According to gardening expert Mike McGrath, author of Mike McGrath's Book of Compost, and radio host of the show You Bet Your Garden, the rule of pruning is that you should NOT prune in fall. That's right, nothing and no exceptions.
Generally, the best time to prune or trim trees and shrubs is during the winter months. From November through March, most trees are dormant which makes it the ideal time for the following reasons: Trees are less susceptible to insects or disease.
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.
Don't prune too late in northern regions.
In areas with cold winters, avoid pruning after the middle of August. If you prune too late, you may stimulate new growth that would not have time to grow thick, protective bark before the killing frosts of winter.
Pruning to remove damaged, dead or diseased parts can be done at any time of the year. Most trees and shrubs, especially those that flower on current season's new growth should be pruned in late winter or early spring before the onset of new growth. (March-April).
Prune evergreen shrubs, such as juniper and yew, in late March or early April before new growth begins. Light pruning may also be done in mid-summer. Avoid pruning evergreen shrubs in the fall. Fall-pruned evergreens are more susceptible to winter injury.
Don't shear the branches of spring flowering shrubs (Forsythia, Lilac, New Mexico Privet, Spirea, Flowering Quince, and others). These shrubs produce flowers on last year's wood, so removing old growth will reduce or prevent flowering. These plants should be pruned immediately after they are done blooming.
An improper cut like a flush cut (cutting too close to the trunk) or a stub cut (cutting too far from the trunk) can cause irreversible damage to a tree. A flush cut removes the branch collar and leaves a large wound in the side of the tree that won't heal properly.
Last chance to trim deciduous hedges to keep them looking tidy over the winter. Climbing roses should be pruned now if not done last month. Shrubs normally pruned hard in the spring such as Buddleja davidii, Cornus alba, and Lavatera, can be cut back by half now, to prevent wind rock and to neaten their appearance.
"As soon as trees and shrubs go into dormancy, pruning can begin," he said. "And you can prune up to the time when buds begin to plump up." From mid-November to mid-March, you can shape and thin your deciduous plants in preparation for the spring season ahead.
Some perennials, like hostas, peonies and daylilies, need to be pruned in fall to avoid winter damage. Plants like these should be pruned after the first few frosts in late fall or early winter. Other perennials like mums and coneflowers are better off being pruned in spring just before new growth comes in.
October sees the start of the dormant season which is the best time to prune lots of deciduous garden trees. You can prune newly planted trees to remove any damaged growth and help balance the shape of the canopy as well as maintain a dominant main leader.
The best time for pruning is February on a frost-free day. In late winter, you can also give your buddleia a drastic pruning to rejuvenate it. In autumn after flowering, you can also prune your butterfly bushes, you should, however, not proceed so radically as in the winter pruning.
Also, do not cut back hardy perennials like garden mums (Chrysanthemum spp.), anise hyssop (Agastache foeniculum), red-hot poker (Kniphofia uvaria), and Montauk daisy (Nipponanthemum nipponicum). Leave the foliage. It's important to protect the root crowns over winter.
Pruning evergreen bushes is best done in late March or early April before new growth begins. Light pruning can also happen in late June or early July. Avoid pruning evergreen shrubs in the fall, since they are more susceptible to winter injury.
Thuja plicata conifers (western red cedar) should be pruned in spring and early autumn. Never trim any type of conifer hedge after August though, as this can lead to the development of bare patches.
The period between the full and new moon (third and fourth quarters) is best for harvesting, slowing growth, etc.
The best time to prune is after flowering. If the plant needs to be renovated, or severely reduced, this can be done late winter and early spring just before growth begins.
When is Late Winter? Late winter is 4 to 6 weeks before spring thaw begins. This could be any time in January to May, depending on your climate. Use your average last frost date and count back.
Pruning Too Early
If you prune too early in the season, there's a good chance you'll snip the buds right off their branches, which means no flowers in the spring. Before you cut, do a little research, like with our pruning guide, or just wait to trim until the plant has finished blooming for the year.