Opuntia aurantiaca Lindl.

Low, divaricately branched, spreading to suberect shrub 0.3–0.6 m high (up to 2 m high when supported by other vegetation). Segments covered by soil may lose thorns and resemble an underground tuber, with fibrous roots. Lower (older) stem segments subcylindric. Stem segments (terminal) easily detached, subcylindric to cylindric or somewhat linear to oblong and somewhat laterally compresed, linear to oblong, 3.5–30 cm long, 1.5–4 cm wide, 1–1.5 cm thick, not tuberculate, bright green, sometimes with reddish purplish tinges, glossy, glabrous. Areoles mostly circular or subcircular to elliptic, 3.5–5 mm long, 2–3 mm wide, filled with white to pale brown wool and glochids, 5–25 mm apart. Leaves succulent, terete, conical, c. 3–4 mm long, caducous. Spines spreading, 1–7 per areole on most areoles, the longer spines 10–40 mm long, 0.6–0.8 mm wide near base, rigid, minutely barbed near apex, red to red-brown, ageing white to brownish, with 1–3, shorter, thin, weak spines. Glochids 0.7–1.5 mm long, pale brown. Flowers 25–60 mm diam.; outer tepals reduced, succulent, green or tinged reddish green; inner tepals mid to deep yellow, spreading, obovate, 10–30 mm long, the apex obtuse with or without a short mucro, or emarginate; staminal filaments pale yellow or pale creamish, anthers pale yellow; style pale yellow or pale creamish, stigma lobes pale yellow; pericarpel (at anthesis) smooth, spiny. Fruit solitary or sometimes proliferating, obovoid to somewhat pyriform, 2.5–3.5 cm long, 1.5–1.8 cm diam., not tuberculate, spiny, green to reddish purple, fleshy but not juicy, indehiscent. Seeds not seen.

Opuntia aurantiaca is an aggressive environmental and economic weed in Australia. It is a weed in pastures, chenopod shrubland, grassland, dry open forest and woodland, on a variety of soil types; common along stream banks where stem-segments are dispersed by floods. It can form extensive, impenetrable stands to the exclusion of native herbs and shrubs, impede the movement of native fauna and stock, and provide habitat for pest species such as rabbits (Sheehan & Potter 2017).