Engelmann & J. M. Bigelow, Proceedings of the American Academy of Arts and Sciences 3: 291, 1856
Opuntia occidentalis occurs in southern CA. Britton and Rose reported that the type locality is the “western slopes of the California Mountains,” between San Diego and Los Angeles. The species also occurs on the islands off the California coast. It is a large plant (up to 4-ft tall) with similarities to O. engelmannii. Cladodes are typically blue-green, 8(10)-inches wide and up to 12-inches long, but they may be larger or smaller. Dark red-brown spines are typical, but they may be pale at the tips. Fruit is reddish purple, occasionally green, and sweet. Seeds are larger than in the similar O. engelmannii. The species forms large single-species thickets. See the original description. Opuntia occidentalis is hexaploid.