The thistle (Carduaceae tribe, family Compositae or Asteraceae) is a spiny herbaceous plant with white or purple flowers. There are 800 species worldwide, 46 in Canada (15 native). Native species (none serious weeds) are mainly "true" or "plumed" thistles of genus Cirsium. The remainder are nonspiny arctic or alpine herbs of genus Saussurea. Naturalized species are mainly common weeds. Common burdock (Arctium minus) has burs which attach themselves to clothing or skin. A chemical irritant in the bur can cause a rash similar to that caused by poison ivy. Canada thistle (Cirsium arvense) is not native but is found in agricultural areas of all provinces. It is variable in appearance and spreads in pastures and grasslands by root fragments. Bull thistle (C. vulgare), a common weed of Québec, Ontario and southern BC, is biennial and spreads by seeds only. Several species of knapweed are regarded as pernicious weeds in the BC Interior. Diffuse knapweed (Centaurea diffusa) and spotted knapweed (C. maculosa) are subjects of major efforts in biological weed control.