May 22, 1. 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 your forsythia%. Jan 03, For rejuvenation pruning or if you’re trying to reduce the overall size of the shrub, it’s best to make your pruning cuts in late winter or early spring when the forsythia is still dormant.
Yes, this will sacrifice some of the spring-time blooms. But you’ll be able to easily see the structure of the plant to determine which stems to remove. Rejuvenate the forsythia in late winter or early spring, before the buds begin to swell.
Cut back the entire plant to within 4 inches of the ground with the disinfected shears. Rejuvenated forsythia won't bloom the first year after rejuvenation pruning, but they should resume flowering within one or two years.