When we say that the plants in these lists are low-water, xeric or drought tolerant, it means they can survive and even thrive under dry conditions. However each plant has its own different characteristics and so not all will tolerate the same degree of drought, nor will they be drought tolerant under the same conditions. Some need part shade, others need protection from late afternoon sun or from hot winds; some benefit from a cool root run under a nearby rock; others must be dead-headed after flowering; some like clay, and some need sand.
Therefore it is necessary for the gardener to watch the plants through the seasons and possibly to try the same plant in different locations to better learn the needs of each plant. Established in the right spot, a plant will not be difficult to grow and will give its best performance on the least amount of water.