The following Abstract is from this link: http://www.npwrc.usgs.gov/resource/1999/loosstrf/loosstrf.htm
Purple loosestrife (Lythrum salicaria L.) is an
emergent aquatic plant of Eurasian origin; with the rise of marine commerce, it
became established on all mid-latitude continents, except South America. L.
salicaria arrived along the northeastern maritime coast early in the 19th
century and has subsequently spread across mid-latitude North American
wetlands. Several modes of colonization or escape were probable, including
ship's ballast, livestock bedding and forage, wool, and purposeful import as
seeds or rootstocks for early gardens and herb beds. The gene pool shared by
these immigrants accumulated over 50 years or more from a wide climatic and
geographic range across Europe and Asia Minor. Rapid gene flow is inferred from
the more or less equal distribution of three forms of flowers (short-, mid-,
and long-style) occurring over Europe and North America. Purple loosestrife's
plant associates in North America are cattails (Typha spp.), reed
canarygrass (Phalaris arundinacea), sedges (Carex spp.),
bulrushes (Scirpus spp.), willows (Salix spp.), and horsetail (Equisetum
fluviatile), in the order mentioned. All of these plants have conspecifics
(or nearly identical taxa) that are native to Eurasia, suggesting that L.
salicaria was strongly preadapted to be a successful immigrant.
The
bisexual flowers of purple loosestrife are insect-pollinated. Self-pollination
is possible, but cross-pollination prevails. The period of bloom in most areas
is from late June to early September. An old plant can produce more than 2 million
seeds per growing season; sexual reproduction is of overwhelming importance.
Seed dispersal is largely by drift in moving water; long distance spread is
possible by seeds imbedded in mud on water birds, trucks or off-road vehicles,
or in the cooling systems of outboard motors. Erratic spread can also occur by
purposeful introduction as a honey bee forage plant or by accidental escape
from horticultural plantings. Seed samples from commercial suppliers of
wildlife cover and prairie restoration plants have contained purple loosestrife
seeds as an impurity. Seed longevity is at least 3 years with viability of 80 +
%. Floating seeds or propagules must lodge on open, moist soil or saturated
organic debris to take root. Once established, purple loosestrife can survive
with 50% of full sun, but declines in vigor at lower light levels. The plants
grow on a wide range of substrates, but are more successful on slightly acid or
neutral soils. L. salicaria has many ways of adapting to a wide range of
habitats. It responds to soil nutrient (P and N) deficiencies by increasing the
root to shoot ratio, to rising water level by the growth of aerenchyma in
submerged stem tissue, and to trampling, cutting, or crushing of stems with
shoot and root buds at the site of damage.
The
spread and dominance of purple loosestrife in North American wetlands has shown
a pattern of exponential increase that corresponds to the rate of exploitation
of these habitats. With the construction of the eastern canal systems, and the
extension of marine commerce into the Great Lakes, L. salicaria
colonized the glacial marshes of the Midwest by 1900. By 1940, it was
established in the Pacific Northwest and had begun to spread onto the Great
Plains; by 1985, Alaska and Montana were the only States north of the 35th
parallel that had not reported purple loosestrife.
The
impact of purple loosestrife on native vegetation has been disastrous, with
more than 50% of the biomass of some wetland communities displaced.
Monospecific blocks of this weed have maintained themselves for at least 20
years. Impacts on wildlife have not been well studied, but indicate serious
reductions in waterfowl and aquatic furbearer productivity. Several declining
species of vertebrates are threatened with further degradation of their
breeding habitats with the continued expansion of purple loosestrife. The
plant's low palatability to livestock makes it a problem in wetland pastures in
the Northeast; it also threatens riparian hay meadows and off-water swales in
irrigated areas in the West. Although L. salicaria can invade relatively
undisturbed habitat, the spread and dominance of this weed have been greatly
accelerated in disturbed habitats. Despite early control efforts in Quebec, and
subsequent work in the United States, little research on purple loosestrife
ecology and control has been done. Glyphosate has been used successfully, but
no effort has been made to measure the impact of this broad-spectrum herbicide
on native plant communities. Enough preliminary work has been done with
biological control of purple loosestrife to suggest that this promises to be a
valuable approach.
On
both regional and local scales, the rate of spread of L. salicaria has
followed the logistic curve. At present, coping with purple loosestrife hinges
on early recognition of the arrival of the weed and a conservative pattern of
marsh management that avoids stressing native plant communities. Early
detection allows local eradication to be carried out with minimum damage to the
native plant community. Some form of integrated control is needed. California,
Illinois, Ohio, Minnesota, and Wisconsin have taken prompt action to deal with
this weed.
The
following pictures and line drawing are from this link: http://plants.usda.gov/cgi_bin/topics.cgi?earl=plant_profile.cgi&symbol=PHAU7
