The best time to trim trees depends on multiple factors, like the effects of insects, the surroundings, and how trees are prone to diseases. Many arborists acknowledge that the best time to trim trees is during winter. Most trees are dormant from November to March, which makes it the ideal time for trimming. During this time, trees are less prone to diseases or insects. Additionally, during the winter months, trees recover faster and will blossom when spring comes. Yet, if you want to save money on tree trimming, winter is the best time.