Frailea castanea

Frailea castanea is a small cactus in the family Cactaceae. Kew places its native range from southern Brazil to northeastern Argentina. LLIFLE also records it in northern Uruguay. It grows in a subtropical climate. Its wild sites include rocky ground within pampa grasslands. LLIFLE records it at 492–984 feet [150–300 metres] above sea level. Plants occur in scattered groups across their range. The species is scarce in Brazil but more abundant in Uruguay.

The stem forms a low disc or small rounded body. Some plants remain single rather than forming clumps. Mature stems reach about 0.13–0.16 feet [0.04–0.05 metres] across. Their colour ranges from brown to deep reddish green. Each stem has eight to fifteen low, flat ribs. Small black spines lie close against the stem surface. Most curve down rather than pointing away from the plant. LLIFLE gives a lifespan of about ten to fifteen years.

The yellow flowers can exceed the width of the stem. They reach about 0.13 feet [0.04 metres] across. Buds may form without ever opening into full flowers. This cactus can set seed within closed flowers through cleistogamy. Flowers that do open need strong heat and bright afternoon sun. Dry fruits detach with ease when ripe. Their thin walls break and free the seeds. Each seed has a raised rim and can float. Runoff may help carry seeds away from the parent plant.

Thick, tuberous roots store reserves that help plants survive drought. These roots also anchor the cactus and draw it down. As the soil dries, the roots shrink and shorten. The stem can then sink level with the soil surface. At times, it drops below ground and becomes hard to find. Flowers may push through soil above the hidden stem. Fresh growth does not always make the plant much larger. Older stem tissue contracts as the plant adds new growth. This cycle helps explain its lasting small size.

Wild plants face pressure from grazing and trampling by cattle. Farming, forestry, fire, and plant collection also threaten some sites. These pressures do not affect every part of its range. The supplied species evidence gives no firm frost limit or watering schedule. It also provides no basis for claims about edibility or toxicity.