Forsythia putting up 10 foot stems.
How should I prune these long stems?
Forsythia has already made the blooms for next spring. Any pruning now will remove blooms but that is your decision. Most folks prune forsythia right after it blooms, removing the oldest stems to the ground. About 1/3 of the stems should be pruned out annually. That may seem drastic but the shrub responds with new growth that grows 5+ feet in a season. And new growth yields more blooms than old wood. It's natural shape is like a fountain; tall stems that eventually arch over to the ground. If your shrub is in a place that doesn't have enough room for the natural state, you can prune it into whatever shape you like. For the largest number of blooms, still remove 1/3 of the oldest stems to the ground annually. If you follow the 1/3 rule, you may find that the overall size works in its current location. Also consider hybrid forsythias that don't grow so tall. Fall is a great planting time for shrubs if you want to make a change.