The genus Ariocarpus occupies a special place in the affections of many a cactophile; the aged, timeworn appearance of a mature plant of this genus evokes a special response from those who regard plants as natural phenomena rather than mere pretty objects in a greenhouse. Bill Weightman takes a closer look at this revered group of plants.

Most mature plants of Ariocarpus in collections are imports, survivors from the time when importing habitat collected plants was legal. This trade has now almost, if not entirely, ceased as far as the UK is concerned, and collectors are looking to seed raising as a practicable method of producing or obtaining plants.

The plants currently considered to comprise this genus have, in recent years, been gathered together from three separate genera, Ariocarpus, Roseocactus and Neogomesia, although the genus Roseocactus has never been accepted by many authorities. Those with an interest in the taxonomic history of these plants will find the story considerably more complicated, with other names appearing; one species even passed as a Mammillaria for a time. The full story is readily available in the literature and will not be repeated here.

More contentious is the distinction, in some instances, between species and varieties. This will be considered when we come to discuss the individual species.

The genus has given rise to rather more myths than most. These centre on the supposed difficulty of cultivation and are largely untrue. In times past, when imports were received, they were almost invariably rootless, and persuading them to produce new roots was a lengthy procedure. The usual method was to pot the plants in gravel or very gritty compost and leave them dry until they rooted. It was sometimes recommended that the dead corky material surrounding the taproot and base of the stem should first be removed. Rooting might take a year or even longer but, providing the  plant was sound, seldom failed. Once rooted and growing the plants are amongst the easiest of cacti to care for. The golden rule is to always remember that they are very slow growing and nothing should be done to try and speed up the process. During the warm summer months they should be watered freely, exactly as for other cacti, being allowed to dry out completely between waterings. The winter rest period should perhaps be extended either end by a couple of weeks compared with other genera, during which time the compost must be kept completely dry. Under these conditions they tolerate cold better than many cacti, certainly down to freezing. The growing medium must be open and well drained. Half normal potting compost and half coarse grit seems satisfactory. Some species in habitat grow in almost pure limestone, but the plants seem very tolerant and it is not necessary to provide these conditions in cultivation. This is demonstrated by some growers successfully employing soilless cornposts which often have a slightly acid reaction.

Ariocarpus have a very late flowering season. First flowers usually appear in late August or early September and the season may extend to early November, by which time the winter drying out process should be well under way. The flowers are attractive and their effect is enhanced by their appearance on such unpromising looking plants. The metaphor of a rose amongst thorns comes to mind, although this should not be laboured because an absence of spines is a characteristic of the genus.

As mentioned, it was widely considered until fairly recently that the only practicable way to obtain a flowering sized Ariocarpus was to purchase, at considerable expense, an imported specimen Doubtless there have always been purists and enthusiasts who have raised these plants from seed for the sheer fun of it, but since the ban on imports they will have been joined by others who see seed raising as the only way of obtaining a plant. The task is not as hopeless as it once seemed. The seed of several species is readily obtainable, and germination rates seem comparable with, or not much lower than, those of other genera. A search among the tubercles of old plants will often reveal hidden seeds resulting from earlier flowerings, and although old this is usually viable. As all cactus seed sowers know, the critical period is between germination and the establishment of a growing seedling. Since Ariocarpus are so slow growing this critical period is rather extended and it is during this time that most losses occur. Once growth commences there is no real problem, but great patience and a realistic appraisal, on the part of the grower, of his expected lifespan are recommended. Even so, a certain professional grower delights in telling the story of a near octogenarian enthusiast earnestly enquiring after seedlings. The minimum time to grow from seed to flowering size is probably ten years, and may well be twice that. More rapid development can be obtained by grafting, but Ariocarpus seem to suffer from grafting more than most other genera in that it promotes atypical growth and is not recommended.

The genus occurs in the Chihuahuan Desert in northern Mexico with one species also being found just across the border in southern Texas. In cultivation, and indeed in habitat, most plants occur as single headed specimens, but all will form multiheaded clumps. The time taken to form these clumps will depend on growing conditions but some must be very old indeed.

Ariocarpus agavoides
This species was discovered in 1941, when it was placed in the monotypic genus Neogomesia. From the beginning it was obvious that it was very closely related to Ariocarpus, and Anderson incorporated it within his revision of the genus. If one accepts as reasonable the amalgamation of Ariocarpus and Roseocactus, and most modern authorities do, then clearly this plant also belongs within the same genus.

It is a small plant, seldom exceeding 5cm. tall. The few, elongated tubercles are greyish-green, and rise from a relatively large underground root. Each tubercle bears a single wooly areole on its upper surface. The pink flowers appear in the centre of the plant, arising from the youngest areoles. As the plant ages the outermost tubercles wither and eventually drop off. The specific name is quite apt, for the plant does bear a superficial resemblance to a small Agave.

Ariocarpus fissuratus

This is probably the commonest Ariocarpus in cultivation in the UK. It is the exception in the otherwise exclusively Mexican genus, occurring in southern Texas as well as south of the border. It is a somewhat variable plant, resulting in the description of a couple of dubious species (see varieties below).

The plant is characterized by its wrinkled, triangular tubercles, each with a central fissure, forming a flat or slightly domed plant up to about 12cm. diameter. The colour is usually grey or slightly greenish. Flowers are produced from the central tuft of white or yellowish wool. They are bright pink in colour and a large plant will produce a succession of up to eight or so blooms, with as many as four being open at any one time.

The popular name of the species is living rock, and it is well deserved. The body closely resembles the rubble in which it grows, both in colour and in texture, and when not in flower it is difficult to find. The resemblance goes further than appearance; the plants are so tough and leathery that stepping on one while wearing heavy boots leaves them quite undamaged.

Ariocarpus fissuratus var. lloydii

This plant has been described as a separate species, but this is hardly tenable. It is distinguished by its taller growth, plants being often almost globular in shape rather than the distinctly flattened form of the type. Individual tubercles are fatter, often less wrinkled and the central furrow does not extend the full length. Also, the lateral furrows on the lower edges of the tubercles, present in the type, are lacking here. The general appearance gives the impression of an overfed and overwatered specimen of A. fissuratus, except that such treatment would almost certainly be fatal! The flower is said to be darker in colour than the type but specimens seen have been indistinguishable.

Ariocarpus fissuratus var. intermedius

Also sometimes considered a distinct species, the claim in this case would seem to be even less substantive than with the previous variety. In appearance it lies between the type and A. fissuratus var. lloydii. It is tempting to consider that all three are simply local forms of a geographically variable species.

Ariocarpus fissuratus var. hintonii

This variety was described as recently as 1989 It is a small plant, growing to no more than 6cm diameter and very flat in shape. The furrowing of the tubercles differs from the type, the flowers are the same. This plant will certainly be sought out by growers seeking to acquire a full set; plants do not yet seem to be available in cultivation, although seed has been offered this year.

Ariocarpus furfuraceus

Although for many years this plant maintained its status as a distinct species it is now usually considered to be a variety of A. retusus It is here treated as such.

Ariocarpus kotschoubeyanus

Along with A. agavoides this species is one of the miniatures of the genus. Usually no more than about 5cm diameter it has the general appearance of a small A. fissuratus but the tubercles are proportionately much smaller and more numerous. They to have a central woolly furrow but are smoother and hardly wrinkled. The plant is usually of a dark green rather than a grey colour. The pink flowers are produced from the central woolly tuft. This species is the one most likely to be seen in collections as a clump.

This plant is the subject of the often repeated and best loved story of the cactus family. Of the original three plants brought to Europe in 1830 by its discoverer Baron von Karwinski, one was sold to a wealthy French collector for 1,000 francs — several times the value of the plant’s weight in gold.

Ariocarpus kotschoubeyanus var. albiflorus

Differs from the type only in having white instead of pink flowers. As such, it perhaps deserves only the status of form rather than variety, but the latter is generally accepted.

Ariocarpus kotschoubeyanus var. macdowellii

Identical to the type in form, but only about half the size, the body growing to 2cm diameter Again, the status of variety could perhaps be questioned.

Ariocarpus retusus

This is one of the larger species of the genus, the plants growing to 20cm diameter. The triangular tubercles are up to 3cm long, glaucous and terminating in a sharp point. The underside has a well defined keel. The flowers are white, sometimes tinged with pink, and are produced from close to but not right in the centre of the plant. This positioning of the flowers was one of the characteristics used in earlier classifications to distinguish between Ariocarpus and Roseocactus

Ariocarpus retusus var. furfuraceus

As mentioned, this variety held its own as a distinct species for many years. It even had its own variety, A. furfuraceus var. rostratus. It is distinguished from the type by its somewhat smaller size, acutely pointed tubercles and the presence (usually) of a small, woolly areole about 1cm from the tip of the tubercle. However, it appears that plants meeting this description can be found intermingled in habitat with A. retusus. The flowers are indistinguishable from the latter. The areoles of the former variety rostratus are alleged to bear weak spines.

Ariocarpus scapharostrus

This species has the reputation of being the most difficult of the genus to cultivate. It is certainly the rarest in cultivation, so perhaps there is some truth in this. If possible, it is even slower growing than its fellows. The tubercles of this plant are elongated, up to 5cm long, with a blunt, rounded end and a keel along the underside, The specific name implies ‘boatshaped’. The colour is a dull, dark green, with the older tubercles often becoming coated with a whitish grey lime-like deposit. Flowers are produced from near the centre and are a rich, dark purple colour. If this aristocratic genus can be considered to have a senior member this plant may be it, although it is, also, probably, the ugliest

Ariocarpus trigonus

Here we have the giant of the genus. The triangular sectioned tubercles rising from the usual bulky tap root can be up to 7cm long, tapering to a sharp point. Plants are occasionally seen up to 25cm in diameter. The proportions of the tubercles vary considerably from elongated and relatively slender to shorter, stouter forms. This, inevitably, has led to the erection of A. trigonus var. elongatus but since intermediate forms exist it is probably better to consider all plants as forms of a variable species.

Flowers arise from near the centre and are yellow. The intensity of colour varies but this coloration is distinctive within the genus. A large specimen plant may have a ring of blooms around the centre, and presents a splendid sight.

In habitat this plant often grows with just the pointed tips of the tubercles visible above the ground, but it is invariably cultivated with the body fully exposed.

Cristate forms of Ariocarpus are occasionally seen. These would certainly be of interest to fanciers of the cristate cult, the deformation of cristation added to the usual rather rugged appearance of these plants can certainly have some bizarre results. Ariocarpus hybrids have been produced; interested readers are referred to work by Neudecker. Such hybrids do not appear to be in general cultivation, but similar experiments could be carried out by any sufficiently enthusiastic and youthful cactophile.

Because of its popularity, information about the genus Ariocarpus can be gathered from all the usual cactus textbooks. The serious enquirer is strongly recommended to seek out as many sources as possible so that a balanced view may be formed on matters such as cultivation. As mentioned earlier, old myths still persist. The appended references are recommended as a source of further study, they contain information not freely available elsewhere.

References

Anderson, E.F. (1965). A Revision of Aniocarpus. C&SJ(US) Vol 37.

Glass, C. & Foster, R. (1974). Aniocarpus - Living Rock Cactus. C&SJ(US) Vol 46.

Needham, L.&D. (1983). Notes on the Genus Ariocarpus. BCSJ Vol 1, No 1.

Neudecker, T. (1985). Crossing Experiments within the Genus Ariocarpus. BCS,J, Vol 3, No 1.

Stuppy, W. & Taylor, N. (1989). A New Variety of Ariocarpus fissuratus. Bradleya 7.

Illustrations (Will open in a new window)

• Fig 1: Ariocarpus agavoides - with long narrow tubercles, giving rise to its name meaning 'Agave-like'

• Fig 2: Ariocarpus fissuratus - note the lateral fissures

• Fig. 3: Ariocarpus kotschoubeyanus - with triangular, flat topped tubercles like a tesselated pavement

• Fig. 4: Ariocarpus retusus - named for the recurving tubercles

• Fig. 5: 20 month old seedlings of Ariocarpus fissuratus growing in a 5cm pot

• Fig. 6: Ariocarpus retusus var. furfuraceus - note the tufted areoles at the end of the tubercles

• Fig. 7: Ariocarpus scapharostrus - probably the rarest and slowest grower

• Fig. 8: Ariocarpus trigonus - the largest growing member of the genus