This cosmopolitan species may have originated in warmer American regions. In addition to being common in fields, it is locally abundant on exposed drying muddy banks of larger rivers and sloughs. Although sometimes considered adventive or even alien in southeastern states (e.g. SE), it was probably present and locally abundant at the time of settlement (Short et al. 1833; Gm). The large edible seeds of Xanthium plants were greedily consumed by the extinct Carolina Parakeet, as recorded in several early accounts from the Ohio Valley and elsewhere (e.g. Audubon 1840, Beckner 1928). There is, however, a diterpene glycoside (carboxyatractyloside) in foliage and fruits (especially spines) that is generally toxic to vertebrates; the parakeets appear to have evolved resistance, but passed on the toxin to predators (Gelabert et al. 2020). There has also been much medicinal use of Xanthium extracts, especially in China (Kamboj & Saluja 2010). Appropriate treatment of intraspecific variation remains uncertain (F, Cr). FNA 21 did not recognize any varieties, and listed only 2n = 36. Included here under var. canadense are plants formerly filed under X. pensylvanicum Wallr., X. italicum Moretti or other obscure names (including commune and saccharatum). Var. canadense is diagnosed here by its relatively large burs with lower parts of prickles spreading-hairy (Cr), but identification here remains tentative in several colls. Typical var. strumarium is supposedly native to tropical America and perhaps southern Europe. It occurs rarely as a waif in northeastern states (Cr); records from Ky. are dubious (M).