Mammillaria petterssonii

Mammillaria petterssonii - another beautiful Mammillaria. This attractive form with yellow spines is common in garden stores. It is a large growing plant, slowly forming a clump of boll-shaped heads. Larger plants, like the one in the picture, will show hundreds of open blooms per time.
Here in Central CA Mammillaria petterssonii can be grown without cover year-round, but will develop somewhat slower.