Engelmann, Memoirs of the American Academy of Arts and Science n.s. 4(1): 51, 1849
Opuntia phaeacantha is a common and wide-ranging plant (CO, OK, NM, UT, AZ, NV, CA, trans-Pecos TX, and Mexico) that forms a low, irregular, sprawling clump, sometimes with only one or two branches. In other areas with more plentiful rain or milder winters, O. phaeacantha has many branches and forms a mat 4(6)-ft across. Opuntia phaeacantha is not particularly stiff (woody), and branches generally run along the ground. O. phaeacantha does not put up vertical stems. Flowers may be yellow with traces of red at the bases of the central tepals. However, flowers are known that are entirely yellow, orange, pink, or magenta. See the original description. Opuntia phaeacantha is hexaploid.
Dave Ferguson says:
Oddly, the original description of O. phaeacantha is muddled and describes two different species. Two different entities were combined in the original description of O. phaeacantha. The first, a smaller plant, is the one that became O. phaeacantha. The second, larger one, was named var. major by Engelmann and it grew in another location; this taxon is the same basic thing as O. dulcis.
In addition to the many specific epithets, many subspecific varieties of O. phaeacantha have been described including var brunnea (= O. camanchica), var major (= O. dulcis), var nigricans (= typical O. phaeacantha), var phaeacantha, and var superbospina. Many of the published varietal names are technically valid, they just don’t apply to O. phaeacantha, or they are absolute synonyms (e.g., nigricans is based on the type specimen of O. phaeacantha. Many garden variants of this wide-ranging species have also been described. See Green and Ferguson (2012) for more information about O. phaeacantha and names). Therefore, distinctive populations have not been properly named. Proper identification and naming of multiple taxa may happen in the future, and perhaps new species will be separated out.
O. phaeacantha is a low, wide-ranging plant (OK to CA, and into Mexico) that forms an irregular clump. Plants may be small and sparsely branched in very cold or dry areas, but they may be full and several feet across in more clement situations. O. phaeacantha is not particularly stiff and cladodes will sag or dimple in drought or cold but generally don’t wrinkle. Flowers are usually yellow, but they may have red centers, or may be entirely orange, pink or magenta. Generally, it is easy to distinguish between O. phaeacantha and other small opuntias because it typically has only 2 strong spines per areole and because the spines are not hat-pin like. Spines may be present only in distal 1/2 to 1/4 part of pad and may number zero to 8. Spines are typically brown or tan, sometimes tan with brown bases. Tip spines may be quite dark. Occasional plants may have very dark spines, white spines, or even yellow spines. Pads may be oblong (ovate or obovate) and less often circular. Another feature that can help distinguish O. phaeacantha from other small opuntias is that it does not put up branches (branches growing upright).