HARDINESS: Zones 4 to 8
PREFERRED SOIL pH: Neutral
PREFERRED SOIL TYPE: Average, well-drained
PREFERRED LIGHT: Sun to partial shade
ATTRIBUTES: Starry lavender, purple, pink, or white flowers in profusion in the fall
SEASON OF INTEREST: Late summer to fall
FAVORITES: 'Purple Dome' for compact habit; A. divaricates for dry shade
QUIRKS: Plants more than 2 ft (0.6 m) tall require support
GOOD NEIGHBORS : Chrysanthemum, golden-rod, rudbeckia, marigold, sneezeweed
WHERE IT GROWS BEST: Sun to part shade in overage soil and a wide range of climates
POTENTIAL PROBLEMS: Fungal leaf diseases
RENEWING PLANTS: Lives several years; dig and divide every 3-4 years
CRITTER RESISTANCE: Good
DIMENSIONS: 1-5 ft (0.3-1.5 mj toll, 1-3 ft (0.3-1 mj wide
Aster in the Landscape
Just when you think there's nothing to look forward to in the garden in fail except the sunset colors of tree leaves, asters burst on the scene with cool blue and silvery pink flowers. While a few species do bloom willingly earlier in the summer, the easiest asters to grow save their fireworks for autumn. The starry blue or pink blossoms with yellow centers are so numerous that they often hide the leaves entirely. And a few stems won't be missed if you bring them indoors to admire in a vase of water.
Asters often bloom in tandem with chrysanthemums, but this partnership works best with compact aster varieties that match the mums in terms of plant form and stature. Taller asters usually require staking, and are thus beloved fence plants suited to tying to posts or open rails. Goldenrod and sneezeweed are always welcome company for fall-blooming asters, as are the last of the marigolds and rudbeckias that manage one final autumn flush of flowering.
Sorting Through the Masses
Most of the asters sold by nurseries are descendants of two North American native species: New England aster (A. novae-angliae) and New York aster (A. novi-belgii), including hundreds of named cultivars. If ease of culture is your goal, begin with a naturally compact cultivar, such as 'Purple Dome', which grows to only 18 in (45.7 cm) tall and needs no pinching or staking. Where you want more height, consider 'Hella Lacy', a 40 in (101.6 cm) tall purple bloomer with excellent cold hardiness and good resistance to fungal leaf disease.
Indeed, disease can be a worry with many asters when they grow in warm, humid climates. If you notice fuzzy or moldy-looking spots on the foliage, trim off affected leaves and prune back or space plants widely to increase air circulation. Or consider growing a disease-resistant aster, such as the shade-tolerant white wood aster (A. diraricatus), which has starry white flowers and is happy under larger shrubs or in the dry shade of trees. Another disease fighter is A. x frikartii, which flowers prolifically with big blooms and is very vigorous.
Set out purchased plants in spring, in soil that is of average fertility. A scant ration of fertilizer can help get new plants going, but avoid feeding asters after summer begins. These tough flowers prefer to fend for themselves, and too much fertilizer can make them grow big, but weak-stemmed and short on flowers.
Pinch back tall varieties at least once, in early summer, to encourage branching. Stake or tie up the stems when they have grown more than 2 ft (0.6 m) tall.
Increasing the Bounty
After only two to three seasons, asters grow into large, crowded clumps. To relieve crowding, either pinch out one-third of the new stems soon after they appear in early spring, or dig up and divide the clump if you want to expand your collection. Small divisions can be dug from the outer edges of a clump well into early summer. If transplanted promptly and given plenty of water, the new divisions will bloom heavily in fall the same season.