Astrophytum myriostigma: Bishop’s Cap, Star Cactus
The highland plateaux of central and northern Mexico are the home of this plant, which is one of the first that beginners should start with. Like most cacti, astrophytums have succulent stems without foliage, the function of the leaves (photosynthesis) having been taken over by the outer surface of the thickened stem.
The thick outer skin is coloured greenish grey to whitish grey and thickly covered with white-felted spots, hence the name myriostigma, meaning with thousands of spots. There are usually 5 pronounced ribs (rarely 3 or 4) with sharp edges on which are spaced round or elliptic areoles with brown wool, which later disappears. The, which appear on the crown, are pale yellow and measure about 5 cm (2 in) in diameter; they are produced at irregular intervals throughout the growth period.
The body of the young plant is almost globose, becoming columnar in age. The variety columnaris has exceptionally tall forms. In cultivation one may also occasionally come across the variety glabrum, with sharper edges and pale green skin.
The species has no spines. In Astrophytum capricorne, however, lovely twisted brown spines, up to 7 cm (2-¾ in) long, grow from the areoles in varying numbers. In the young plants they are flexible, but in older specimens they readily fall.
What is the best compost for cacti? In their native habitat cacti often grow in heavy, compacted ground but the substrate is usually very porous. In cultivation, therefore, they should be provided with a well-drained compost, such as John Innes potting compost No. 1 with additional fine gravel thoroughly mixed in.
A thick layer of stones should be spread over the bottom of the pot or container before filling it with the compost. Also becoming widespread nowadays is hydroponic cultivation in sterile granular matter (for example crushed brick) with water, to which nutrients are added from time to time. Cacti grown in this way do very well. In general thefor cacti should be neutral or only slightly acid (pH 6.0 to 7.0), the only exception being astrophytums, which require an alkaline substrate containing lime.