applying herbicide with a paintbrush (instead
of a spray bottle) ensures that the chemical is confined to the stem.
It is important to recheck the eradication area again approximately two
weeks later and treat any plants that were missed and are still alive.
- Contributed by Richard Staffen, Bureau of Endangered
Resources, Wisconsin Dept. of Natural Resources
A Purple Loosestrife Biological Control Manual for Educators (WDNR)
Also see Wisconsin
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A Role for Native Weeds
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