The easiest desert cacti* to get to flower in the home are the small globular ones (Gymnocalyicum, Rebutia, Echinopsis, Mammillaria, etc.). Prickly pear cacti (Opuntia), with their flat pads, and large columnar or barrel cacti are either difficult to get to flower or take decades before they are mature enough to bloom, but many globular cacti are mature and ready to flower when they are less than 2 years old.
A Summer and Fall Outdoors
To get a globular cactus to bloom, start by putting it outside for the summer, after proper acclimatization, of course. That’s because sunlight indoors passes through a glass window and glass filters out the ultraviolet rays. Ultraviolet light is not needed by most plants and is in fact even harmful to many, but desert cacti actually grow best when they receive ultraviolet rays. So a summer outdoors really does them a great deal of good.
Video by Greg Krehel showing Echinopsis flowers opening in slow motion: just beautiful!
Water and fertilize your cactus regularly throughout the summer, like any other container plant, as this is its growing season. However, when autumn begins, it’s time to place the plant in a spot protected from rain, perhaps under the overhang of the roof or behind a window frame leaning against a wall. That’s because a period of drought is one factor that stimulates bloom in many small cacti. You can, of course, water when the soil is thoroughly dry, but otherwise, it’s best keep the rain off it throughout the fall. Keep your cactus outside as long as possible in the fall and don’t bring it back indoors until there is a real threat of frost, as cool or even cold nights are the second factor that stimulates cacti to bloom.
When the plant has had a few months of cold nights and drought, bring it indoors and place it in front of a window, preferably in a cool spot (although by now, many small cacti will have had their required cold treatment and wouldn’t be too bothered by a bit of warmth). Through the winter, continue watering only when the soil is really dry.
Flowering should occur somewhere between mid-winter and late spring, depending on the species and the growing conditions. If it still hasn’t bloomed by spring, put it outdoors again for the summer … where it will likely bloom in short order.
*Epiphytic cacti or jungle cacti, like Rhipsalis and Christmas cacti (Schlumbergera) are also easy to get to bloom … but aren’t desert cacti and shouldn’t be treated as such.