A fast draining one. If in pots, use cactus mix. If in the ground, mix sand and compost in until the top dries out the day after you water-- usually about one part sand and one part compost to 3 parts native soil--unless your native soil is already sandy. In nature, this species goes dormant in the summer, and likes to dry out then, so if you have rainy summers and chilly winters, it may be hard to perennialize it in your area.