Mammillaria (mam-mil-lar-i-a) One of, if not the, most popular and largest genus of cacti in the world. Their attractive, tuberculate forms fill collections both large and small and their variation keeps them interesting. You could dedicate an entire collection to Mammillaria without fear of boredom.
Form: They're rather short, manageable cacti, all of which are tuberculate. As such, they lack ribs and opt for many raised bumps instead. Each of these bumps are tipped by an areole which sprouts spines of varying length.
Colors: The stems themselves are green, nearly without exception. However, the spines and hairs are a different story. They display and enormous amount of variation and describing it in broad strokes here wouldn't do the genus the justice it deserves.
Foliage: All of the foliage in this genus is organized into spines. There are no photosynthetic leaves.
Flowers: Relatively small, with a couple exceptions, but always numerous. Many Mammillaria develop a crown of sorts. This crown is made of many flowers that form a ring around the tip of the plant. Most flowers are magenta to white and typically feature a darker colored midstripe.
Light: They do their best growing and flowering in full sun conditions. Place in a south facing window or grow outside if possible.
Soil: Pick a gritty, well-draining mix like a cactus and succulent soil (available at garden centers) or mix your own (more info).
Water: Water deeply enough for water to run out the container’s drainage hole. Allow soil to completely dry before watering again. Keep dry in winter.
Hardiness:Mammillaria are not frost hardy and should be brought indoors to a sunny window sill if there is a chance of frost. (What’s my zone?)
Propagation: A few of them produce offsets and branches that can be rooted with ease. However, all of them can be grown via seeds as well.
NOTES FROM THE NURSERY
This cactus species got its superstar status because of its many members and because it's easy to grow. They are very forgiving but, like many cacti, they are prone to root rot. They've given us no trouble in our greenhouses and they've flowered quite profusely.
Speaking of their flowers, once pollinated, they form beautiful, edible fruits. They're fairly small but they're packed with tiny, black seeds. These seeds have a decent germination rate and, with a little TLC, can grow into healthy cacti themselves.
These cacti are, nearly without exception, endemic to Mexico. Surprising no one, they especially love the arid and semi arid areas but some even make a home for themselves in the wetter jungles. Unfortunately, due to their popularity, many Mammillaria are endangered due to poaching and over collection.