These round, bubble-like cacti have pronounced tubercles (thus the genus name) which bear spines at the tip. Some plants may be globose, others cylindrical. Some are solitary; others may branch at the base, along the stem, or at the tip; and some form large clumps. The spines may be quite ornamental. They can be stout or bristly or feathery or pectinate (appearing in comb-like units). The central spines on some Mammillarias are hooked, and this feature can be useful for identification.
Many species flower reliably every year starting at a fairly young age, and this is a big part of their appeal. Some flowers are relatively shy (smallish and somewhat closed) but others are comparatively large and open wide. A few plants make red, tubular flowers specialized for hummingbird pollination. Mammillaria flowers tend to form rings around the tips of the stems, emerging from the previous season’s growth. + Show More
This genus is widespread in Mexico (and especially Baja California) but a few species also occur in the southwestern US, central America and the Caribbean, just into South America. The greatest diversity is found in dry or seasonally dry climates. These cacti often occupy rocky terrain and exposed locations where they do not experience wet feet.
As with most cacti, Mammillarias demand strong light for good form and good health. They do not tolerate deep shade, but they can do well for many years on a sunny windowsill. Your Mammillaria should “see” the sun for hours a day year round, and the more the better indoors.
These plants enjoy gritty, fast-draining soil with at least half rock and do well in containers on the small side (matching the diameter of the plant or the size of the roots without a lot of extra space, especially at the bottom). They may prefer unglazed terra cotta containers because of the way they breathe. Overwatering or overpotting can lead to disastrous results. Be sure to allow the soil to go dry at depth before watering. Avoid watering when temperatures are on the low side.
Depending on the plant, Mammillarias may be grown from seed or from offsets. They may not be self fertile (ie. two individuals in flower may be required to produce seed). At least one plant (M. dioica) is dioecious, meaning it has distinct sexes. The seed is small and very young plants do not tolerate the soil drying out much, thus they typically receive overhead protection for some time (months) after germination in cultivation. + Show More
The modern concept of Mammillaria includes genera that used to be distinct, including (most recently) Cochemiea (Baja California natives with long, tubular, actinomorphic red flowers) and Mammilloydia (one species, candida, whose seeds have unique features). It excludes other plants with somewhat dimorphic areoles like Coryphantha (grooved tubercles, blooms at the tip) and Escobaria (also blooms at the tip).
John Pilbeam: Mammillaria: Now and Again (2017)