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Purple loosestrife occurs in all but 6 states of the continental United States . It is found along the Atlantic coast from North Carolina to Maine  and is scattered but spreading in the western United States . Purple loosestrife occurs most commonly in the United States in the Midwest and Northeast, corresponding closely with the geographic extent of the Wisconsin glaciation [81,125]. It is distributed across the southernmost tier of Canadian provinces from Newfoundland to British Columbia, with northern limits generally around 51Â° N . The greatest concentrations in Canada are in southwestern Quebec, southern Ontario, southern Manitoba, and in British Columbia's lower Fraser Valley . The Plants Database provides a map to purple loosestrife's distribution in the United States.
Considered native to Eurasia , purple loosestrife has a widespread circumpolar distribution throughout the northern hemisphere, except in extremely cold and arctic regions [111,129]. Although the precise origin of purple loosestrife colonization in North America is unknown, it was well established by the 1830s within coastal wetlands along the New England seaboard, having likely been introduced via ship ballast soil. Further introductions are thought to have occurred intentionally by early American horticulturalists. Initial spread of purple loosestrife into the interior of eastern North America occurred primarily via routes of maritime commerce, such as canals, rivers and the Great Lakes. Spread into the arid West appears to be closely related to development of irrigation systems within that region .
The following biogeographic classification systems are presented as a guide to demonstrate where purple loosestrife could potentially be found based on reported occurrence and on biological tolerance to factors likely to limit its distribution. For instance, because purple loosestrife does not tolerate salt water, classifications describing a variety of salt marsh habitats are excluded from these lists. Additionally, many of these classifications are named for predominantly upland habitats that nevertheless contain sometimes-substantial wetland areas where purple loosestrife could potentially occur. Precise distribution information is lacking because of gaps in the understanding of biological and ecological characteristics of non-native species and because introduced species may still be expanding their habitable range. Therefore these lists are speculative and may not be complete.
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- 125. Stuckey, Ronald L. 1980. Distributional history of Lythrum salicaria (purple loosestrife) in North America. Bartonia. 47: 3-20. 
- 129. Thompson, Daniel Q.; Stuckey, Ronald L.; Thompson, Edith B. 1987. Spread, impact, and control of purple loosestrife (Lythrum salicaria) in North American wetlands. Fish and Wildlife Research 2. Washington, DC: U.S. Department of the Interior, Fish and Wildlife Service. 55 p. 
- 132. U.S. Department of Agriculture, Natural Resources Conservation Service. 2008. PLANTS Database, [Online]. Available: http://plants.usda.gov/. 
- 46. Haber, Erich. 2001. Invasive plant data summary and control options: Purple loosestrife. In: Invasive plants of Canada: Guide to species and methods of control, [Online]. Available: http://www.magi.com/%%7Eehaber/lyth_sal.html [2002, January 25]. 
- 79. Mal, Tarun K.; Lovett-Doust, Jon; Lovett-Doust, Lesley; Mulligan, G. A. 1992. The biology of Canadian weeds. 100. Lythrum salicaria. Canadian Journal of Plant Science. 72(4): 1305-1330. 
- 81. Malecki, Richard. 1995. Purple loosestrife. In: Non-native species. In: Laroe, Edward T.; Farris, Gaye S.; Puckett, Catherine E.; [and others], eds. Our living resources: a report to the nation on the distribution, abundance, and health of U.S. plants, animals, and ecosystems, [Online]. Washington, DC: U.S. Department of the Interior, National Biological Service (Producer). Available: http://biology.usgs.gov/s+t/noframe/x193.htm [2002, February 6]. 
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