Is it time for tree trimming? As a general rule, experts say to trim trees once a year during the dormant season, which varies by species. However, certain circumstances call for immediate trimming. If you have branches hanging over your roof, tree trimming is a task that cannot wait. ​Overhanging branches are a hazard, and ignoring the issue could result in costly damage. How Overhanging Branches Can Cost You When branches grow to hang over your rooftop, they become threats