Mammillaria petterssonii Hildm.
Mammillaria apozolensis, Mammillaria huiguerensis, Mammillaria obscura
Mammillaria petterssonii is a small cactus with a globular, usually solitary, light green stem with woolly axils and large 3-angled tubercles. The stem grows up to 4.8 inches (12 cm) in diameter. The tubercles bear 6 to 7 long central spines surrounded by 10 (or more) fine radial spines. The central spines are up to 0.8 inches (2 cm) long, orange-brown, become whitish with age, and have a dark tip. The radial spines are white and up to 0.4 inches (1 cm) long. Flowers are pinkish-red to white, with reddish midveins, up to 1 inch (2.5 cm) long, and have a diameter equal to the length.
USDA hardiness zones 9a to 11b: from 20 °F (−6.7 °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 Mexico.
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