Pruning Overgrown Forsythia

My forsythia is overgrown. When and how far can I prune it back?

Enjoy the blossoms and then get out the saw. Prune spring flowering shrubs such as forsythia, lilac and bridal wreath spirea once they finish flowering. Forsythias can be pruned back to several inches above the ground level. The resulting growth is often larger than the plant you started with. Consider spreading the pruning over several years to avoid this excessive growth. Remove one third of the older stems to ground level. You can reduce the height of the remaining stems by one third. Make these pruning cuts just above a healthy bud or where branches join. By the end of three years you will have a smaller plant and young healthy growth. 

Related

Article

Spring Flowering Shrubs Blooming in Fall