General
Scientific classification of prickly pear cactus places them in the family Cactaceae, and genus Opuntia. U.S. Department of Agriculture resources show 59 species of Opuntia, with some of them having a few different varieties. Characteristics that separate them from other cactuses include the flat, pad-like nopales, and clusters of tiny, barbed spines known as glochids.
Wild Species
Prickly pear cactuses grow extensively in the wild, and can tolerate a range of growing conditions. Examples include the less commonly found Opuntia Santa Rita, which can have blue/gray nopales with a tinge of purple. Opuntia polyacantha grows in many U.S. states, and has the common name plains prickly pear. This species has five different varieties, with these typically having green nopales, red fruit and yellow flowers. Other species include Opuntia basilaris, commonly known as beavertail prickly pear, and Opuntia ammophila, which also bears the more descriptive name devil's tongue.
Cultivated Species
Nopales used for food typically come from cultivated prickly pear cactus, with farms commonly growing Opuntia ficus-indica for this purpose. This plant also goes by the name Barbary fig, and typically has large, green nopales, red or yellow flowers and red/purple fruit. Cultivation of other species focuses more on the beauty of the nopales and flowers for ornamental purposes. This includes Opuntia engelmannii, with this species commonly known as the cactus apple, and having six named varieties.
Spineless Species
Some prickly pear plants can grow with spineless nopales. Examples of this include Opuntia ellisiana, which has the common name tiger tongue. This plant is native to Texas, and the large, green nopales grow without spines. They can have a few of the smaller glochids, although these tend to be poorly developed.