O. engelmannii is generally shrubby, with dense clumps up to 3.5 metres (11 ft) high, usually with no apparent trunk. The pads, or cladodes, are green (rarely blue-green), obovate to round, about 15–30 cm long and 12–20 cm wide. In the
Sonoran Desert, terminal pads face predominantly east-west, so as to maximize the absorption of solar radiation during summer rains. The cladodes make up the stem and are the primary photosynthetic structure.
O. engelmannii typically grows in high altitude desert, where lower atmospheric pressure increases water stress and may make gas exchange difficult , leading to a reliance on CAM photosynthesis for more efficient gas exchange and less water lost in the process with some taproots acting as water storage.
Opuntia species tend to be colonized by arbuscular mycorrhizal fungi. Mycorrhizae colonize
Opuntia plants more in the summer months than in the winter months and when there is plenty of water around. The flowers are yellow, occasionally reddish, and 6-9 cm in diameter and length. The flowers are hermaphroditic with inferior ovaries. Flowering is in April and May, with blooms lasting 1-2 days each. Single-day blooms open around 11AM and remain open for 6 hours. Blooms lasting 2 days tend to open around 10AM and remain open until 4:30PM on day one. On the second day they open from 8:30AM to 2:00PM. This results in an average total bloom time of 12 hours. Pollinators include
solitary bees and
sap beetles. Most pollinators of
O. engelmannii are polylectic (pollinate unrelated species), but several known pollinators have specialized for
Opuntia ),
Andrenidae, and
Megachilidae, primarily consisting of solitary bees. The seeds can remain viable in the soil for at least 19 months prior to germination. In its North American range,
O. engelmannii seeds are often distributed by coyotes. == Uses ==