Strombocactus disciformis (DC.) Britton & Rose
Strombocactus disciformis subsp. disciformis, Mammillaria disciformis (basionym), Ariocarpus disciformis, Pediocactus disciformis
Strombocactus disciformis is a rare cactus with strong, turnip-like root and small, sunken, roughly spherical stem covered with spirally arranged overlapping tubercles, each with a spine-bearing areole at its tip. The stem is up to 3.2 inches (8 cm) tall, blue-green with a grayish tinge. The spines are 4 to 5, erect, dark grey at the tips, pale grey at the base and up to 0.8 inches (2 cm) long. The flowers are shiny cream-colored, up to 1.4 inches (3.5 cm) in diameter and come in early spring from new growth at the crown.
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 allow for increased water storage. The flowers emerge from the axils of these tubercles on the previous year's growth, which accounts for their interesting halo effect. It's imperative that the cactus is not 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
Strombocactus disciformis is native to central and northeast Mexico.
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