For a cactus that's colorful out-of-bloom as well as in-bloom, grow rainbow cactus (Echinocereus rigidissimus, Echinocereus pectinatus var. rubrispinus). The short, cylindrical stem is densely covered with comb-shaped spines in alternating bands of red, pink, white and gray. In spring, bright magenta flowers at least 2 inches wide appear. Slow-growing plants rarely branch, so rainbow cactus is grown from seed. It is suited for container growing because of its need for excellent drainage.
For best spine color and stem shape, grow rainbow cactus in the sun. In areas with hot summer temperatures and intense sun, partial shade in the afternoon is acceptable. The cactus is native to the southwestern United States and northern Mexico, growing in mountainous areas between 4,000 and 6,000 feet elevation. Plants are subject to hot, dry summers and increased levels of ultraviolet light because of the higher elevation, so you can grow them where more tender plants would fail.
One of the cold-hardier cacti, rainbow cactus tolerates short exposures to temperatures of 10 degrees Fahrenheit if the plant is kept dry, making it hardy in U.S. Department of Agriculture plant hardiness zones 7 through 11. Leo Chance, author of "Cacti and Succulents for Cold Climates," reports rainbow cactus marginally hardy in USDA zone 5, enduring temperatures of minus 20 degrees Fahrenheit when protected with a thick mulch. If you grow rainbow cactus in a container, it will not be as cold hardy as if it is in the ground. For colder winter areas, keep the cactus and its soil dry in winter.
Grow rainbow cactus in an open, fast-draining soil mix. You can use purchased cactus and succulent potting mix, but mix 1 part perlite with 2 parts of the potting mix to increase the drainage. Rainbow cactus reaches 12 inches high -- very rarely 18 inches -- so the plant is best enjoyed up close in a container where its color and markings are easily visible. Give the cactus a pot just slightly bigger than the plant's diameter for good drainage and so soil can dry out quickly between watering. In USDA zones 8 through 11, plants grow outdoors in well-drained garden beds such as rock gardens and mounded soil areas or raised beds in xeriscape gardens.
During the growing season, water rainbow cactus thoroughly and then let the soil dry out before watering it again. It is best to err on the side of underwatering for rainbow cactus, since it is susceptible to root rot if soil is too moist. Test the soil with your fingers or a soil probe halfway down the pot or in the garden bed at 2 inches into the soil. If you find moistness at that level, don't water the plant. Rainbow cactus goes dormant for the winter, so water it less frequently then, withholding water if the plant is in the ground.