The prickly pear cactus is just among thousands of species of the now ubiquitous succulent plants. For plants that are between 2 and 4 feet in height, the recommendation is to wait to water until the soil conditions become dry 3 inches below for plants under 4 feet. Plants over 4 foot should wait until it dries to 4" before watering. Regardless of height, you will want to saturate the soil thoroughly, remembering this must be good draining soil.