ABSTRACT Opuntia cespitosa (until recent taxonomic splitting, identified as O. humifusa) is an endangered cactus in Canada, is found in only two populations nationwide after some local extirpations. The larger site is at Point Pelee National Park (PPNP-ON) at the northern edge of its range has little published data. Data were collected for plant size, overlying cover (the species is sensitive to high shade), reproductive variables, and various cladode (pad) characteristics to assess response to local conditions. The PPNP-ON population was considered conspecific to many populations throughout the eastern USA until recently. A southern Humifusa clade population in Florida was sampled to confirm the recent separation of these populations into distinct species. A variety of parametric (e.g., ANOVA) and nonparametric (chi-square, Spearman’s correlation) tests were used to determine relationships across variables. This study contributes to our foundational knowledge of an endangered species. Results for this species in its last major Canadian population include that site conditions affect variables such as etiolation, in addition to shade. Plants are taller and have larger pads in Florida, supporting an upcoming taxonomic revision which would split the Floridian populations from the Ontario plants. Past work also suggests that Opuntia pads develop phototropically to maximize PAR (photosynthetically active radiation) receipt. Results show that while pads are statistically more parallel than perpendicular to the pad from which they grow, the offset is random. Thus physiological limitation of orientation is possible, or if there is a phototropic response, it occurs at the pad/areole scale.