By Lynn Ocone

It's hard to imagine a fall garden without asters. When little else is blooming, their cheery flowers of lavender, blue, pink, purple and white brighten the garden like colorful constellations against the tawny yellows and browns of autumn. Indeed, aster, the Latin word for star, aptly describes the starry flower heads.

Asters are easy to grow and are versatile, with sizes and growth forms ranging from the six-inch-high Aster alpinus to the towering seven-foot A. tataricus. Small, compact varieties are attractive massed, while large asters punctuate borders with intense color. One Maryland nurseryman says that when the New England asters (A. novae-angliae) are blooming, customers often think they are shrubs because each one produces such a huge mound of color.

Top Garden Asters

Here are a few of the best fall-blooming asters. The USDA hardiness zone ratings are not absolute, but rather a guide, since many factors besides temperature affect hardiness. The exact bloom time will vary by region, and flower color may vary with soil type.

New England asters (A. novae-angliae) are native through much of the country, from Vermont to Alabama, and west in North Dakota, Wyoming and New Mexico. They prefer moist soil and sun.

'Purple Dome', a recent introduction by the Mt. Cuba Center for the Study of Piedmont Flora, in Greenville, Delaware, is especially noteworthy. The plant has an outstanding compact habit to 18 inches tall and 36 inches wide. In late summer and autumn it is covered by semidouble purple blossoms. The flowers obscure the lower foliage, hiding any leaf damage caused by insects or disease. Easily mixed in borders and beds, 'Purple Dome' combines beautifully with Sedum 'Autumn Joy' or with the low-growing goldenrod, Solidago sphacelata 'Golden Fleece'. Zones 3 to 8.

'Alma Potschke' has brilliant rose-red flowers on stiff stems to three feet. It blooms September through October.

Other Choice Asters

White wood aster (A. divaricatus) is a native of eastern North American forests. Unlike most asters, it blooms well in dry shade. It's especially valuable in naturalistic woodland gardens, where it brightens dark corners with sprawling clouds of one-inch white daisies in September and October. The plants reach two to three feet. Their shiny, deep purple stems contrast beautifully with the flowers. Zones 3 to 9.

Heath aster (A. ericoides) is native in the eastern United States, from Maine to Georgia, and west to Minnesota, South Dakota and New Mexico. Tiny flowers in white, pink or lavender bloom in great profusion, smothering dense, bushy plants to three feet. Flowers sprays are particularly good for cutting. The plants tolerate some drought, though in dry climates they need supplemental water. Plant in a sunny spot. Zones 4 to 9.

'Blue Star' is a soft-textured plant with a myriad of starry bluish daisies with yellow centers.

'Esther' is one of the pink forms with pastel, orchid pink flowers.

'Monte Casino' (also sold as A. pringeli 'Monte Casino') is the popular white florist aster. The plants reach four feet tall and spread three feet. Each stem produces long fountains of tiny, solitary flowers. In mild-winter climates, such as zone 10 in southern California, plants go through two full bloom cycles: the first in spring and the second in fall. Cut back after the first bloom to maintain vigor. More tender than others of this group, 'Monte Casino' grows in zones 6 to 10.

Aster frikartii is a bushy, multistemmed aster with fuzzy dark gray-green leaves. Native to the Himalayas, it reaches 2 1/2 to three feet high and produces abundant two- to 2 1/2-inch lavender-blue, gold-centered daisies. This aster generally blooms from summer into fall. In the mild-winter western U.S., it blooms practically year-round if spent flowers are removed. Cut stems are useful in flower arrangements. The plants need sun and very well-drained soil.

This plant received mixed reviews from several of my consultants. Often recommended for zones 5 to 10, it is most successful in zones 7 and above. In zones 5 and 6, provide a winter cover of evergreen boughs. Only gardeners in the coastal West report consistently good results with this plant, and even there it is prone to whiteflies. But when it is good, it is really good, cloaking itself in a blanket of lavender. 'Monch' and 'Wonder of Staffa' are selections with a slightly deeper lavender color.

Aster laevis looks different than its relatives described here. Native to both the eastern and western U.S., it has shiny green leaves that grow in a clump something like a dandelion. Dark maroon stems reach three feet or higher and are cloaked with one-inch lavender-blue flowers in fall. This aster is mildew resistant and easy to grow given well-drained soil and full sun. 'Bluebird', a variety just introduced this year, is touted as being free from mildew and other foliage diseases. Zones 2 to 8.

Aster oblongifolius angustatus 'Raydon's Favorite' displays a profusion of blue-purple blossoms in September and October. Plants are three feet high and two feet wide, and the leaves have a pleasant minty fragrance when crushed. Zones 5 to 9.

How to Grow Asters

Spring is the favored planting time in all regions. Most species grow best with full sun and well-drained, fertile soil.

To minimize mildew, provide good air circulation by spacing plants liberally. Interplant with spring-flowering bulbs such as daffodils, or with wildflowers, to fill gaps early in the season before the asters fill out.

To promote dense, compact growth and eliminate the need to stake, pinch back growth tips two or three inches once or twice in spring and early summer before flowers begin to develop. Or shear asters back by half in spring.

The fastest-growing asters require yearly division in spring. Others need dividing every three years or so. After lifting and dividing, replant sections from outside the clump and discard the old center. Propagate by division or by stem cuttings in spring or early summer.

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