May begins the bloom period for Columbine 'McKana' hybrids and wild columbine. Daylilies also offer tremendous durability and have few insect or disease problems. The veronicas with cultivars of 'Sunny Border Blue,' 'Icicle' and 'Blue Peter' can be relied on year after year. Purple coneflower Echinacea purpurea, False Dragonhead Physostegia virginiana and Balloon Flower Platycodon grandiflorus are durable long bloomers. For late season color Asters offer many colors.
Asters are generally much more dependable for late season bloom than chrysanthemums.
Gardens have a rhythm of their own. We may fill in the note, but we can't completely orchestrate it. The best we can do is hum along.