Answer: Forsythia develops flowers on the most recent growth so if you prune them back now they will have the rest of the summer to grow and they should put on a spectacular flower show next winter. I would simply prune them back into a hedge shape. I don't know how large they are now, but to keep them healthy you won't want to remove more than about one third of the growth in any one pruning session. If they are much overgrown and your really want to reduce their size, do so over a 2-3 year period. This way you won't overly stress them and they will continue to grow and flower nicely for you. After they have finished flowering next spring you can prune them back again. Eventually you'll have the thickest, fullest, most spectacular forsynthia ever. Enjoy!
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