Mammillarias make up the largest part of my potted cacti collection, and this Bird’s Nest Cactus, (Mammillaria decipiens ssp camptotricha) is one of the 21 Mammillaria species I currently have potted. I like its long, flexible white spines, which become denser in strong sunlight. It is currently flowering, and the tiny white flowers will barely poke through the spines when completely opened.
Most of my potted Mammillarias are exposed to full sun about eight months of the year, but I do move them into an afternoon shade location in late May through early September. I do that because some of the pots get super-heated in direct sun and I don’t want to chance cooked roots or have them permanently scarred from sunburn. I also have about seven more species of Mammillaria planted in my garden, and all are exposed to full sun. They are doing fine.
The one critical thing in caring for Mammillarias (or all cacti for that matter) is careful attention to watering in the winter when cacti are subject to rot from too much water. Many cacti don’t require any water when they are in a dormant period, which could be either summer or winter, depending on the genus or extremes in temperature.
All the Mammillaria species I have so far are easy to find because they are the easiest to grow, and that's what the nurseries stock. I always keep my eye out for something rarer when I’m in a cactus nursery, or even on line. If I wanted to collect them all, I would have to start my own nursery as there are between 200 to 300 species in this genus, although many are not commercially available.
One would think that Arizona nurseries would carry more species since most of the genus is native to the Southwest and Mexico, but that’s not the case. As a cactus lover, I think my expectations are unrealistic. I’m not sure the average person looking for a cactus specimen much cares about the genus, species, subspecies, and etcetera. I think they are mostly interested in whether the cactus is easy to care for and what color the flower will be.