See notes under sulcatum, which is difficult to distinguish in several colls. T. erectum varies much in flower color and petal shape, but segregates other than sulcatum are not generally recognized in recent treatments (W). Several scattered colls. from Ky. have been referred to var. album (Michx.) Pursh, and one to var. flavum Eason (HARL for WKY). Occasional plants with reddish centers in flowers might be confused at first sight with T. undulatum (D. Dourson, pers. comm. and photos).