Gymnocalycium castellanosii Backeb.
Gymnocalycium bozsingianum, Gymnocalycium ferox, Gymnocalycium hybopleurum var. ferox
Gymnocalycium castellanosii is a small cactus with a solitary, green or bluish-green stem that has 10 to 12 ribs and 5 to 10 heavy spines per areole. The stem is globose to flattened-globose and grows up to 4 inches (10 cm) tall and up to 6 inches (15 cm) in diameter. It has 5 to 8 radials and no (rarely one or two) central spines. They are slightly curved back, whitish, greyish or brownish with a darker tip and up to 1 inch (2.5 cm) long. Flowers are white to light pink with a pink or reddish throat, funnel-shaped, up to 1.8 inches (4.5 cm) in diameter, and appear in spring and summer.
USDA hardiness zones 9b to 11b: from 25 °F (−3.9 °C) to 50 °F (+10 °C).
How to Grow and Care
Some Gymnocalyciums are shade-seeking in the wild, among shrubs or grasses, while others grow completely exposed. Some will, therefore, need a light shading from the sun in the hottest months, but to overdo, this will result in the loss of flowers.
The balance of the potting medium should be sufficient to allow good drainage so that the plants do not sit in soggy soil for more than a day or two after watering.
Watering in the summer months, while the plants are growing well can be frequent (weekly for small plants in small pots), but always allowing the compost nearly to dry out before rewatering. Watering in the winter months at all is unwise and certainly not necessary. The difficult times are spring and autumn.
Those species which produce offsets can be readily propagated by cuttings. Gymnocalycium seed germinates well when fresh and will keep for a few years if stored in cold conditions.
Learn more at How to Grow and Care for Gymnocalycium.
Gymnocalycium castellanosii is native to Argentina.
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