The senita (pachycereus schottii
) is by far the rarest of Arizona's big three cacti (the others being saguaro
and organ pipe
), since it cannot tolerate even the slightest frost. Plants are found only in a small narrow band along the southern edge of Organ Pipe Cactus National Monument
; the best place to see them is at Senita Basin, on the sheltered, south-facing slopes of the Puerto Blanco Mountains. Like the organ pipe, the senita is much more widespread in Mexico, where frosts never occur. Even in the national monument, many specimens show damage as a result of cold weather spells.
Pachycereus schottii forms extensive clusters, of a hundred or more stems, up to 20 feet tall, branching from the base - like the organ pipe, but easily distinguished as they have fewer ribs (five or six), shorter, lighter and more widely separated spines, and because the top portion of each stem is covered by dark, hairy tufts, after which the plant is named; senita ('old' in Spanish) comes from the resemblance to an old man's beard. The short spines allow the bright yellowish green stem to be prominently visible. Flowers are borne along the stems, unlike the saguaro and organ pipe which bloom from the tips.
A spineless variant known as the totem pole cactus, pachycereus schottii var monstrose, is even less frost-tolerant and is native to Baja California, but may often be seen in desert cities of the Southwest, where it is used as a landscaping plant.