Perennials are the gift that keeps on giving, season after season, unlike annuals, which you have to plant every year. Some perennials are more carefree than others, though. Forgetful gardeners will love drought-tolerant pasque or the delicate-looking but durable penstemon. For hot and dry climates, we like brilliant sedum, and, yes, yarrow (don’t dare call it a weed!).