Incidence of Canada Thistle, Cirsium arvense (Asteraceae), Along Interstate Highways in Tennessee

Canada thistle, Cirsium arvense, is a noxious weed in Tennessee, where it poses a significant threat to some of the state’s agricultural and natural areas. Tennessee represents part of the southern edge of the distribution of Canada thistle in North America. Because highways serve as possible corridors for the spread of exotic and invasive species, the interstate highway system was monitored throughout Tennessee in 1997 and 1998 for Canada thistle. This exotic plant species was found at nine of 229 sampling sites in three of 47 Tennessee counties. Six sites were located near the state’s northern border with Kentucky in Campbell County; two sites were located in Washington County in upper eastern Tennessee; and one site was located in middle Tennessee in the Western Highland Rim region in Hickman County, the westernmost occurrence of Canada thistle in the state. The occurrences of this plant species in Campbell, Hickman, and Washington Counties represent new documented county records. Each of these Canada thistle sites occurred within the <190 day frost-free zone in Tennessee and the USDA Plant Hardiness Zone 6. Each Tennessee county with a previous report of Canada thistle is at least partially included in the 190 day frost-free zone or Zone 6. Since its discovery in Tennessee in 1948, Canada thistle has only spread to a new county in the state approximately every five to seven years, which may be a result of climatic factors.