Astrophytum asterias aka Sand Dollar Cactus

Astrophytum asterias is a small cactus. It belongs to the Cactaceae family. People call it sea urchin cactus or silver dollar cactus. It is native to southern Texas and northeastern Mexico. Kew lists Nuevo León and Tamaulipas within this range. It grows in dry shrubland, grassland, and dry riverbeds. Wild plants often grow beneath shrubs or beside rocks. Sand may hide much of the stem. This helps it blend into the soil. Farming, collecting, and habitat loss have cut wild plant numbers.

Its single stem forms a flat disc or low dome. Mature plants stand about 0.07–0.23 feet [0.02–0.07 metres] tall. Plants in gardens often span 0.26–0.33 feet [0.08–0.10 metres]. Wider plants can reach about 0.66 feet [0.20 metres] across. The stem is grey-green or dull green. It usually has eight broad ribs, split by narrow grooves. Small tufts of white hair dot its surface. Round, woolly areoles form rows along each rib’s centre. These are the pads from which spines can grow. Adult plants lack spines. Seedlings may have tiny spines for a short time.

Yellow flowers open at the stem tip in daylight. Their centres range from orange to red. Flowers span about 0.16–0.23 feet [0.05–0.07 metres]. Some reach 0.30 feet [0.09 metres]. Each lasts just one or two days. Wild plants bloom in spring. With enough water, grown plants may bloom through warmer months. Oval or round fruits follow, with wool and small spines. They may look green, pink, or grey-red. The fruits dry out and break off near their bases. Their shiny seeds range from dark brown to black.

This cactus grows slowly and suits a greenhouse. It can grow on its own roots. Use a mineral mix that drains well. It should have little organic matter. Rich compost can make the stem stretch. Give little water while it grows. Let the soil dry before you water again. Keep it dry through its cool winter rest. LLIFLE gives a winter resting range of 5–15°C. Dry plants may withstand brief cold near −5°C. They can bear this cold only if they stay dry.

Its small form and lack of spines give it ornamental value. Good drainage and care with water matter more than frequent feeding. Wet soil in winter can cause rot. Dampness also harms its fine roots. Older plants may grow weak roots and become prone to disease. Repot about every two years to renew the growing mix. You can grow it from seed, but young plants need different care. Seedlings dislike harsh light and dry conditions.