Cute in its rounded form but lined with prickly spines, the mammillaria cactus (Mammillaria mystax) is native to the hot, dry Mexican landscape. It adapts well to being a houseplant if given a fast-draining soil that is fertile but sandy and coarsely-textured; not over-watered; and given very bright indirect light or direct sunlight in a window. Always kept on the dry side, the soil can be doused with half-strength liquid fertilizer and slightly more frequent waterings in the heat and intense sunlight in summer.
Locate the mammillaria in a very bright window that receives either direct sunlight for hours each day or one that is very bright with indirect light. Avoid shady and cool northern window exposures as they are too dim, even in summer.
The soil must be coarse and sharply draining. Use pre-made cactus potting media or mix regular houseplant potting media with sand, perlite and small aggregate stones like gravel or pumice. The container in which it grows must have drainage holes in the bottom.
Water sparingly in the fall, winter and spring. Allow the soil to become very dry before adding room temperature water, adding just enough water so that it soaks in and dribbles out the bottom of the container. Do not overwater.
Increase watering slightly in the warmth and more intense sun rays of summer. Again, do not overwater, but more frequent waterings can occur in summer without as great of a risk of overwatering. Always stop adding water to the soil once you see the water draining out of the container's bottom drainage hole.
Consider including a half-strength application of a houseplant fertilizer in a watering of the mammillaria in late spring and midsummer. Follow directions on the product label, but dilute the amount of fertilizer used. For example, if a normal solution is 1 tsp. of fertilizer per quart of water, dilute it so you add only 1/2 tsp. of fertilizer per quart of water.
Re-pot the cactus only after it has become overly large or pot-bound in the container. There is no need to re-pot the plant each year; its slow growth will warrant re-potting about once every two to four years. The less direct sunlight, heat and water it receives, the more slowly it grows.