About the genus
Discocactus was described by Pfeiffer in 1837. The genus has around 12 species, many of which are extremely variable. The name Discocactus comes from the Greek diskos meaning a disc or quoit. They occur in widely scattered locations, predominantly sandy or stony, and often extremely limited in extent, in Brazil and parts of Bolivia and Paraguay. The habitats are generally low altitudes with a tropical climate.
Discocactus has two stages of growth. The initial phase may last for many years as plants produce a stem, incapable of flowering or fruiting, the size of which can be small, as in Discoctus horstii, to large in numerous species including Discocactus boliviensis. Later they produce a cephalium, where the underlying body begins to produce dense hair with bristles and often longish fine spines. They can then flower and fruit.
The flowers are white or ivory-coloured and develop rapidly, in one day, opening in the evening for one night. They are often strongly sweetly scented, have a long tube with scales and open widely, and can be produced several times in the summer.
How to grow them
This genus is not for the faint-hearted. They require higher temperatures over winter and plenty of light and appear to be less forgiving than Melocactus. A temperature of 15°C is about the lowest they will endure and ideally higher.
They all come from areas without lime, so require a moderately acid soil. A mix of one part soil-based compost like John Innes 2 or 3, one part acid peat-free compost and two parts coarse grit has been successful. However, plants have also been grown well in an acidic peat-free compost and grit, mixed 1:1. There have been recent moves to inorganic media like pumice, lava and perlite with considerable success. Repotting of the adult plants should be done only if necessary and carefully as the any damage to the roots can be deleterious. It is best done in summer when the damage is less likely to cause fungal rot.
In the late spring until late summer, they should be watered frequently, providing they show evidence of growing well. Rainwater is preferred. They can be fed at each watering with the same fertiliser as other cacti and succulents. There is debate about winter watering, but, if done, it should be with caution.
Discocactus seed if available, germinate well and quickly in the same conditions and compost as for any other cacti. The seedlings are large when they emerge and generally grow strongly. They require bright light, though not full sun, to prevent them becoming weak and succumbing to fungal rots. In suitable conditions they can grow reasonably quickly, but a cephalium cannot be expected, and so flowers and fruits, for at least ten years. However, most of the best ones now seen are grafted as seedlings and then develop rapidly and can develop a cephalium within five years or so for the smaller species. They take being grafted very well and often produce fine specimens.
Plants are susceptible to root and stem mealy bugs and an infestation can develop quickly. Use the same treatments as with other cacti. The only other problem is fungal rotting and light and good ventilation help to minimise this.
Text and photos by John Arnold
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