Figure 9 - Changes in field frequency of Canada thistle in the Prairie provinces from the 1970s to the 2000s, from the Prairie Weed Surveys (Leeson et al. 2005).

The map illustrate the changes or increases in field frequency of Canada thistle in the Prairie provinces.

Each province is divided up into ecodistricts or in some cases groups of ecodistricts. Each ecodistrict is shaded one of six colors where brown represents more than a fifty percent change in field frequency of Canada thistle, maroon a twenty point one to fifty percent change, dark red a ten point one to twenty percent change, red a zero point one to ten percent change, light red represents absence from the area and white indicates that the species not surveyed.

Data for the map illustrating field frequency changes in the 1990s are as follows. Areas with more than fifty percent change include most of Manitoba, central Saskatchewan, and central and eastern Alberta. These areas largely correspond to the Boreal Shield and Boreal Plains ecozones. Ecodistricts in south-western Saskatchewan were not surveyed in this time period.