Jan 08, Like many perennials, spring is the best time for pruning forsythia. Do this later in the spring after the blooms fade and the leaves are starting to grow. If you wait until summer or fall to trim forsythia bushes, you’ll find that the number of flowers that you get the next spring will be much less than desired.
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%. Prune these spring flowering shrubs soon after they have bloomed. Top prune the plant to improve the shape and reduce its size, but be careful not to remove more than 30% of the top growth.
Some summer pruning can also be done on particularly vigorous plants, but not after mid-July.
As the plant matures, some “renewal pruning” should be done. 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.