After “how?", the second most-asked question we get about pruning is “when?” (Or, "Can I prune this now?") The rule of thumb is to prune immediately after bloom for flowering shrubs, in late winter or early spring for non-blooming shrubs (particularly for heavy pruning), and not after mid-August for any shrubs.
But as in all rules of thumb, there are special cases. Damaged or diseased areas should be pruned whenever the problem is noticed. Shrubs prone to ice damage like wax myrtle and butterfly bush might be reduced in late fall or early winter to avoid breakage.
Light thinning or shaping can be done almost any time, including fall, on shrubs that have developed uneven growth since their last shaping. This can be seen on many types of shrub and is the result of vigorous summer growth. These stems look like they’re sticking out of the top of an otherwise nicely shaped shrub, and often the growth habit does not match the rest of the shrub.
To correctly prune this type of growth, follow it back to where it meets the branch it grows from. Make the cut as close to the parent branch as you can. Removing these all the way back to where they meet the branch they grow from does not encourage new growth.
Do not prune by cutting those stems to the outside of the shape of the shrub. That will stimulate new growth that may cause two problems: it will not have time to harden off enough to avoid winter damage, and this new growth will once again “stick out” and need pruning again.
Use this same type of pruning cut if you want to use branches from your evergreen trees and shrubs for holiday decoration. Removing holly, pine or magnolia branches this way won’t cause any problems.
Never shear a shrub in fall (or ever, actually, but that’s another article) and leave major pruning or renovation for late winter/early spring, or immediately after bloom for spring-flowering shrubs.