Weedy Thistles of the Western United States

Abstract

The majority of thistles found in the western United States are native species that pose little or no threat as weeds. However, thistle species introduced from Europe and Eurasia during the past two centuries have proven to be very aggressive and invasive, spreading rapidly throughout much of the West, crowding out and replacing desirable plant species in cropland, pastures, rangeland, and forests. Nineteen thistle species have been declared noxious according to state weed control laws or seed purity laws. Though Canada, bull, musk, and Scotch thistles are most common, all noxious species are known to occur in the West and will become widespread if not controlled. More intensive field monitoring, mapping, and early control programs are required to stop the spread of thistles on public and private lands. Programs to help more individuals recognize noxious thistles will be key to the success of all thistle control efforts.