Q: My clumps of oriental poppies and creeping moss phlox leave boring gaps in my beds after they have finished flowering in the spring. What can I plant to fill in after them? — Diana Poisson, Montreal, Quebec
A: Because the foliage of the Oriental poppy (Papaver orientale) dies back to the ground shortly after flowering, it’s easy to hide the gap this perennial creates by surrounding it with big, lax perennials that bloom in midsummer or later. Tall perennial phlox (Phlox paniculata, not to be confused with your creeping moss phlox, Phlox subulata) is a stalwart fill-in. P. paniculata ‘David,’ which has mildew-resistant leaves and shining white flowers, is one of the best, along with ‘Norah Leigh,’ whose strongly variegated leaves give you a spot of color before and after its pink flower show. A companion planting of asters, physostegia, black-eyed Susans, or coneflowers will also do the trick.
Moss phlox’s postbloom blahs present a bigger challenge than the Oriental poppy’s disappearing act, because the leaves of P. subulata (however lackluster they may look) continue growing nearly year round and need sun the whole time. Any part of the plant that is shaded will die out. Ring it with low to midheight, later-flowering perennials; if the patch still seems too broad and boring, dig a few holes in the phlox carpet and fill them with more interesting compact perennials. Scabiosa, for instance, is a neat, mounded plant that blooms for a long time, especially if you cut its spent flowers. Any of the creeping thymes, with their gorgeous foliage, would work well alongside or amid the phlox. So would golden oregano (Origanum vulgare ‘Aureum’), practically any dianthus with handsome silvery foliage, or a low-growing sedum such as ‘Vera Jameson.’ For a colorful last hurrah, I’d tuck in a few autumn crocuses or colchicums.