Opuntia Monacantha is a fast-growing shrubby or tree-like cactus with a short trunk and glossy green oblong to obovate stem segments with irregular margins and tapered towards the base. It grows up to 20 feet (6 m) tall. The trunk is up to 8 inches (20 cm) in diameter. Stem segments are 12 inches (30 cm) long and 5 inches (12.5 cm) wide. The spines (1 or 2) are slender, unequal, brown and grey, and up to 1.6 inches (4 cm) long. Flowers are yellow to deep orange-brown, with reddish outer perianth-segments, up to 3 inches (4.5 cm) long and 4 inches (10 cm) across. The fruits are pear-shaped, reddish-purple, up to 3 inches (7.5 cm) long, and 2 inches (5 cm) in diameter.
The current mother plant flowers and fruits prolifically.
Source: World of Succulents