Griffiths, Bulletin of the Torrey Botanical Club 43(2): 89, 1916
Opuntia semispinosa is a coastal prickly pear cactus from southern California and Mexico that has been variously lumped into O. occidentalis, O. littoralis, or even other opuntias of coastal California. We accept it as a stand-alone species. At one time there was much interest in the plant as a possible source of spineless Opuntia germplasm because some pads are spineless over part or all of the of the pad (hence “semi” spinosa), but this hope was not realized. Plants my reach 1 or 1.4 m tall and 1 or 2 m wide. Originally the plants grew in great thickets, but land use eroded this habit and this opuntia now often grows singly or as a few individuals. This prickly pear can present as an untidy collection of branches and irregular cladodes because the branches are angular. Some plants are compact whereas others spread. Segments of this Opuntia may be oval or obovate and or even pointed at both ends (almost rhomboid). The prickly pear’s spines are white, but larger centrals may darken to brown at the base. Larger spines are often angular and twisted. Interestingly, areoles can sometimes be wider than long, 5 mm wide and 6-8 mm wide. Flowers are yellow but may have a tinge of orange. Filaments are yellowish or pale green and the style is red and large, up to 8 to 10 mm in diameter. The stigma is pale green. The outer sepals are often recurved in bud. The fruit is 35 x 50 mm and obovate with a flat umbilicus. The fruits of this prickly pear are spiny. O. semispinosa was initially collected at Santa Barbara, CA, San Pedro, CA, and near Tija Juana, Mexico. See the original citation.
O. semispinosa is not a prickly pear generally found in gardens due to its size.