Hedge trimming should be done bi-annually.
Spring – hedges are trimmed to remove the winter damage, and prepare them for their growth phase.
Late Summer / early Fall (Jul-Sept) – is the best time for a second trimming, after maximum growth, when you can shape the hedges, so they will maintain their shape during their slower rate of growth.
If you have a lot of hedges, if there is some risk involved (i.e. elevated or sloped surface), or you just want to make sure the job is properly done, you should hire a professional arborist. For large jobs, it is almost always more economical (and much faster) to hire a professional.
Note: Pruning too late in the fall may risk damage to the hedges because they may not have time to heal the cuts before the frost sets in. The best time for a second trimming is late summer after the hedge has slowed down it’s rate of growth.