Discocactus horstii is a charming little cactus described by Buining and Bredero in 1973. The relationship of the genus to other genera is not well understood and is disputed. All species are characterised by the apical cephalium which has led some botanists to believe that it is closely related to the genus Melocactus. Early DNA work supports this, but that theory is now questioned. Its morphology, growth pattern and nocturnal, scented flowers are quite different from Melocactus. The body of melocacti stop growing as soon as the cephalium develops. Discocacti, on the other hand, continue to grow after cephaliating. Also the cephalium of the plant does not grow to the enormous lengths as with some species of Melocactus. Consequently, its systematic position is still undecided.
Fig. 1 Discocactus horstii. Don’t remove dead flowers unless they come off easily, otherwise the plant will be damaged.
The genus Discocactus was described by Pfeiffer in 1837, with the type species designated as insignis, which now refers to placentiformis. There are about 12 species. Those accepted in the update (illustrations) to the New Cactus Lexicon are bahiensis, boliviensis, catingicola, diersianus, ferricola, hartmannii, heptacanthus, horstii, patuliformis, placentiformis, pseudoinsignis and zehntneri. The genus has suffered from synonymy like most cacti and other names that may be encountered include araneispinus, boomianus, buenekeri, crystallophilus and flavispinus, to name only a few.
The plants are endemic to Bolivia, Paraguay and primarily in Brazil, frequenting cerrado and caatinga vegetation regions. Most are solitary and produce offsets only when damaged. Although some species grow to as much as 26cms in diameter (diesianus), horstii reaches only 6cms at best. They have been and still are extensively collected. Consequently, discocacti are classed as vulnerable or in danger.
Fig. 2 Discocactus placentiformis. The generic name refers to the flattish, round shape and derives from the ancient Greek word diskos meaning a disc.
Like most species of the genus, horstii can be tricky to keep alive for a long time and even harder to maintain without the body marking. This dwarf cactus is almost always grafted as it is possibly slightly more difficult than other species of the genus. Discocacti require fairly high temperatures. To be safe something in excess of 17°C is a minimum, although the may survive down to 14° if day temperatures are high. Like melocacti, discocacti do not requite very strong sunlight but can benefit from it. They can easily scorch, however, if suddenly exposed to strong solar radiation. Soil should be very gritty. Experiments growing the plants in pure pumice have been successful. As pumice contains none of the necessary nutrients, regular feeding is essential.
George Thompson
Useful references
Buining, A. F. H. Discocactus, Succulenta, Claremont (1975).
Hunt. D. (editor) The new cactus lexicon, DH Books, Milborne Port (2006).
Hunt, D. (editor) The new cactus lexicon: Illustrations, DH Books, Milborne Port (2013).
Taylor, N. and Zappi, D. Cacti of eastern Brazil, Royal Botanic Gardens, Kew (2004)
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