Highly sought-after slow-growing cactus from Mexico with unusual spineless, flattened tubercles. White 5cm flowers in summer. Indoors only in warm bright position in well drained potting mix. Ariocarpus retusus is one of the largest and fastest-growing species of the genus known for its slow growth. The stem is grey or blue-green, flattened, globose, rounded on top, up to 5 inches (12.5 cm) tall and up to 10 inches (25 cm) in diameter. Despite its slow growth, often taking 10 years to reach flowering age, it is a desirable cactus for cultivation, having attractive flowers and an interesting form. The flowers are white to pink, occasionally with reddish midribs and up to 2 inches (5 cm) in diameter. USDA hardiness zones 9b to 11b: from 25 °F (−3.9 °C) to 50 °F (+10 °C).