Asters should be pruned when the terminal bud appears, which often occurs when the plant reaches about 6 inches. Pruning or pinching an aster involves removing the terminal bud, which allows lateral buds to grow.
To pinch an aster, squeeze the stem until the tip breaks off. Pruning shears are also effective tools for this process. Pinching asters allows the stems to grow stronger and more compact, which requires less staking. Pinch asters two or three times during the growing season to encourage continued growth. Discontinue pruning by July 4 to prevent delayed flowering.
Asters bloom in the fall and are perennials, meaning they will return year after year. To protect asters through cold winter seasons, prune the stems down to about 2 inches above the ground and cover them with mulch. When spring arrives, cover with a thin layer of compost and a fresh layer of mulch.
Asters require a good balance of moisture. If it is in an area that does not receive at least 1 inch of rainfall in a week, it will need to be supplied with additional water. Be careful not to over-water, as too much water can cause the plant to lose foliage or prevent flowering. Remove a portion of the plants every 2 to 3 years to prevent overcrowding and to encourage growth.