Griffiths, Annual Report of the Missouri Botanical Garden 21: 165, 1910
Opuntia alta is a large arborescent species; some ancient plants grow to 10- to 15-foot tall. It forms a tree that typically has one main trunk that can be 15-inches across in mature specimens. The cladodes are generally of uniform shape and on a plant and may be subcircular or ovate to obovate. Spines are yellow and 1/2- to 3/4-inches long. It is found in Texas along the Rio Grand River and in parts of the adjacent South Texas Plains. It grows with O. gomei and O. lindheimeri. O. alta has lemon-yellow flowers that are smaller than those of O. lindheimeri. Sometimes the flowers are greenish-yellow or pale. Britton and Rose reported O. alta is one of the tallest, if not the tallest, Opuntia species in the United States. See the original citation. O. alta is diploid.
Very large plants of O. lindheimeri could conceivably be misidentified as O. alta. However, O. alta forms a distinct trunk and mature plants are much taller than O. lindheimeri. Moreover, O. alta may have yellow-green or pale-green stigmas, whereas O. lindheimeri flowers have decidedly green stigmas. Moreover, the fruits of O. alta are smaller than those of O. lindheimeri and are often spherical (sometimes egg shaped). Unlike O. lindheimeri, O. alta fruits have prominent areoles. O. alta does not naturally occur in central, eastern, or western TX and would not be confused with O. lindheimeri in those areas.
One O. alta plant grown from material collected near Laredo, TX survived for several years outdoors in a protected situation at the Rio Grande Botanic Garden in Albuquerque, NM. It suffered damage each winter–but it survived for a dozen years.