When we returned from France, our front yard was a vague hummock. So I built low rock terraces across the front. I wanted to define the public and private spaces in an open, inviting way. What plants would contribute to that effect?

On the first private-area terrace, I kept plants low, so you could see farther in. Initially a mix, sempervivums in the sunny middle: some with a white cobweb between the tight fractal spirals of succulent petals, some with red tones, some big and some tiny. Along with them, small sedums trailed over the rocks; my favorites were the pink-white-and-green leafed, and the one that looked like a spill of tiny blue beads. At the edge of garden it was easy to see miniatures. Eventually however I took out the collage, to unify the effect across the whole expanse — using ‘Ralph Shugert,’ a hardy variegated periwinkle.

Still I keep trying taller accents amidst the periwinkle, to add more life to the arrangement without sacrificing the welcoming openness. Verbena-on-a-stick, I thought would be perfect: four foot tall, but you can see through it fine because it bears so few leaves. However its stiff, spiky stems still looked like an unfriendly barrier. In contrast, a clump of ‘Autumn Joy’ sedum (two feet tall but a much more substantial plant), feels like a calm, friendly sentinel greeting visitors.

On the top terrace, farthest into the yard, I planted the whole bed with fancy-colored iris. They rose a substantial three feet, and formed a gentle fence that was so beautiful in June, it was more an offering than a barrier.

Now however (twelve years later), a linden tree shades the bed too much: the iris don’t flower much. For a few years I postponed moving them by getting the tree thinned — but now I have my eye on a spot in the back yard where no trees will interfere.

What could I replace them with? Like the lower bed, it would look best with one plant throughout. Phlox could take the range of sun and shade there now. Last year, however, when I let a pretty volunteer phlox grow among the iris, it developed terrible mildew. I don’t want to have to water that bed. (Mildew sounds like it comes from damp, but drought stress is a common cause.) Resistant varieties? Add compost?

Design adds one more challenge to finding the right plant for the right spot. But it’s worth it.