This is a widespread weed in temperate North America that was present in Ky. early after settlement (Short et al. 1833). Rafinesque (1936, 1:31) expressed uncertainty about its naturalized versus native status in North America. The species is generally tetraploid (2n = 32), but other chromosome numbers have been reported (Matsk et al. 2003): 2n = 16, 40, 48.