Mammillaria gracilis fragilis, Cactus, Thimble Cactus: Mammillaria gracilis v. fragilis has a delicate and charming. It features small round bodies that are densely covered with white spines, interlaced together. These give the impression of white "thimbles." It forms large mats quickly, growing vigorously in clusters. Expect to see creamy yellow flowers in the late winter. To grow this one well, you will need gritty succulent or cactus potting soil, as well as bright light and good airflow.