See entry for Opuntia polyacantha
We recognize 7 varieties of O. polyacantha; the varieties are real, but at least 1 has not been published properly (var rhodantha).
var polyacantha: Low spreading plants usually with rounded pads (shape varies somewhat); areoles small, close together; spines usually numerous with one stout terete central spine that is usually around an inch long (occasionally another one or two centrals that are likely to be flattened) with several smaller spines radiating around the central; spine color is highly varied, but the basis for several of the synonymous names; fruit tends to be roughly round with short stout spines; mostly in grasslands, but occasionally into scrub and woodlands in mountains; found on the Great Plains and in the Rockies, from Alberta, Saskatchewan and MN southward to central NM and the panhandles of TX and OK; eastward (formerly?) into IA and MO, to southern ID and eastern UT; similar plants in the Columbia Basin between the Blue Mountains and southern British Columbia, seem to represent this variety too, and may just be a westward continuation of populations in ID and MT.
var juniperina: Very much like var. polyacantha in most traits, but spines fewer and often only a few in upper areoles; lower areoles usually with only one or two tiny spines or no spines; fruit often nearly spineless; found in mountainous areas of the Colorado Plateaus and Southern Rocky Mountains, but rare in the Front Range region; found in southern WY, CO, eastern UT and northern AZ, and northern and central NM; similar groups of plants are scattered into ID and north into northern WY and in the Black Hills also in higher plateaus and mountain country of northern AZ; replaced to the west by var. utahensis (the division is the north-south-trending central high plateaus and mountains of Utah); usually grows in open conifer woodland, mountain scrub, or in areas of sagebrush.
var. schweriniana: Tends to be a miniature replica of vars. polyacantha, juniperina, or hystricina; grows at high elevations (above the others) in the Southern Rocky Mountains in eastern UT, CO, northern NM, southern WY, with pockets of similar plants further north in WY and in the Black Hills. Also a few patches at high elevations in the San Juan drainage in northeastern AZ.
var hystricina: Very much like var. erinacea, but joints usually not so thick, stem segments broader, lower, and more openly spreading in habit (habit as in most varieties); found in areas with at least some summer rainfall in cold winter desert and grassland from eastern NV and southwestern UT to southwestern CO, central NM, and across most of northern AZ.
var erinacea: Areoles usually large, relatively close together; stems with joints often very thick, often elongate; plants tend to form upright clumps; spines many, usually mostly slender and terete, and can be stiff or hair-like, even on the same plant (some eastern populations have mostly plants with very thick stiff relatively short spines), varied in color; fruit usually more elongate than in var. polyacantha, often reddish when ripe (but soon fading to brown or tan as they dry). Found mostly in rocky areas in Mojave Desert. Also on hot rocky south slopes in Great Basin Desert north almost to (or perhaps into) Idaho. Also in hotter areas, mostly in canyons, up the Colorado and San Juan Rivers into southern UT and almost to the Four Corners in AZ (but was mostly behind dam building, and wiped out by reservoirs in these areas). Grows in southeastern CA, NV, northern AZ, perhaps in northeastern Baja California del Norte (unsure). Plants at high elevations in the Mojave Desert and southern California sometimes spread more and sometimes are nearly spineless, tend to look vaguely like var. utahensis, and perhaps another variety could be recognized there.
var utahensis: Areoles usually smallish, far apart; stem segments most often thickish and elongate, sometimes a bit lumpy (usually not) and light often yellowish green in color; spines usually pale white to yellowish in color and slender. Fruit with areoles far apart, sometimes spineless. Found mostly in lower mountains in woodland or scrub, or sometimes in sagebrush areas. Grows in eastern CA (rare), NV, western UT, and perhaps into southern ID and southeastern OR.
var rhodantha: Areoles large, far apart; stem segments usually very thick and elongate, tend to be “lumpy”, most often dark or dull waxy green; spines usually stout,varied in color, with annulate markings. Fruit with areoles prominent and far apart, usually with stout short spines, but sometimes spineless. Found mostly in desert in Colorado Plateaus northwest half of Great Basin, and lapping into Southern Rockies, Snake River Plains and Wyoming Basin. Grows in UT, western CO, southwestern WY, southern ID, southeastern OR, NV, and UT.
All of the above blend into one another anywhere they meet or overlap distributions, and it is often very hard to “label” a plant to a proper variety. I do think however, that all seven of the above are constant enough over large areas, that they can be called varieties.
Other plants that have been proposed as varieties of O. polyacantha, but which we treat as species herein are: