Griffiths, Annual Report of the Missouri Botanical Garden 21: 165, 1910
What is Opuntia alta?
Opuntia alta is a large arborescent prickly pear cactus. Some ancient plants grow 3 to 4 m tall. It forms a prickly pear tree that typically has one main trunk that can be as much as 0.5 m across in large specimens.
The cladodes are generally of uniform shape on this prickly pear and may be subcircular or ovate to obovate but seldom elongate. Spines are yellow and 1 to 1.5 cm long. This Opuntia is found in South Texas along the Rio Grand River and in parts of the adjacent South Texas Plains. It grows with other prickly pears including O. gomei and O. lindheimeri. This prickly pear 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.
O. alta might be unsuitable as a plant for most gardens due to its large size. However, small specimens may be grown for a years with appropriate pruning. The smallish flowers and fruits are plentiful.
Dave Ferguson says:
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. Also, O. alta may have yellow-green or pale-green stigmas, whereas O. lindheimeri flowers have decidedly green stigmas. Moreover, the fruits of this large prickly pear are smaller than those of O. lindheimeri and are often spherical or egg shaped. Unlike O. lindheimeri, the larger fruits of the larger Opuntia 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. Thus, with some protection, this Opuntia can survive substantial cold.