Cactus requires sandy soil because this is the native soil that cactus roots are made to handle. Any addition of loam, peat or other organic matter will cause the soil to hold too much water and can result in root rot. That being the case, grow plants that also do well in sandy soil. As a general rule, native desert plants are best, but ask a nursery about plants that do not require a nutrient-rich soil.
Cactus requires full sunlight for optimal growth; the plants grow very slowly. Any plants that will accompany the cactus should also grow well in full sunlight. The primary rule is to avoid plants that easily wilt or that may become sunburned from intense summer sunlight.
Cactus is the trademark plant of the desert because its requires very little water. In fact, cactus can go as long as three months without water. Plants selected as cactus companions should be drought resistant, which means they will not die off if forced to go long periods without water.
The best trees for growing with cactus include acacia, palo blanco and anacacho orchid tree. For shrubs, consider mallow, honeysuckle, barbary and any of the bird of paradise plants. Flowering ground cover options would include lantana and iceplant.