Most deciduous trees and shrubs are best pruned while they are dormant, after the leaves have dropped and before the buds open in spring. On the Western Slope that window runs through the winter months and into early spring.
Why dormant season
Without leaves, the full branch structure of a tree is visible. Crossing limbs, weak attachments, and dead wood are easy to identify, and cuts can be placed precisely. Insects and disease organisms are largely inactive in cold weather, so fresh cuts are less exposed than they would be in summer. When growth resumes in spring, the tree directs that growth into the branches that remain.
Snow and wind
Removing dead, weak, or poorly attached limbs before heavy snow reduces the chance of a branch failing onto a roof, fence, or driveway. Wet spring snow is often what brings down a limb that has been weak for years.
Fruit trees
Fruit trees are pruned while dormant to open the canopy to light and air, remove crossing and upright shoots, and keep the tree at a height that can be maintained and harvested. Late winter, before bud break, is the standard time for this work.
What should not be pruned in winter
Shrubs that bloom in spring, such as lilac and forsythia, set their flower buds the previous summer. Pruning them in winter removes this year's flowers. Those shrubs are pruned shortly after they finish blooming instead, and we will tell you which plants on your property fall into that group rather than prune them on the wrong schedule.
Summer work is covered under Summer Pruning. Dormant pruning pairs well with a dormant oil application in late winter.