Suppl. Pl. Succ., 82. 1819.
Shrubs, low, 10–25 cm, with ± prostrate branches. Stem segments not easily detached, green, elliptic to narrowly to broadly obovate to circular, 4–27 × 2–18 cm, low tuberculate; areoles 4–14 per diagonal row across midstem segment, subcircular, 3–6 mm; wool tan to brown. Spines at all or only distal areoles of stem segment, terete to flattened, stout to acicular to bristlelike, straight to curling, of 1 or 2 kinds; if 1 kind: 0–18 per areole, spreading and curling in various directions, sometimes straight, erect, ascending to deflexed, yellow to dark-brown to black, turning gray, pink-gray to gray-brown, longest (35–) 40–90 (–185) mm; if ± 2 kinds: major spines (0–) 1–5, reflexed to porrect, yellowbrown to brown to gray, longest 20–150 mm; minor spines (0–) 5–11, deflexed, white to white-gray, longest 4–16 mm. Glochids inconspicuous, in narrow, tidy crescent at adaxial edge of areole or in broad, brushy crescent and tuft, yellow to reddish, aging brown, to 10 mm. Flowers: inner tepals yellow to magenta throughout, 25–40 mm; filaments white, yellow, or red to magenta (flowers may superficially appear bicolored); anthers yellow; style white to pale-pink; stigma lobes green. Fruits tan to brown, ± cylindric, 15–45 × 12–25 mm, dry at maturity, glabrous, sometimes burlike; areoles 10–33, each or only distal areoles bearing 3–16 spines, 4–20 mm. Seeds tan to gray, flattened, warped, oblong to subcircular, 3–7 × 2–4 mm; girdle protruding 1–2 mm.
Alta., Sask., Ariz., Calif., Colo., Idaho, Kans., Mont., N.Dak., N.Mex., Nebr., Nev., Okla., S.Dak., Tex., Utah, Wyo., n Mexico
Varieties 5 (5 in the flora).
Populations of Opuntia polyacantha with spines few or absent (especially var. hystricina) were the basis for several names including O. juniperina, O. utahensis, and O. rhodantha.
|1||Areoles 4-6 per diagonal row across midstem segment; stem segments 4-7 × 2-3(-5) cm, elliptic to very narrowly obovate; Texas, New Mexico, Mexico||Opuntia polyacantha var. arenaria|
|1||Areoles 6-14 per diagonal row across midstem segment; stem segments 6-27 × 5-18 cm, obovate to circular or, if elliptic, longer than 9 cm; widespread in w United States and Canada||> 2|
|2||Spines of 2 kinds: major spines 1-3 per areole, usually deflexed on terminal stem segment, but sometimes ascending at stem segment apex; minor spines 1-3 per areole, deflexed, white, subtending major spines||> 3|
|2||Spines of 1 kind, grading in length, from ascending to deflexed, 7-18 per areole, longest spines of terminal stem segment porrect to ascending, shorter spines deflexed to porrect||> 4|
|3||Longest spines 2-4 cm; areoles 6-13 mm apart; stem segments 6-12 cm; Great Plains, n and e Great Basin Desert||Opuntia polyacantha var. polyacantha|
|3||Longest spines 4.5-12 cm; areoles 12-30 mm apart; stem segments 10-27 cm; Arizona and Utah border||Opuntia polyacantha var. nicholii|
|4||Spines usually dark brown, those of proximal stem segments like those of terminal stem segments, porrect to ascending, straight and stiff; fruit areoles 11-21, spiny but not burlike||Opuntia polyacantha var. hystricina|
|4||Spines usually pale, those of proximal stem segments not like those of terminals stem segments, more numerous, longer and more reflexed, straight or flexuous and/or hairlike or threadlike; fruit areoles 20-33, spiny, forming dense bur||Opuntia polyacantha var. erinacea|
"longest" is not a number.