Almost no long-blooming-season plants can carry your garden all the way from early summer through to the end of fall. Annuals are exhausted, and perennials such as coneflower (Echinacea) and black-eyed-Susan (Rudbeckia) attract birds with seedheads, not flowers. Members of the mint and salvia families, however, continue to bloom in spite of waning light and heat. Several other perennials and a shrub begin their blooming seasons late in summer and continue until frost.
Agastache is a perennial genus of the mint family, with approximately 30 species. Foliage resembles that of mint, and agastache is often called "hummingbird mint" or hyssop. Plants grow to between 2 1/2 and 3 feet tall and half as wide. Agastache is drought and heat tolerant and grows best in full sun. "Just Peachy," "Blue Fortune," and "Shades of Orange" suggest the broad color palette of the tubular-flower spikes that attract hummingbirds and pollinators. Anise hyssop (A. foeniculum) is named for its fragrance, and giant hyssop (A. rugosa) smells like mint and licorice and can grow as tall as 4 feet. Different varieties of agastache can be grown in U.S. Department of Agriculture plant hardiness zones 4 through 11.
Michaelmas daisy (Aster novi-belgii) reaches the height of its bloom around the end of September, or the religious feast day of St. Michael. Growing to a height of approximately 18 inches, this bushy perennial is covered with purple-to-blue daisylike flowers and continues blooming till frost. This aster species is hardy through USDA zone 8.
According to Edward Higgins, writing in "Fine Gardening," garden chrysanthemums (Dendrathema x grandiflora) are photoperiodic, which means that blooming is triggered by the shortening of days and lengthening of nights in late summer and fall. A fall classic, chrysanthemums come in a huge variety of colors and flower forms ranging from fuzzy buttons to multipetaled daisies. Pinching or cutting back stems that have no flower buds several times during the summer will increase fullness of foliage and flowers. Garden chrysanthemums are hardy in USDA zones 8 through 10a.
A member of the Aster family, goldenrod is often blamed for fall allergies when other plants such as ragweed are at fault. "Golden Fleece" (Solidago sphacelata "Golden Fleece") is a cultivar hardy in USDA zones 4 through 9. Reaching heights of 2 feet, "Golden Fleece" expands slowly to a 3-foot cluster, making it a better garden plant than older, more aggressive varieties. Flower clusters are a brilliant yellow, and deadheading produces more blooms.
The large Salvia genus contains many perennial varieties. The largest number of sages are hardy in USDA zones 5 through 10, with some hardy through USDA zone 11. Generally drought tolerant, salvias send up abundant spikes of predominantly blue and purple flowers, although some varieties have pale lavender or bright red blooms. Plants generally reach between 1 1/2 and 4 feet high, although some Southwestern native varieties grow to 6 feet. Most salvias bloom steadily through summer and fall until felled by frost. Some varieties known for late displays are "Limelight" (S. mexicana "Limelight"), a 6-foot tender perennial with purple flowers beginning in August; pitcher sage (S. azurea) with sky-blue August flowers; and small-leaved salvia (S. microphylla var, neurepia), a 4-foot shrub with bright red flowers that bloom most abundantly in late summer and fall.
Sedum, or stonecrop (Sedum spp.), has gained popularity with gardeners in USDA zones 3 through 9 as reliable contributors of fall color. Plants of 1 to 2 feet in height have fleshy leaves that mark their membership in the succulent family. Tolerant of heat and dry soil, sedums typically form flower-bud clusters in midsummer, usually pale green or white (pink in the case of red-leaved varieties), which intensify in mostly pink colors as they open in late summer and early fall. Blooms draw pollinators until frost, and the darkened dry blossom clusters can contribute winter interest. "Autumn Joy" and "Black Jack" are two long-standing varieties of an ever-growing group of sedums.
For those with more garden space, a fall-blooming camellia species (Camellia sasanqua) produces a large display of color. Hardy in USDA zones 7 through 10, these glossy-leaved evergreen shrubs grow at a moderate rate to heights of 6 to 10 feet and similar widths, depending on variety. Flowers are single or multipetaled, in shades ranging from white ("Alba") to pale pink ("Apple Blossom," "Showa-No-Sakae"), deep pink ("Kanjiro") and red ("Bonanza"). A few members of the species, including bright-red "Yuletide," bloom in winter.