The best time to prune asters is not in the fall after they've bloomed. Pruning asters is best done in the summer. According to Burpee Seeds and Plants, prune your asters in early summer with hedge clippers to about 18 inches high, which will encourage the plant to branch out and will also encourage more blooms.

When should asters be cut back?

Prune By Cutting Back

Cut back aster plants in late autumn, after the first hard frost. Snip off the stems 1 to 2 inches above ground level. Rake up and discard the pruned matter into a green waste bin to discourage insect pests from colonizing the soil around the asters.

Do you cut back asters UK?

Asters do not strictly need to be pruned, but there are some good reasons to do it. One is simply to maintain a shape and size that you like. Especially if you have rich soil, these flowers will grow abundantly. Pruning them back can prevent the need to stake them and give the plants more pleasing shapes.