Full sun, all day, once acclimated. Indoors that means the brightest south or west window, and it is still not quite enough; a grow light helps in winter.
Water
A deep soak when the pot is bone-dry and the weather is warm, every two to three weeks in summer. From November through February, nothing at all for most species.
Soil
Mostly mineral: two-thirds pumice or scoria, one-third cactus mix. The white-spined and woolly species want it even leaner.
Dormancy
Hard winter dormancy. Cool, bright, and completely dry; the body shrinks and that is fine. Watering a dormant cactus is the surest way to lose it.
Many take a light frost if dry; the Mexican globulars generally do not. Assume 40°F as a floor unless we have told you otherwise.
Feeding
Low-nitrogen cactus fertilizer at half strength two or three times over the summer. Nothing else.
Repotting
Every three to four years, or when the plant reaches the rim. Spring. Keep it dry for a week before and after.
Watch for
Soft, dark patches at the base mean rot from cold water. Red or bronze color is sun stress and usually harmless. Mealybugs in the root zone show as white cottony bits at the surface.
Myrtillocactus cristata 8" Pot A · Cacti · family notes · botanicwonders.com/care/myrtillocactus · Botanic Wonders, 2705 Sarver Ln, San Marcos, CA 92069 · (760) 519-9987