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.
How to Trim Forsythia…. There are a few things to remember before you begin pruning. Forsythia’s have a natural arching habit in an irregular rounded shape and forsythia pruning should allow for this natural growth. Pruning forsythia into a formal hedge will remove most of the shrubs flowering potential. In a mature forsythia shrub, cut at least one-fourth to one-third of the oldest, thickest branches close to the ground. For the very oldest and most overgrown forsythia, pruning should be brutal, cropping the entire shrub to about 4 inches from the ground. New shoots will emerge and with careful trimming, forsythia can be renewed and rejuvenated. Within two years, you’ll have a new shrub. Younger forsythia shrubs are easier to maintain with regular care. Take out the oldest branches to make room for new shoots. This also allows more light into the center of the forsythia. Trimming back any straggling growth will go a long way in neatening the look of your shrub. Spend just a few minutes each spring on forsythia pruning and your reward will be many years of spring-is-here smiles.
All that being good…it’s totally acceptable to allow this gorgeous shrub to grow in a uncontrolled fashion….My personal preference is somewhat out of bounds of neat and tidy!!