We've had desert plants die of drought here because they were not adapted to our long DRY summers. It doesn't rain in most of California from the end of April into the first of November. Nada, not a little as folks back east say, no measurable rain on bad years from March until December. But it does rain a lot in winter, so many desert species drown in our winter. Moreover, our California plants drown in most other states that get summer rain. So again, the best and most drought plants for your area are going to be the ones that have adapted to your climate, native in your area. If you are in a coastal area these plants may pick up as much as half of their moisture from the morning fogs. If you're on a mountain top the plants may have adapted to blowing clouds. If you're in a desert they have adapted to the five minute downpour and will spring into life that day or the next because their large and shallow root system picks up every drop that hits the ground. Chaparral, forest, many inland coastal plants and even some of the desert species belong to a cooperative where they share their resources and cycle the moisture among themselves for the greater good of all. (That's one of the reasons weed control is so important; weeds do not share.) These drought tolerant plants need to be planted together for that to work; they can't share if they do not recognize each other. Nor can the garden be truly drought tolerant if you plant a bunch of non-native plants in with the native plants. If you wish to mix non-native with native, plant clusters of each with a walk way in between.