Sedum was my gateway genus to rock gardening. In high school, I spent a few summers working at a green roof nursery owned by some family friends. Long hours planting and weeding stonecrop-filled modules gave me an appreciation for those hardy, water-wise plants. Planted en masse, they created a glowing tapestry of yellow, pink, white, and green that defied the summer heat and winter cold. I went home with pockets full of cuttings, which Mom allowed me to plant in a...