About the genus
The genus Ariocarpus was discovered in Mexico in 1838 and the initial species was retusus. The species are now generally considered to be retusus (and various local forms and subspecies including furfuraceus), fissuratus (including subsp. lloydii), trigonus, kotschoubeyanus, agavoides, scaphirostris (scapharostrus) and bravoanus (including subsp. hintonii). A further possible species, confusus, looks like retusus but often has sharp, upwards-pointing tubercle tips like trigonus. All but fissuratus are found only in Mexico, in the central northern plateau. Only fissuratus extends northwards into south-western USA (Texas).
All species have a large tap root, are slow-growing and have large, showy flowers. These are usually deep pink in fissuratus, kotschoubeyanus, scaphirostris, bravoanus, agavoides, confusus and some forms of retusus though paler pink and white flowered forms exist in some species. A. retusus usually flowers white, some times with pink tinges, and trigonus has yellow flowers. The flowering season is generally in September and October.
All species are normally spineless and, as they mature , the plants produce wool around the central tubercles, from which the flower buds poke through.
How to grow them
In habitat, Ariocarpus grow in calcareous soils, mainly in limestone and some in gypsum, mostly including sandy clay and with little organic material. Similar conditions can be replicated in cultivation with a very open and free-draining compost of sieved John Innes compost, plenty of sharp sand, (preferably) limestone chippings and perhaps some perlite, vermiculite or similar. Some magnesium and other trace elements can be added to the compost. There are some growers who successfully omit organic ingredients completely. The plants can then be watered like the rest of the collection, preferably from below so that water is drawn upwards by capillary action to avoid water lodging in the wool, the central tubercles and around the neck. Allow the plants to dry out thoroughly between waterings. Nutrients can be added occasionally to the water by a general balanced fertiliser including trace elements, and extra magnesium can be added (sparingly) by including some Epsom salts.
The watering season should be selected cautiously, waiting till the weather is warmer, perhaps in early April and ending in late September or early October. The plants prefer a very dry atmosphere, particularly around flowering time, so that the dying flowers can dry off thoroughly and not rot down into the plant body. Therefore extra ventilation and perhaps some heat are advisable until all flowers have dried off. Plants should be repotted only when the roots have filled the pot completely and the tap root is starting to bulge the sides, and should only be moved up one pot size to avoid areas where the compost has few roots and will take longer to dry out.
Propagation is by seed-raising, preferably in a heated propagator with the pots enclosed in polythene to ensure consistent moisture at the roots. In winter, the plants are usually fine down to freezing or even below if thoroughly dry and in a dry atmosphere.
Text and photos: David Quail
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