Noxious and highly invasive.
Ethnobotanic: Immigrants might have deliberately introduced L. salicaria for its value as a medicinal herb in treating diarrhea, dysentery, bleeding wounds, ulcers, and sores, for ornamental purposes, or as a source of nectar and pollen for beekeepers (Hayes 1979; Jones 1976; Malecki et al. 1993; Stuckey 1980). In states where it is permitted, purple loosestrife continues to be promoted by horticulturists for its beauty as a landscape plant and for bee-forage. Purple loosestrife has been of interest to beekeepers because of its nectar and pollen production. However, honey produced from it is apparently of marginal quality (Feller-Demalsy & Parent 1989).
Horticultural: Horticultural cultivars of purple loosestrife (Lythrum spp.) were developed in the mid-1900s for use as ornamentals. Initially, these were thought to be sterile, and therefore safe for horticultural use. Recently, under greenhouse conditions, experimental crosses between several cultivars and wild purple loosestrife and the native L. alatum produced hybrids that were highly fertile (Ottenbreit 1991; Ottenbreit & Staniforth 1994). Comparable, subsequent experiments performed under field conditions produced similar results, suggesting that cultivars of purple loosestrife can contribute viable seeds and pollen that can contribute to the spread of purple loosestrife (Lindgren & Clay 1993). Ottenbreit & Staniforth (1994) indicate that such results suggest the need to prohibit cultivars of this species.
Noxiousness: Purple loosestrife grows most abundantly in parts of Canada, the northeastern United States, the Midwest, and in scattered locations in the West. Although this species tolerates a wide variety of soil conditions, its typical habitat includes cattail marshes, sedge meadows, and bogs. It also occurs along ditch, stream, and riverbanks, lake shores, and other wet areas. In such habitats, purple loosestrife forms dense, monospecific stands that can grow to thousands of acres in size, displacing native, sometimes rare, plant species and eliminating open water habitat. The loss of native species and habitat diversity is a significant threat to wildlife, including birds, amphibians, and butterflies, that depend on wetlands for food and shelter. Purple loosestrife monocultures also cause agricultural loss of wetland pastures and hay meadows by replacing more palatable native grasses and sedges (Mal et al. 1992; Thompson et al. 1987).
Having a noxious weed designation in some states prohibit its importation and distribution, but it is readily available commercially in many parts of the country. Lythrum salicaria has been labeled the “purple plague." because of its epidemic devastation to natural communities. The species is included on the Nature Conservancy’s list of “America’s Least Wanted -The Dirty Dozen” (Flack & Furlow 1996).
Impact/Vectors: Naturalized purple loosestrife was relatively obscure from the time of its introduction into North America in the early 1800s (Pursh 1814) until 1930, when a significant increase in populations invading wetlands and pastures was documented (Strefeler et al. 1996b). Reasons for the apparent sudden colonization and spread of this species include the disturbance of natural systems by human activities including agricultural settlement, construction of transport routes such as canals, highways, and perhaps, nutrient increases to inland waters (Mal et al. 1992; Malecki et al. 1993). Absence of natural enemies and ornamental use are other possible causes for purple loosestrife’s rapid expansion in North America (Thompson et al, 1987). Recently created irrigation systems in many western states have supported further establishment and spread of L. salicaria (Malecki et al. 1993).
The acquisition of adaptive characteristics from native species of Lythrum may have enhanced purple loosestrife’s invasive success. It will hybridize with Lythrum alatum, a widespread, native North American species, in natural settings. Under certain circumstances fertile hybrids are produced that can cross with weedy purple loosestrife. Such interspecific hybrids could serve as a “hybrid bridge” for the transfer of adaptive traits from native L. alatum into weedy populations of purple loosestrife (Anderson & Ascher 1993; Strefeler et al. 1996b).
North American naturalized populations of purple loosestrife often form monospecific stands, whereas, in its native Eurasian habitat the species comprises 1-4% of the vegetative cover (Batra et al. 1986; Strefeler et al. 1996b). Purple loosestrife causes annual wetland losses of about 190,000 hectares in the United States (Thompson et al. 1987; Mal et al. 1997). The species is most abundant in the Midwest and Northeast where it infests about 8,100 hectares in Minnesota, 12,000 ha in Wisconsin, over 12,000 ha in Ohio, and a larger area in New York State. Recent distributional surveys document the occurrence of monocultures in every county in Connecticut, where it has been found in 163 wetland locations (Ellis and Weaver 1996; Ellis 1996). At the Effigy Mounds National Monument (EFMO), combined populations of purple loosestrife cover an area of 5 to 10 hectares growing in regularly disturbed sites. This species has a major visual impact on the vegetation of EFMO, and it has the potential to invade and replace native communities endangering the areas' primary resources. (Butterfield et al. 1996). In response to the alarming spread of this exotic species, at least 13 states (e.g., Minnesota, Illinois, Indiana, Ohio, Washington, and Wisconsin) have passed legislation restricting or prohibiting its importation and distribution (Malecki et al. 1993; Strefeler et al. 1996b).
Numerous studies demonstrate the aggressive and competitive nature of purple loosestrife. Fernald (1940) reported a loss of native plant diversity in the St. Lawrence River floodplain following the invasion of purple loosestrife and another exotic, Butomus umbellatus L. Gaudet and Keddy (1988) report declining growth for 44 native wetland species after the establishment of Lythrum. Among the species tested, Keddy (1990) found that purple loosestrife was the most competitive. His hierarchical rank, arranged from most to least competitive, illustrates the dominance of this invasive weed over many common natives: Lythrum>Cyperus>Juncus> Eleocharis> Mimulus>Verbena. In the Hamilton Marshes adjacent to the Delaware River, annual above-ground production of L. salicaria far exceeded all other plant species’ production combined.
Purple loosestrife provides little food, poor cover, and few nesting materials for wildlife (Mann 1991). Waterfowl nesting becomes more difficult as clumps of L. salicaria restrict access to open water and offer concealing passageways for predators such as foxes and raccoons (Mal et al. 1992). Non-game species, including black terns and marsh wrens, also lose nesting sites when purple loosestrife infests their normal habitats. Balogh and Bookhout (1989a) report that dense stands of purple loosestrife provide poor waterfowl and muskrat habitat. Red-wing blackbirds appear to be the only species to cope with changes in wetlands caused by purple loosestrife (Balogh and Bookhout 1989a). In many areas where L. salicaria populations have increased, wildlife species have declined. While some studies may fail to demonstrate cause and affect relationship, they firmly establish circumstantial evidence implicating that Lythrum’s invasion is responsible for major changes in wetland communities (Mal et al. 1992).
Purple loosestrife prefers moist, highly organic soils but can tolerate a wide range of conditions. It grows on calcareous to acidic soils, can withstand shallow flooding, and tolerates up to 50% shade. Purple loosestrife has low nutrient requirements and can withstand nutrient poor sites. Under experimental, nutrient-deficient conditions, the root/shoot ratio increased and provided purple loosestrife with a competitive advantage over the native species Epilobium hirsutum. Survival and growth of L. salicaria was greatly improved by fertilizer treatment and greater spacing between plants. Such results suggest that excessive use of fertilizers and the release of phosphates, nitrates, and ammonia into the environment has enhanced the success of Lythrum (Mal et al., 1992; Shamsi and Whitehead, 1977a and b).
Purple loosestrife flowers from July until September or October. Flowering occurs 8-10 weeks after initial spring growth. The lowermost flowers of the inflorescence open first and flowering progresses upward. The capsules mature in the same sequence and the lowermost will ripen and disperse its seeds while flowering is still occurring further up the inflorescence (Butterfield et al. 1996). Thompson et al. (1987) estimated that on average, a mature plant produces about 2,700,000 seeds annually. Purple loosestrife seeds are mostly dispersed by water, but wind and mud adhering to wildlife, livestock, vehicle tires, boats, and people serve also as agent. Seeds are relatively long-lived, retaining 80% viability after 2-3 years of submergence (Malecki 1990). Welling & Becker (1990) investigated seed bank dynamics in three wetland sites in Minnesota and noted a mean density of 410,000 seeds per square meter in the top 5 cm of soil, which was more than all other species combined.
Spring-germinated seedlings have a higher survival rate than summer-germinated seedlings. Seedlings that germinate in the spring will flower the first year, whereas, summer-germinated seedlings develop only five or six pairs of leaves before the end of the growing season. Since its seeds are small, weighing about 0.06 mg each and carry little food reserves, germination must occur under conditions where photosynthesis can occur immediately. A strong taproot develops quickly in seedlings and persists throughout the life of the plant. The aerial shoots die in the fall and new shoots arise the following spring from buds on the rootstocks. Shoots destroyed by fire, herbicides, or mechanical removal can also regenerate from the rootstock. As plants mature, they produce more and more aerial shoots forming very dense clumps of growth. Purple loosestrife can spread vegetatively by resprouting from stem cuttings and from regeneration of pieces of root stock (Mal et al. 1992). Rhizomatous growth is insignificant in purple loosestrife (Shamsi & Whitehead 1974a; Thompson et al. 1987).
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