Late summer can be a depressing time for the garden. Most plants are preparing to go into dormancy and look tired and ratty from summer heat. There are a few perennials that only begin to shine in autumn, with bright flowers that continue until the last hard frost of the season. Perennials for the late summer garden are usually very hardy and easy to grow.
Aster novae-angliae (New England Aster) is so densely covered in autumn flowers it appears to be entirely blue. Daisy-like blooms appear in August and September, and often persist well into cold weather. This is a rather large perennial, growing up to 4 feet tall. Stems can be pinched back in early summer to control height and promote dense, bushy growth. After blooming, foliage can be trimmed again to remove old growth. Asters require full sun to bloom well, and moist but well-drained soil. Hardy in USDA Plant Hardiness zones 4 through 8.
Chrysanthemum (Hardy Garden Mum) not only comes in a wide array of flower colors, it continues blooming often well past the first fall frost. Nurseries sometimes sell florists mums that are labeled as garden mums. Florist mums are not reliably hardy, and have a noticeably different form. Hardy garden mums are short, dense and very bushy. Florist mums bear flowers on very long stems, which give them a lanky appearance. Hardy mums should be planted early enough to become well-established before autumn arrives, as this will increase winter hardiness. Mums need full sun and regular water, with well-drained loamy soil. A light coating of mulch in late fall will help protect the plant from cold damage. Resist the urge to prune in late fall after the foliage has died back. Pruning should be done lightly in early spring once the plant reaches 4 to 6 inches high. Pinch the new growth back lightly to encourage flower buds and a dense growth habit, and remove any dead branches from last season. Most mums are hardy in zones 4 through 9, although some cultivars will even grow in zone 3.
Gaillardia grandiflora (Blanket Flower) puts out a brightly colored show of daisy-like flowers that continue until cold weather arrives. Gaillardia blooms are shades of yellow, red and orange, often in combination. Good drainage is very important with this plant. Heavy clay soil or excessive winter rains can kill it. Full sun and some light watering during very hot weather will keep this plant healthy. Butterflies are very attracted to this perennial. Pruning off the spent flowers will prolong blooming well into fall. The plant is hardy in zones 4 through 10.
Rudbeckia fulgida (Black-Eyed Susan) is another striking perennial with flowers similar to a daisy. Depending on variety, blooms may be yellow, orange or a combination. Flowering begins in late summer and continues through October. Rudbeckias are a wild flower native to the eastern United States, which makes them very hardy and tolerant of almost any soil, including clay. Some supplementary water during summer will promote flower bud formation. Although these plants will tolerate some shade, full sunshine is best. They are hardy in zones 3 through 9.