Strombocactus disciformis (DC.) Britton & Rose
Strombocactus disciformis subsp. disciformis, Mammillaria disciformis (basionym), Ariocarpus disciformis, Pediocactus disciformis
Strombocactus disciformis is a rare cactus with a turnip-like root and a small sunken spherical or flattened stem covered with spirally arranged overlapping tubercles, each with a spine-bearing areole at its tip. The stem is blue-green with a grayish tinge, up to 3.2 inches (8 cm) tall and up to 3.6 inches (9 cm) in diameter. Spines are erect, dark grey at the tips, pale grey at the base, and up to 0.8 inches (2 cm) long. Each areole bears 4 to 5 spines. Flowers are shiny cream-colored, up to 1.4 inches (3.5 cm) in diameter, and appear from the new growth at the crown in early spring.
USDA hardiness zones 10b to 11b: from 35 °F (+1.7 °C) to 50 °F (+10 °C).
How to Grow and Care
To encourage better flowering, allow the plants to enjoy a cooling period in the winter and suspend watering. Unlike many other cacti, which use their ribs as storage devices, Mammillaria feature raised tubercles, from which spines emerge. When you water, the tubercles will expand to increase water storage. The flowers emerge from the axils of these tubercles on the previous year's growth, which accounts for their interesting halo effect. The cactus mustn't be exposed to prolonged dampness and sitting water. Never let your cactus sit in a dish of water. Lastly, make sure to fertilize during the growing season for the best results.
Repot as needed, preferably during the warm season. To repot Mammillaria, make sure the soil is dry before repotting, then gently remove the pot. Knock away the old soil from the roots, making sure to remove any rotted or dead roots in the process. Treat any cuts with a fungicide. Place the plant in its new pot and backfill with potting soil, spreading the roots out as you repot. Leave the plant dry for a week or so, then begin to water lightly to reduce the risk of root rot.
See more at How to Grow and Care for Mammillaria.
This species is native to central and northeast Mexico.
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