Gymnocalycium denudatum (Link & Otto) Pfeiff. ex Mittler
This species is native to Brazil (the Rio Grande do Sul), Uruguay (Melo), and probably Paraguay.
Gymnocalycium denudatum is a small cactus with a dark green, spherical, usually solitary stem with 5 to 8 ribs with low tubercles, hardly apparent, and clusters of spines often hugging the stem contour and looking like long-legged spiders. The stem grows up to 1.2 inches (3 cm) tall and 3.2 inches (8 cm) in diameter, or a little taller in cultivation. Spines are all radial, usually 3 to 5, ivory white to pale cream-yellow, and up to 0.8 inches (2 cm) long. Flowers are white, up to 2 inches (5 cm) long, up to 3 inches (7.5 cm) in diameter, and appear in spring and summer. Fruits are oblong and green.
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. Therefore, some will need light shading from the sun in the hottest months, but to overdo, this will result in the loss of flowers.
The potting medium's balance 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 allow the compost to dry out nearly 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.
See more at How to Grow and Care for Gymnocalycium.
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