Description: Succulent herbs or shrubs, often much branched from the base, sometimes with a definite trunk, often spreading and forming clumps; ultimate segments mostly flat or compressed, rarely ± cylindrical; roots mostly fibrous. Areoles on segments bearing glochids, spines, hairs and flowers.
Leaves usually small, ± terete, falling early.
Flowers solitary, hypanthium bearing areoles with small leaves, spines and glochids often present. Outer perianth segments usually green passing into brightly coloured petaloid inner segments.
Fruit dry or juicy, often edible, ± ovoid, spines present or absent; seeds covered by a hard, bony aril.
Distribution and occurrence: World: c. 150 species, North & South America. Australia: 15–20 species (naturalized), all mainland States.
Many species have become established and spread in pest proportions. All species except O. ficus-indica have been declared noxious weeds in N.S.W.
Text by G. J. Harden
Taxon concept:
Taxa not yet included in identification key
Opuntia schickendantzii
| | Key to the species | |
| 1 | Segments mostly 2–4 cm wide, 4–10 cm long, ultimate segments usually flattened, sometimes rounded under moist conditions, stems more or less terete | Opuntia aurantiaca |
| Segments mostly more than 5 cm wide, and usually more than 10 cm long; all segments flattened | 2 |
| 2 | Segments finely hairy between areoles; spines mostly absent; glochids often very obvious, mostly yellow to brown, usually more than 2 mm long | 3 |
| Segments glabrous between areoles; spines mostly present; glochids present, not yellow-brown, mostly less than 2 mm long
Back to 1 | 4 |
| 3 | Shrubs mostly less than 60 cm high, usually low and creeping; segments 10–15 cm long, pubescent, areoles small, usually 5–10 mm apart; glochids 5–10 mm long, golden-brown; spines absent | Opuntia microdasys |
| Shrubs more than 3 m high, erect; segments 15–30 cm long, tomentose to velvety; areoles larger, at least 20 mm apart; glochids yellow, 1–2.5 mm long; spines sometimes present on younger segments
Back to 2 | Opuntia tomentosa |
| 4 | Low spreading plants, mostly less than 50 cm high; many-segmented, segments relatively small, mostly less than 12 cm long; spines mostly present on some areoles, usually whitish or brown, 5–50 mm long, or spines absent | 5 |
| Erect or clumped shrubs, mostly more than 50 cm high, plants often tree-like; few-segmented and spreading in young stages, segments mostly more than 12 cm long; spines numerous, few or absent from the areoles
Back to 2 | 6 |
| 5 | Areoles far apart, usually 4 in a diagonal line across the segment; areoles without woolly hairs, glochids scattered around edge of areoles, spines absent from many areoles or 1 erect spine, more than 30 mm long, present at some areoles | Opuntia humifusa |
| Areoles relatively close, 5 or 6 in a diagonal line across the segment; areoles filled with brown woolly hairs, glochids restricted to upper end of areoles; spines often absent from lower half of segments, 1–3 spines less than 20 mm long mostly present on upper areoles
Back to 4 | Opuntia sp. sensu I.Telford (1984) |
| 6 | Segments covered with spines, most areoles with at least 2 large spines, rarely only 1 | 7 |
| Segments with relatively few spines, most areoles without spines or irregularly with only 1 or 2 spines at some areoles
Back to 4 | 11 |
| 7 | Spines numerous, usually at least 3 at most areoles | 8 |
| Spines usually 1 or 2 at most areoles on ultimate segments, rarely more
Back to 6 | 10 |
| 8 | Areoles relatively close, 8–10 in a diagonal line across the segment; spines 3–20, the longest 2 or 3 more or less flattened, 1 or 2 reflexed bristles at the lower end of areoles | Opuntia streptacantha |
| Areoles widely spaced, mostly 4–6 in a diagonal line across the segments; spines 2–12, all terete, reflexed bristles absent
Back to 7 | 9 |
| 9 | Segments obovate, bright green, not glaucous; spines dark brown with age, 2–8, 2–4 cm long, sometimes to 7 cm long | Opuntia elatior |
| Segments more or less circular, bluish green, glaucous; spines brown or yellowish at base, white above, mostly 8–12, to 5 cm long, spines sometimes absent
Back to 8 | Opuntia robusta |
| 10 | Tree-like plant; segments bright, shiny green; spines yellowish to dark red-brown | Opuntia monacantha |
| Spreading bushy plant, often forming large clumps; segments dull green or bluish green; spines yellow
Back to 7 | Opuntia stricta |
| 11 | Segments glossy green, more or less spathulate; flowers more or less orange | Opuntia elata |
| Segments bluish to dull bluish green, rarely dull green, oblong to circular, not distinctly spathulate; flowers yellow
Back to 6 | 12 |
| 12 | Segments usually less than 30 cm long | 13 |
| Segments usually more than 30 cm long
Back to 11 | Opuntia ficus-indica |
| 13 | Bushy, clumped plant, distinct trunks absent, clumps usually less than 1.5 m high; segments mostly less than 20 cm long, mostly dull bluish green to green but not glaucous | Opuntia stricta |
| Tree-like plant with distinct trunk, often 2–3 m high; segments usually 20–25 cm long, bluish green, glaucous
Back to 12 | Opuntia robusta |
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