Discocactus horstii
| Light | Very bright light with a little shade from the harshest afternoon sun; the flat body sits near soil level in habitat |
|---|---|
| Water | Warm and careful; keep evenly slightly moist in active growth on a warm mineral mix, dry and warm in winter |
| Soil | Very fast-draining, mostly mineral mix (see Soil and potting mix) |
| Temperature | Warmth-loving; keep well above freezing, comfortably in USDA zones 10–11 and warmer at the roots |
| Propagation | Seed (see Propagation — seed); mature seedlings are often grafted to build strength |
| Toxicity | Not documented as toxic to cats and dogs, though reliable data for this species are lacking; keep out of reach |
Discocactus horstii is a tiny, flattened, many-ribbed cactus from Brazil, prized by collectors as one of the smallest and most exacting members of the genus Discocactus. Its disc-shaped body carries a large number of very narrow ribs edged with minute, comb-like (pectinate) spines pressed flat against the surface, and mature plants develop a woolly cephalium from which fragrant white nocturnal flowers open. It is widely regarded as one of the most beautiful and most demanding cacti in cultivation.
Description
Discocactus horstii is a small, solitary, strongly flattened plant, typically only a few centimetres across and sitting low and disc-like against the ground. The body is divided into many slender, closely set ribs — more numerous than in most globular cacti of its size — giving the surface a finely fluted, almost gear-like outline when viewed from above.
Each areole bears tiny, tightly appressed spines arranged in a neat comb (pectinate) pattern, so the plant feels smooth rather than sharp and reads as a low, sculpted rosette. At maturity the crown forms a woolly, bristly cephalium, the flowering structure characteristic of the genus. From this crown the plant produces slender white flowers that open at night and carry a strong, sweet fragrance, a feature shared across Discocactus.
Distribution and habitat
The species is native to Brazil, where it grows in the campos rupestres of Minas Gerais. In habitat it is found in open, sun-baked sites among quartz gravel and sand, often nestled almost flush with the substrate so that only the flat top is exposed. The climate there is warm, with a distinct wet and dry rhythm, and the plants root into extremely sharp, fast-draining mineral ground.
Wild populations are very restricted and have suffered from over-collection and habitat pressure, which is a large part of why the species carries strict international protection (see Legal status below).
Cultivation
Discocactus horstii has a reputation as a connoisseur's plant, and most losses come down to two things: cold and stagnant moisture. It wants genuine warmth, especially at the roots, and a very open, mostly mineral mix that never stays wet. Give it strong light so the low body keeps its tight, flattened form.
During the warm growing season water carefully — the plant appreciates moisture when it is actively growing and warm — then allow the mix to dry before watering again, and keep it dry and warm through the cool months to avoid rot. Because the roots resent both chill and sourness, many growers keep the plant slightly warmer and drier than they would a more forgiving globular cactus. See Watering and Repotting for general technique.
Own-root plants are notoriously slow and touchy, so a large share of cultivated D. horstii are grown grafted (see Propagation below), which sidesteps the temperamental root system.
Propagation
Seed is the primary method; the fine seed is sown on a warm, sharp, mineral surface and kept humid until germination, much as for other members of the genus. See Propagation — seed for a full walkthrough.
Because seedlings on their own roots grow slowly and can be difficult to keep, growers very commonly graft young plants onto a vigorous rootstock to build size and vigour, later growing choice plants on more slowly once established. The species rarely offsets, so division and offset propagation play little part.
Common problems
- Rot — the classic killer; cold combined with moisture, or a mix that holds water, rots the sensitive roots and base.
- Sulking after repotting — the fine root system re-establishes slowly; keep freshly repotted plants warm and barely watered until new roots take (see Repotting).
- Etiolation and loss of form — too little light makes the normally flat body dome up and soften its outline.
- Pests — mealybugs (including root mealybugs) and red spider mites are the usual offenders; see Pests and diseases.
Legal status
Discocactus horstii has a very limited natural range and is assessed by the IUCN Red List as Vulnerable, and the entire genus Discocactus is listed on CITES Appendix I — the strictest level of international protection. In practical terms this means wild-collected plants cannot be freely traded across international borders, and international movement of specimens requires the appropriate CITES permits. All cacti are additionally covered by CITES at the family level (Cactaceae, Appendix II).
Nursery-propagated plants raised from seed are, by contrast, produced legitimately by specialist growers; collecting or trading wild-taken plants is both damaging to the fragile wild populations and unlawful. Growers should keep documentation of legally propagated stock, particularly when moving plants between countries.
See also
- Discocactus — the genus overview
- Grafting · Soil and potting mix · Watering · Repotting · Propagation — seed
- Pests and diseases
- CITES