(Photograph = Opuntia orbiculata, New Mexico)
Introduction
It is difficult to tell many Opuntia species apart. There are multiple aspects to consider, what is the shape of the pads, where is it growing, what is it growing with, what is the overall look (small bush, large bush, crawling along the ground), and the color, taste, and shape of fruit, etc.
The Details
One feature that can help identify species is spines. But, these can be variable within a species.
O. orbiculata is identifiable throughout its range, from southern OK, into south Texas, and west to Arizona, and maybe into California. Though it has an overall look that is constant, the spines can vary in number and color. They may be white, or they may be yellow turning to light brown or brown. Spines generally have no dark brown or black at their bases, though the areoles may be very dark.
Spines may be 4-sided and have edges (like a kitchen match), 3-sided, or even flat. There may be 1 to 4 spines per areole or there may be none. Spines may twist or bend, or not, and they may be 3- to 4-cm long or less than 1-cm. Plants without spines were named O. dillei, but this name is considered synonymous with O. oribiculata.
In all of these cases, the pads are round or sometimes shovel shaped or ellipticial. The plants are large relative to O. phaeacantha or O. gilvescens. The overall color of the plants has a bluish cast, but this can be absent, especially in extreme conditions.
Alternate Explanation: Perhaps the different spine forms represent two or more different varieties or even species. I don’t think this is the case, but there is much more to learn about Opuntia in the USA.
Related Reading: Thorns, Spines, and Prickles
