This is said to have originally been a "riverside pioneer in eastern North America" (FNA 4), but it is now a cosmopolitan weed. There has been confusion between hybridus (2n = 32) versus retroflexus and powellii (2n = 34), which have proven to be distinct species. Hybrids have been reported but may be generally sterile. It appears that retroflexus became "very common" in Ky. during the 18th century (Gm). A. hybridus (as "paniculatus") was "frequent in good ground, but not as common" in 1914 (Gm); now it is much more abundant. A. hybridus has generally narrower inflorescence branches than retroflexus (and often more reflexed to nodding versus just erect to reflexed at tips), shorter bracts (mostly 2-3.5 mm versus 3.5-5 mm) and shorter tepals (mostly 1.5-3 mm versus 2.5-3.5 mm), but these are acuminate to aristate versus obtuse to emarginate (F, FNA 4). Stems of hybridus are generally glabrous or glabrescent except when young; those of retroflexus remain moderately to densely pubescent (FNA 4). Both species can have reddish hues in various plant parts but these are highly variable characters, even within populations. A. hybridus is reportedly more often reddish in inflorescences (especially bracts), but retroflexus may be more consistently reddish at stem bases. FNA 4 stated: ".. hybridus is extremely variable. In particular, there are numerous North American specimens with subobtuse tepals and thick inflorescences, suggesting hybridization with A. retroflexus. In Europe such presumably hybrid forms are known as A. ×ozanonii Thellung."