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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. If you wait to prune your forsythias until after the springtime, the number of flowers that your plant yields next season will be considerably less. This is a good idea if your bush is large and unruly and you want to tame it back. You may sacrifice a season of flowers, but your forsythia bush will look much better the next year. Young forsythias need little to no pruning. Only when they mature should you prune them once a year. If you want to help maintain the shape of the plant as it grows, you can snip off a few longer stems once a year, though this is not required. Letting the plant take its natural shape helps it establish a healthy root system.
Prune forsythia plants right after they finish blooming in the spring. Prune in the summer, fall, or winter to reduce the number of flowers. Avoid pruning your forsythia when the plant is 1-3 years old.