How to Get Indoor Cacti to Bloom
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 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 can flower when they are less than 2 years old.
To get a globular cactus to bloom, put 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 lot of good.
Water 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 any 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 rains 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 stimulate 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, most small cacti will have had their required cold treatment and wouldn’t be to bothered by a bit of warmth). Continue watering only when the soil is really dry. Flowering should then occur somewhere between mid-winter and late spring, depending of 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.