The best time to prune Forsythia is right after it blooms - sometime in April or May.
The best way to prune Forsythia is to selectively cut old canes out from the base of the shrub. Shearing with hedge trimmers is not advised. This takes away the natural form of the Forsythia which is part of what makes it so beautiful.
Then let the forsythia grow during the summer and it will produce next years flowers on the new canes.
Please never prune Forsythia (or any spring-flowering shrub) in the fall. Check out this blog post about pruning shrubs.
|Forsythia blooming in Salt Lake City - pruned correctly provides sprays of color!|
|Forsythia in the spring garden in downtown Salt Lake City|
|sheared Forsythia can't achieve it's natural open form|
|Forsythia blooming in Utah|