This plant is not on the Vermont invasive species list. That could be because it hasn't yet spread enough in the state. According to the USDA National Agriculture Library, Canada thistle was introduced in the United States from Eurasia in the 1600s. There is an interesting map that this site furnishes for each invasive species that shows the spread of the plant across the continent. You can select to view year by year or "alltime." It was interesting to watch the spread of Canada thistle across the country. Canada thistle looks very much like spotted knapweed (Centaurea stoebe). I look at the leaves and the base of the blossom to tell the difference.Spotted knapweed is an invasive in Vermont.