Broom snakeweed (Gutierrezia sarothrae) and threadleaf snakeweed (G. microcephala) are aggressive weeds common on western rangelands from Canada to Mexico. The two species, which are similar in appearance, are distinguished primarily by the number of ray flowers per flower head. Broom snakeweed usually has three or more ray flowers per head whereas threadleaf snakeweed has only one or two. In the southwest, they are often collectively referred to as perennial snakeweeds.