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General Information About Purple Loosestrife
(Lythrum salicaria)
Purple loosestrife (Lythrum salicaria)
is a Eurasian native perennial wetland plant which is responsible
for the degradation of a considerable amount of wetland habitat in
the United States. Invasion of North American wetlands by purple
loosestrife began in the early nineteenth century when the plant
was introduced both as a contaminant of European ship ballast and
as a valued medicinal herb for treatment of diarrhea, dysentery,
and ulcers. By the 1830’s, purple loosestrife was well established
along the New England seaboard. The continued expansion of the
plant across the country coincided with an increase in national
and regional transportation systems, commercial distribution of
the plant for horticultural uses, and regional propagation of
plant seed for growing bee forage.
Growth Habit
Purple loosestrife prefers to grow in marshes,
ponds, stream banks, ditches, and lake shores; occasionally it can
be found in upland areas. It can grow to two meters (about six
feet) tall and has spikes of five-petaled reddish-purple flowers.
The plant’s leaves occur opposite each other along a square stem.
A single mature plant can produce more than 2.5 million seeds
annually. The seeds are long-lived and easily dispersed by water
and in mud adhering to aquatic wildlife, livestock, and people. A
strong rootstock serves as a storage organ, providing resources
for plant growth if the above ground stems are cut, burned, or
killed by application of foliar herbicides. All of these
characteristics make a very aggressive plant extremely difficult
to control.
In Washington, purple loosestrife is classified
by law as a noxious weed. This law requires a landowner to control
the spread of the plant or prevent any seed production by the
plant. Because purple loosestrife grows so aggressively, over time
large stands of the plant take over an area and replace the native
plant species. This results in eliminating the natural foods and
cover essential to many wetland inhabitants including waterfowl.
In addition, purple loosestrife can grow in and around irrigation
systems and impede the flow of water. The Winchester wasteway, an
irrigation system in Grant County, Washington, is reported to have
one of the largest stands of purple loosestrife in the country.
Purple loosestrife is a cause for grave concern among water users
and purveyors.
Management
No native herbivores or pathogens in North
America are known to suppress purple loosestrife. Current means of
plant control include mechanical cutting or application of
herbicides. Exciting research into using biological controls on
purple loosestrife is currently being conducted. These efforts are
focused on using insects from the plant’s native range in Europe.
Several types of insects including a root-mining weevil and a
leaf-eating beetle have been introduced into Washington in an
attempt to control purple loosestrife. The root-mining weevil lays
eggs into the stem of the plant and the developing larvae work
their way down into the roots. There the larvae feed extensively
on the root tissue, weakening the plant from within. The adult
weevils also feed on the newly formed purple loosestrife leaves.
The leaf-eating beetles affect the plant in two ways: the adults
feed on newly formed leaf tissue and the larvae feed on bud, leaf
and stem tissue.
The introduction of these insects on purple
loosestrife on the Winchester wasteway is beginning to pay
dividends. In 1997, 1998, 1999, 2000 the insects had significant
impacts on purple loosestrife stands; defoliating the plants and
preventing seed set. Observers reported that the plants appeared
dead or dying and looked as if they had been sprayed with
herbicides. County weed board staff and others collected insects
from the wasteway in July 1998, 1999, and 2000 and dispersed them
to purple loosestrife locations throughout Washington.
Identification
Purple loosestrife is sometimes confused with
fireweed (Epilobium angustifolium) a pink-flowered herb
that inhabits dry often disturbed sites such as clear-cut areas
and roadsides. It is also confused with hardhack (Spiraea
douglasii), a woody shrub also with pink to purple flowers.
Neither fireweed or hardhack have square stems.
- Purple loosestrife has vivid purple-pink
flowers and blooms in summer and early fall.
- The stems are generally square.
- The leaves are usually oppositely arranged
and are lance-shaped.
Source: Washington State
Department of Ecology |
