Add a thin layer of compost (or a portion of balanced fertilizer) with a 2–inch layer of mulch around the plants every spring to encourage vigorous growth.
If you receive less than 1 inch of rain a week, remember to water your plants regularly during the summer. However, many asters are moisture-sensitive; if your plants have too much moisture or too little moisture, they will often lose their lower foliage or not flower well. Keep an eye out for any stressed plants and try a different watering method if your plants are losing flowers.
Stake the tall varieties in order to keep them from falling over.
Pinch back asters once or twice in the early summer to promote bushier growth and more blooms. Don’t worry, they can take it!
Cut asters back in winter after the foliage has died, or leave them through the winter to add some off-season interest to your garden.
Note: Aster flowers that are allowed to mature fully may reseed themselves, but resulting asters may not bloom true.
CHOOSING AND PREPARING A PLANTING SITE
Asters prefer climates with cool, moist summers—especially cool night temperatures. In warmer climates, plant asters in areas that avoid the hot mid-day sun.