This diploid (2n = 20) has a wide northern and western range, and occurs in varied open damp habitats, sometimes becoming weedy. R. mexicanus is a closely related tetraploid (2n = 40) with a southwestern range. There may also be some confusion or intergradation with altissimus. Further verification is desirable in Ky. (FNA 5). R. triangulivalvis differs from altissimus in its narrower leaves (ca. 1-4 cm versus 3-5.5 cm), widest near middle (versus below middle); its smaller inner tepals (mostly 2.5-3.5 x 2.5-3 mm versus 4.5-6 x 3-4.5 mm); and its smaller achenes (ca. 1.7-2.2 mm long versus 2.5-3.5 mm).