The best time to trim trees depends on the species and climate. For most trees, the dormant season (late fall to early spring) is ideal to minimize stress and promote growth. However, for some species, such as oak and elm, pruning during the growing season can increase the risk of disease or insect infestations. Additionally, pruning flowering trees should be done after they have finished blooming to avoid removing potential flower buds. It's always best to consult a certified arborist to determine the specific needs of your trees and the optimal time for trimming.