Forsythia produces flower buds on current season's growth, so if you want to maximize the flower show, prune shrubs shortly after they finish flowering. All pruning should be completed before mid-July in order to give plants enough time to put on new growth and develop flower buds.

When should forsythia be pruned?

When to Prune Forsythia

The best time for trimming forsythia is in the spring after the blooms fade. Pruning forsythia in late summer or fall will reduce the number of flowers in the spring since these shrubs bloom on old wood and set their flower buds soon after new growth appears.

What to do with forsythia after flowering?

The timing is easy – always do it after flowering. It's important to prune then as forsythia flower on shoots grown the year before flowering, so if you prune too late you'll be removing the flowers for next year. Shape by removing about a quarter of the length from all stems.