Cactus is most often planted in desert landscapes individually rather than in groups. Most plants, however, including cacti, are more aesthetically appealing when planted in groupings.
In nature, it's common for a species of cacti to colonize an area. These colonies contain plants of different ages and sizes set in a random pattern with different spacings between individual plants. You can recreate such a natural colony by selecting and planting cacti of one species in various sizes.
Small- to moderate-size cactus groupings work best for urban gardens. Just a few plants in odd numbered groupings of three, five, seven or nine are most pleasing to the eye and in keeping with the scale of most home landscapes.
The most attractive groupings of cactus are those made with species that have rounded or spreading forms. These include golden barrel, fishhook barrel, and devil's tongue barrel.
Low, spreading forms of cactus also provide an attractive "clustering" effect in a landscape. Hedgehogs, clarets cup and flying saucer spread by basal branching. All have beautiful flowers.
When selecting a location for your cactus colony, choose an area that has midday and afternoon shade during the intensely hot summer months. Avoid any location subject to reflected heat, such as along south- and west-facing walls. Most imported cactus species, such as golden barrels, do better with some sun protection. An ideal planting spot is in the light shade provided by a palo verde tree.
Obtaining plants of a single cactus species in a range of sizes may require visiting several garden centers. Although they'll be more expensive, it's important to have just a few larger specimens in the mix to imitate the randomness of an actual colony.
Summer is the best time for planting cactus. Cactus will establish and grow faster with high temperatures and monsoonal rains.
Keep in mind the ultimate size each cactus will obtain over time. It's OK to crowd cactus a little. Just remember that some may have to eventually be removed and relocated if they become overcrowded.
Work the soil to prepare for planting the colony. Cacti prefer well-drained soil, so if a hardpan of caliche is present it should be removed or another site chosen. Turn the soil a foot or so deep using a spade. Remove rocks and stones as you go. Do not add organic amendments to the soil. Cactus is adapted to and grows best in our native desert soils. After loosening the soil, dig holes large enough to accommodate the root systems of the cactus, plant and fill back in with the soil that was removed.
Water the soil thoroughly immediately after planting. After this initial watering, irrigate once every three or four weeks during hot weather. In the winter, water only if normal rainfall does not occur, and then only sparingly. Cactus is more prone to problems from overwatering than underwatering.