Cut them low to the ground with a handsaw.
Jun 22, 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 (10 cm.) from the shrubnotch.clubted Reading Time: 2 mins.
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 shrubnotch.club: Emma Erler. Jan 03, Forsythia are vigorous shrubs that quickly bounce back from this type of rejuvenation pruning (provided that the shrub is healthy to start with). Keep in mind that forsythia are large shrubs so even with rejuvenation pruning you’re likely to quickly end up back where you started; they can grow 8 feet stems in a single season!Estimated Reading Time: 4 mins.
May 22, Prune forsythia plants right after they finish blooming in the spring. Wait for your yellow flowers to fade and for the leaves to grow. At this point, the plant starts to develop next year's buds, and the plant carries the buds through the fall and winter season.
This is the best time to prune 95%.