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Spotted knapweed is native to eastern Europe, though it now occurs in western and central Europe. It was introduced to North America, probably as a contaminant in alfalfa (Medicago sativa) seed and/or ship's ballast, in the late 1800s [113,135,159]. In 1920, the distribution of spotted knapweed in North America was limited to the San Juan Islands, Washington. By 1980 it had spread to 48 counties in the Pacific Northwest. Between 1980 and 1998, the known range of spotted knapweed included 326 counties in the western United States, including every county in Washington, Idaho, Montana, and Wyoming [173]. Although it is reported to occur in 45 of the 50 states [82,209], spotted knapweed is found primarily in the northwestern states and southwestern Canada. The Plants database provides a map of spotted knapweed's distribution in the United States. The following table reflects estimates of spotted knapweed acreage as reported by state or province in 1988 and again in 2000 (from [33]):

State/Province 1988 2000
Arizona not reported 1,800
California not reported 5
Colorado 2,500 2,500
Idaho 2,293,000 2,300,000
Montana 4,721,069 3,818,450
Nevada not reported 5000
New Mexico not reported 500
North Dakota 0 1,160
Oregon 3,000 784,000
South Dakota 2,500 1,898
Utah 500 2,000
Washington 29,070 500,000
Wyoming 100 15,000
Alberta 0 scattered
British Columbia not reported 50,000
Total   7,482,313

The decrease in acreage reported in Montana is attributed to improved inventory methods during the past decade. Although inventories are more common and more accurate, 50% of these states reported only 50% accuracy, while 31% reported 51 to 75% accuracy, and 2 states reported 75 to 100% accuracy [33]. Watson and Renney [220] reported that spotted knapweed was abundant in British Columbia, common in Ontario, Quebec and the Maritimes, and observed in southern Alberta in 1974.

Information on the distribution of spotted knapweed is limited for most North American states and provinces in which it occurs. It is commonly listed as occurring on roadsides and other disturbed areas in the Adirondacks [93], New England [168], the Northeast [45], Michigan [215], Illinois [123], Nebraska [163], the Great Plains [48], the Blue Ridge region of North Carolina, Tennessee, and Virginia [230], West Virginia [195], the Carolinas [147], and Florida [5,232].

Specific information on the plant communities in which spotted knapweed occurs is also limited outside its primary area of occurrence. The following lists reflect ecosystems and cover types in which spotted knapweed is commonly found, although the lists are not exhaustive.


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