Care for indoor cacti by watering them when their top half-inch of soil is dry, fertilizing them during the growing season and periodically forcing them into a dormant season. These desert succulents have different needs than most houseplants.
Plant cacti in a cactus potting soil mixture, or make your own mix by adding equal parts sand, peat and garden soil. These plants require adequate sunlight, so avoid placing them in a northerly facing window. If necessary, supplement the sunlight with additional bright light so the plants receive a minimum of 12 hours of light each day during the growing season. Keep the temperature between 65 and 90 degrees Fahrenheit and the humidity between 10 and 30 percent. Feed the plants 5-10-10 fertilizer every two to three months, except after transplanting or during the dormant season.
Since most homes remain at a constant temperature, it is necessary to force cacti into a dormant period. Move the plants to an area where temperatures remain between 45 and 55 degrees Fahrenheit. Water them sparingly, but provide enough moisture so the plants do not shrivel. Once a cactus grows to within 1/4 inch of the edge of its pot, it is time to transplant it. Once it completes its dormant period, move it to the next-sized pot.