Frailea castanea

Origin and Habitat: South America
Description: Frailea castanea (a.k.a. Frailea asterioides) is a very singular miniature plant, and one the of most fascinating cacti with a disk like, flattened to globose body.
Stem: It is diminutive in size and several remain solitary, never outgrowing 4-5 cm diameter. Stems are chocolate brown or dark reddish-green with 8-15 flat ribs.
Spines: 3-15 minute, short, black spider-like, appressed on the plant body and usually all bent downward.
Flower: Sulphur yellow, sometimes larger than the plant itself (4cm diameter). But don't be disappointed when the easily produced buds fail to open. Fraileas are cleistogamous meaning that their flowers produce seed without even opening. Without the need for pollination, hence the buds rarely reach full bloom and remain closed. They will open only in great heat in the hottest, brightest, afternoon sun, if at all.
Notes: Seasonal growth and contraction. Frailea castanea, like many other diminutive cacti. These tuberose roots have the function of storing enough sustenance so that plants can survive the periods of drought that they have to endure. The tuberose contractile roots continually pull the plants deeper into the ground as the stem elongates so the plants remain subterranean or at an appropriate level in the ground
Cultivation and Propagation: Frailea castanea is a summer grower species relatively easy to grow. Characteristically, during the dry season plants retract completely under the ground.
Growth rate: Fraileas are relatively short-lived plants, only lasting few years (rarely more than 10-15 years in cultivation) and, possible annuals in habitat! In fact they reseed readily around the base of the mother plant from self-set seed if kept reasonably moist during warm weather. It is a
good idea to keep restarting them from seed; what looks like a healthy big plant may suddenly die of old age without warning.
Watering: Requires careful watering to keep plant compact. Water sparingly from March till October, the thin, fibrous roots suffer if there is humidity, therefore the plant should be watered only when the surrounding terrain is dry. Keep dry as soon as the temperature starts dropping in October and keep it perfectly dry in winter at temperatures from 5 to 15 degrees centigrade.
Hardiness: They need a minimum temperature of 5-10° C (but occasionally temperatures of a few degrees below 0° are not dangerous if kept on the dry side prior to, and during, cold weather). It tends to rot in winter during the resting phase, if kept wet. In the rest period no high atmospheric humidity!!
Sun Exposure: Light shade to full sun, its colour tends to richer and darker when grown in light shade. In a shaded position the plants grow faster, but are not flat shaped and dark coloured.
Uses: It is an excellent plant for container growing. It always looks good and stays small.
with stronger sunlight. But plants are sometine grafted to accelerate growth, but the grafted plants are typical rather tall growing, compared with plants on their own roots that are usually more flat to the ground.

Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz