MOST ROCK GARDENERS seem to design a corner to grow choice alpines in troughs. In the right spot, with the right mix of soil, troughs seem to facilitate growing a wider range of tiny plants and challenging alpines than in the ground. Choice androsaces and drabas will persist for many years in a trough where they disappear in conventional rock gardens. Troughs invariably include a few Sempervivum species, which are bona fide alpines after all, and occasionally a species of Sedum or Rhodiola. Over the long haul, gardens exhibit a sort of Darwinian selection: more and more succulents seem to persist as the more delicate tiny saxifrages and primulas disappear one by one. Why not design a trough just to feature the succulent toughies? Like alpines, most succulents seem to enjoy trough culture as much and more than growing in the ground, and perhaps for similar reasons!