Natural communities that are threatened by Canada thistle include non-forested plant communities such as prairies, barrens, savannas, glades, sand dunes, fields and meadows that have been impacted by disturbance. As it establishes itself in an area, Canada thistle crowds out and replaces native plants, changes the structure and species composition of natural plant communities and reduces plant and animal diversity. This highly invasive thistle prevents the coexistence of other plant species through shading, competition for soil resources and possibly through the release of chemical toxins poisonous to other plants. Canada thistle is declared a "noxious weed" throughout the U.S. and has long been recognized as a major agricultural pest, costing tens of millions of dollars in direct crop losses annually and additional millions costs for control. Only recently have the harmful impacts of Canada thistle to native species and natural ecosystems received notable attention. <www.invasive.org>
In 100 Years of Change in the Distribution of Common Indiana Weeds by William and Edith Overlease (2002) reported that Canada thistle noted in Indiana in 1899 (Coulter’s Catalogue of Indiana Plants) “to be found in many parts of the state; more abundant north”. In 1940 (Deam’s Flora) Canada thistle was noted to be “infrequent to frequent in the lake area and is more or less local south of this area”. Canada thistle was reported in Carroll, Cass, Elkhart, Jasper, Kosciusko, LaGrange, Lake, La Porte, Marion, Marshall, Miami, Monroe, Morgan, Montgomery, Newton, Noble, Porter, Putnam, St. Joseph, Starke, Steuben, Wayne and Wells.
In 2002, Overlease reported Canada thistle in the following 89 counties in Indiana: Adams, Allen, Bartholomew, Benton, Blackford, Boone, Brown, Carroll, Cass, Clark, Clay, Clinton, Crawford, Daviess, Dearborn, Decatur, DeKalb, Delaware, Dubois, Elkhart, Fayette, Floyd, Fountain, Franklin, Fulton, Gibson, Grant, Greene, Hamilton, Hancock, Hendricks, Henry, Howard, Huntington, Jackson, Jasper, Jay, Jefferson, Jennings, Johnson, Kosciusko, LaGrange, Lake, La Porte, Lawrence, Madison, Marion, Marshall, Martin, Miami, Monroe, Montgomery, Morgan, Newton, Noble, Ohio, Orange, Owen, Parke, Pike, Porter, Posey, Pulaski, Putnam, Randolph, Ripley, Rush, St. Joseph, Scott, Shelby, Spencer, Starke, Steuben, Sullivan, Switzerland, Tippecanoe, Tipton, Union, Vanderburgh, Vermillion, Vigo, Wabash, Warren, Warrick, Washington, Wayne, Wells, White, and Whitely.