Set A. x frikartii 15 to 18 inches apart, others 18 to 24 inches apart in sun. A. frikartii will also grow in light shade and has an exceptionally long blooming period that starts in summer. In general, asters prefer fertile soil, but will tolerate poor soils as well. Soil should also be well-drained, but take care to provide ample moisture, particularly in fall when plants are in bloom.
Pinch or even shear back several times during the early part of the growing season to promote vigorous, compact growth. Divide every 3 years or as necessary, replanting the younger, more vigorous pieces from the outside of the clump.
To control mildew naturally, avoid overhead watering, and plant away from hedges or fences to assure satisfactory air flow. If you must, a monthly spray with wettable sulfur will ensure mildew control.