How to Grow and Care for Mammillarias

Introduction

This very collectable genus of mostly Mexican cacti is popular because of its diversity (about 170 species), its flowers (often appearing in colorful halos near the top of stems), and its colorful fruit (often bright pink or red). Mammillarias make great container plants given strong light, excellent drainage, and discipline with the watering can.

Varieties

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.

Subsequently fruit may appear in numbers, emerging from axils as small, fleshy, chile-shaped objects. The fruit from several species is edible, sometimes delicious, best harvested when it is fully extruded from the axil and separates easily from the plant. Some Mammillarias are cryptocarpic, meaning that fruit and seeds are produced within the stem of the plant, hidden from view, and seeds only emerge (or can be extracted) later. Fresh seeds from these plants may not germinate well until some time has passed.

One distinguishing (but not unique) feature of Mammillaria is the presence of dimorphic areoles, an anatomical feature where spines are borne at the tips of the tubercles but flowers and fruit appear at the base. The axils, from which the reproductive structures emerge, sometimes bear fur or bristles distinct from the spines at the tips. These axillary bristles or wool may be quite variable, even within a species.

In addition to the plants which grow substantial above-ground stems, there are also plants which are near-geophytes, at least in times of drought. These plants barely emerge above ground level, and they may have enlarged, tuberous or napiform roots. They are especially vulnerable to rot as a consequence of overwatering.

A small group of Mammillarias branches dichotomously (by division of the growth center) and these plants (eg. M. parkinsonii, M. karwinskiana, muehlenpfordtii) have a unique appeal because of their appearance, which can be spectacular in multiheaded old age.

Origins

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. All but 2 species are found in Mexico; almost 90% of species are endemic to Mexico; and a third of species are restricted to a single state in Mexico. These cacti often occupy rocky terrain and exposed locations where they do not experience wet feet.

Care

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.

Propagation

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.

Some plants form extended clumps over time (eg. M. plumosa) and these clumps can be carefully divided to yield separate plants, already rooted and ready to go once they have had a chance for broken stems to heal. Some plants are routinely propagated from cuttings, like the very common (and very well behaved) M. elongata. Cuttings need time to heal and they need to be watered with restraint until they show signs of growth on top.

Relatives

The late-20th century concept of Mammillaria described above includes genera that used to be distinct, including (most recently) Cochemiea (Baja California natives with long, tubular, zygomorphic 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).

Recent genetic studies have rearranged these genera, and as of 2021 the Mammilloid group consists of 3 genera: Mammillaria, Cochemiea, and Coryphantha. Almost all of the straight-spined species still fall in the newly reduced Mammillaria, almost all of the hooked-spine species now fall in a greatly expanded Cochemiea (including the plants with zygomorphic flowers from the former version), and Coryphantha will probably be expanded to include Escobaria (details remain to be seen).