Mammillaria
Mammillaria is one of the largest and best-loved genera in the cactus family, containing roughly 200 species of small, globular to short-cylindrical plants native mainly to Mexico. The genus is instantly recognisable by its tubercles — the nipple-like bumps, arranged in neat spirals, that give the plants their name (from the Latin mammilla, "little nipple") and, importantly, by the way its flowers emerge not from the tubercle tips but from the woolly axils between them, often forming a tidy ring or "crown" around the top of the plant.
Their compact size, generous flowering and forgiving temperament make Mammillaria a staple of windowsill collections and one of the friendliest groups of cacti for a beginner to start with.
Description
Most Mammillaria are small plants, from a couple of centimetres to perhaps 15–20 cm across, forming either solitary heads or dense offsetting clumps. The defining feature is the tubercle: instead of continuous ribs, the body is covered in individual conical or cylindrical projections arranged in spiralling rows. Each tubercle is tipped by an areole bearing the spines, while a second growing point sits in the axil at its base — and it is from these axils that flowers and offsets arise.
Spination varies enormously across the genus. Some species are clothed in soft white hair or fine radial spines that give a woolly, silvery look; others carry stout, hooked central spines (the "fishhook" mammillarias) that catch on anything passing by. Many carry a distinctive milky or watery latex when a tubercle is cut, a trait once used to help divide the genus.
Flowers are typically small, funnel-shaped and borne in a ring near the crown, in shades of white, cream, yellow, pink and magenta. In many species the display is followed by showy fruits — slender, club-shaped berries, often bright red or pink, that persist for months and are as ornamental as the flowers themselves.
Distribution
The genus is centred on Mexico, which is home to the great majority of species and the heart of its diversity. From there the range extends north into the southwestern United States and south through Central America to parts of Colombia and Venezuela, with a few species reaching the Caribbean. Mammillaria occupy a wide spread of habitats — arid scrub, rocky limestone slopes, grassland and open woodland — from near sea level to high mountains, which is part of why the genus is so varied and so adaptable in cultivation.
Notable species
- Mammillaria hahniana — the "old lady cactus", wrapped in long white hair and a ring of pink flowers.
- Mammillaria elongata — the "lady finger" or "gold lace" cactus, forming clusters of slender, densely-spined fingers.
- Mammillaria plumosa — the "feather cactus", cloaked in soft, feathery white spines.
- Mammillaria gracilis — the "thimble cactus", a fast-clumping favourite whose offsets detach at a touch.
- Mammillaria bocasana — a soft, silky-haired clumper with hooked central spines and cream flowers.
- Mammillaria spinosissima — the "spiny pincushion", variable and freely flowering.
- Mammillaria zeilmanniana — the "rose pincushion", among the earliest and most reliable to bloom.
Cultivation
As a group, Mammillaria are among the easiest cacti to grow, which accounts for their popularity. Give them a bright position — a sunny windowsill or greenhouse bench — as most species colour up and flower best in strong light, though the very hairy, white-spined kinds appreciate a little shade from the fiercest summer sun to avoid scorching. Grow them in a free-draining, mostly mineral mix and water thoroughly through the growing season, always letting the soil dry out completely before watering again. See Watering for general technique.
The single most important thing is a dry, cool winter rest: keep the plants nearly bone-dry and cool from late autumn through winter. This not only prevents rot — the commonest cause of loss — but is also what triggers the spring ring of flowers. Repot every couple of years into fresh gritty mix; see Repotting.
Propagation
Many Mammillaria propagate themselves with almost no effort. The clumping species produce abundant offsets that can be twisted or cut away, dried for a few days to callus, and set on gritty mix to root — the method covered in Propagation — offsets. Some, like the thimble cactus, drop segments so readily that stray offsets root wherever they land.
Species that stay solitary are usually raised from seed, which germinates readily on a warm, humid mineral surface; see Propagation — seed and Propagation — cuttings for related methods.
In cultivation
Mammillaria have long been collector favourites, and a number of ornamental forms circulate in the hobby. Crested (cristate) and monstrose forms, in which the growing point fans out into brain-like ridges or irregular lumps, are prized oddities and are sometimes grafted to keep them growing strongly. There is also great natural variation in spine colour and density within many species, which growers select for. On the whole, though, the genus is loved less for exotic cultivars than for the sheer reliability and charm of the ordinary plants — a well-grown clump ringed with flowers is hard to beat.
Common problems
- Rot — nearly always from overwatering or watering during the winter rest; the plant softens and browns, often from the base upward. A dry cool winter and a gritty mix are the best prevention.
- Etiolation — too little light makes the body pale and stretch, and flowering suffers.
- Pests — mealybugs (white fluff tucked among the tubercles and axils) and root mealybugs are the usual troublemakers, with red spider mites a risk in hot, dry, still air. See Pests and diseases.
See also
- Astrophytum — another beginner-friendly globular genus
- Soil and potting mix · Watering · Repotting
- Propagation — offsets · Propagation — seed · Grafting · Pests and diseases