Opuntiads of the USA

Dave Ferguson Writes:

O. polyacantha does sometimes produce hairy-spined plants similar to O. trichophora, but usually in any given location they look more like other plants of their respective varieties than they do like O. trichophora. The one most similar to O. trichophora is probably var. erinacea, which produces the occasional plant or population of plants of the "ursina" type. These are in the Mojave or Great Basin, and O. trichophora is not. O. poly. vars. hystricina and polyacantha can produce hairy plants too, but again, they usually look more like their respective varieties than like O. trichophora. A few things that seem to be true. O. polyacantha (regardless of variety) can produce plants with non-yellow flowers, while O. trichophora apparently cannot. Hairy plants of O. polyacantha (depending somewhat upon variety) usually have at least some stout rigid spines, and the spines on the fruit are usually stiffer and less likely to be hair-like. Often (again depending somewhat on variety) O. polyacantha plants have areoles a bit raised (on slight elevations of the pad), with the areoles larger and further apart.

Plants from the Rockies to the west end of the Oklahoma Panhandle on rock faces and steep slopes in what is the Raton Section of the Great Plains (mesa and canyon country), appear to be O. trichophora. There are also very "trichophora-

like" plants in some of the canyons along the Front Range in Colorado (especially in Larimer and Boulder Counties), but these appear to be local forms within O. polyacantha var. polyacantha (they need more study).

Even nearly spineless varieties of O. polyacantha (notably var. juniperiana) sometimes produce some long hair-like spines on older pads (mostly near the lower edges), but they are clearly not O. trichophora; they are distinguishable at a glance.

O. trichophora is found through the entire Colorado Plateaus (northern limit apparently in Flaming Gorge in Wyoming), on the Pacific side and the south half of the Atlantic side of the Southern Rockies, and south through New Mexico into west Texas and north Chihuahua. South of about I-40, give or take, it seems to be the only dry-fruited Opuntia present, but some populations are in question, as they look a lot like O. polyacantha var. hystricina or var. polyacantha. I haven't seen anything that I would call O. trichophora from the Great Basin nor Mojave regions, but it may sneak into the eastern edge of these areas.

In theory (needs tested further) O. trichophora is always diploid, while O. polyacantha is always (?) tetraploid (some counts are hexaploid, but may be plants of other species misidentified or confused as O. polyacantha).