Gymnocalycium horstii aka Cactus

Gymnocalycium horstii is a cactus in the Cactaceae family. The RHS calls it spider cactus. Kew lists its native range as Rio Grande do Sul, Brazil. The RHS gives the same range for this species. LLIFLE reports a wider range across nearby South American countries. These include Uruguay, eastern Paraguay, and northeastern Argentina. Kew places the plant in a subtropical climate. LLIFLE describes its home as rocky outcrops in pampa grassland. It grows at heights up to 984 feet [300 metres].

The plant has a round body with few spines. Its smooth green stem has a glossy surface. LLIFLE gives a stem width of about 0.66 feet [0.20 metres]. Some stems reach 0.82 feet [0.25 metres] across. These figures describe individual stems, not the whole clump. Plants often grow alone but may form clusters with age. Deep grooves mark the broad ribs on the stem. The ribs have rounded surfaces with somewhat sharp edges. The sparse spines range from straight to slightly curved. They are pale yellow to white and vary in form. One spine points down, while the others extend sideways. Spines can reach about 0.10 feet [0.03 metres] long.

The flowers may be white, pink, or pale purple. Some have only a faint pink tint. They can reach 0.36 feet [0.11 metres] in both length and width. Their size contrasts with the plant’s compact, rounded body. The fruit has an egg shape and blue-green skin. The supplied species account gives no clear flowering season. It also gives no evidence for edible parts or medicinal use. The sources do not establish safety for people or pets.

LLIFLE describes this species as easy to grow. It grows in summer and needs moderate to generous watering then. The account notes that it needs more water than many cacti. It recommends light shade and warns that strong sun can scorch it. A summer feed rich in potassium forms part of its care. Dry plants can withstand brief cold down to about −4°C. LLIFLE lists seed as the main means of propagation. Offsets form only rarely. In the wild, populations are sparse and split into isolated groups. Grazing, farming, fire, and illegal collecting place pressure on them.