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Spread, Impact, and Control of Purple Loosestrife (Lythrum salicaria) in North American Wetlands

Impact on Agriculture

Purple loosestrife is not a likely threat to cultivated crops. The large perennial rootstocks of L. salicaria lie within the upper 30 cm of the soil profile and are thereby susceptible to any form of crop culture that includes annual tillage. Moreover, because purple loosestrife seed is primarily distributed by flowing water, upland crops are secure from infestation even in the presence of lowlands heavily infested with the species. The establishment of L. salicaria adjacent to wild rice (Zizania aquatica)-growing areas of northern California (D. Barbe, personal communication) opens the possibility of the infestation of wild rice stands. In an earlier report from this region, Howell and True (1966) stated, "Dr. T. C. Fuller tells us that the plant has become so rampant at the last station [roadside ditch, 2.5 miles south of Oroville, Butte Co.] as to require agricultural control since it threatens to invade rice fields." We have also received a report from Wisconsin (unpublished Purple Loosestrife Task Force Newsletter, October 1986) of concern that wild rice beds on the Lac du Flambeau Indian Reservation might be threatened by L. salicaria.

Wetland Pasture

Although Fernald (1940) and Muenscher (1955) recognized the threat that L. salicaria posed to native wetland plants, the first report of this plant as an agricultural weed came from the pastures along the floodplain of the St. Lawrence River between Montreal and Quebec (Louis-Marie 1944). In 1942, Quebec's Ministry of Agriculture requested Louis-Marie to begin a study of the growing complaints of farmers and villagers, particularly in the region of Lac St.-Pierre, where it "usurps pasturage from more than 800 cows." After two seasons of field work, Louis-Marie estimated that the infestation covered 5 square miles. He also reported that the livestock forage value of lower meadows, where L. salicaria had established monospecies stands, was negligible. This difference in palatability between L. salicaria and native forages (grasses and sedges) is one of the keys to purple loosestrife's success as a pasture invader. The early foliage of L. salicaria is palatable to livestock (also to white-tailed deer, Odocoileus virginianus); the mature plant is much less frequently taken and therefore has a growth advantage over its heavily clipped associates. The wetland pastures that typically occur along floodplains and on the peripheries of glacial marshes throughout New York, Wisconsin, northern Iowa, and southern Minnesota are highly vulnerable to L. salicaria invasion.

Wild Hay Meadow

The cutting of wild hay has been in slow decline over the past 50 years; however, the number of hectares harvested changed very little from 1969 to 1974, and has remained well above 2,832,900 ha (7 million acres) over the United States (U.S. Bureau of the Census [USBC] 1977b). Figure 18 displays the millions of metric tons of wild hay harvested in six regions of the northern United States in 1974. Several aspects of these statistics need to be considered before evaluating the possible impact of L. salicaria on wetland or riparian hay marshes. First, in Fig. 18, wild hay is excluded from those States in which purple loosestrife does not occur or is not a threat. Second, the wild hay statistic does not separate wetland and riparian hay meadows from upland sites that would not be susceptible to L. salicaria invasion. From our field observations, we judge that lowland meadows make up more than half of the totals shown. Third, the degree that purple loosestrife will be able to invade wild hay marsh is difficult to estimate. In the absence of selective grazing pressure that occurs in meadows used for pasture, wild hay fields that are mowed annually will be more resistant to invasion by alien species. That L. salicaria will indeed invade wild hay meadows has already been demonstrated at sites in the Northeast, Midwest, West, and Pacific Northwest. At each of these sites, purple loosestrife has invaded ungrazed stands of grass or sedge. Last, the most useful concept to be drawn from Fig. 18 is that the potential damage to wild hay is inverse to the present distribution of purple loosestrife infestations. Wild hay cutting has been in decline in the Midwest and Northeast where it had a wide variety of uses including dunnage for ship cargos, fiber for carpets, and forage and bedding for livestock. With the coming of improved upland forage crops, wild hay fields in these middle and eastern areas have reverted to marginal pasture, with many thousands of hectares passing out of agricultural usage. The picture is very different in the western ranges that have traditionally produced most of our feeder cattle. The quantity and quality of the western wild hay harvest determine the growth and survival of overwintering yearlings and the health and success of the spring calf crop. The largest blocks of wild hay in the United States lie in western riparian meadows; the next 10 to 20 years will determine the impact of L. salicaria on this forage resource.

GIF-Graph of wild hay harvests in 1974
Fig. 18. Millions of metric tons (dry weight) of wild hay harvested in 1974 in six regions of the northern United States (USBC 1977b).

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