Griffiths, Proceedings of the Biological Society of Washington 27(6): 26, 1914
What is Opuntia riparia?
Opuntia riparia is a prickly pear cactus that grows in low, wide thickets in southern Arizona. It rarely reaches waist-high, but occasional cactus stems may reach to 1 to 1.2 m tall.
O. riparia plants form open, hemispherical shrubs generally to about 30 cm tall, occasionally some branches reach up higher. Many plants may occur together to form thickets. Pads are often obovate, elongate, or elliptical, averaging about 20 cm long. The dark areoles are noticeably large and white spines are present in most of them. There are 2 to 6(8) white spines per areole with one or two small ones that are deflexed. Spines may or may not darken at the base. Spines on this Opuntia are 1 to 2 cm long, stout, thick, and noticeably angular in cross section.
Flowers are a rich, bright yellow. The style is white and the dark green stigma is large (up to 1 cm across when expanded). Ovaries and fruits may be somewhat to distinctly elongate, with ovaries usually turbinate and fruits usually ovoid. Fruits may sometimes be narrowed or tapered at the base. The seeds are not large but are larger than those of O. engelmannii.
Ploidy is not known.
O. angustata and O. riparia are similar and may, in fact, represent the same taxon. O. riparia occurs farther south than O. angustata (near Superior, Arizona and south into Mexico).