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.
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.
According to Ramsey County Master Gardener Volunteers, February is the best time to prune your trees. In February, harmful tree diseases are at a minimum and deciduous trees have already “hardened off”, meaning they have prepared for winter dormancy.
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 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.
As a general rule, prune spring-blooming trees immediately after they flower. Prune trees that bloom in the summer or fall in late winter or early spring before the end of their dormancy. Routine pruning of dead or dying branches can be done at any time.
For most trees, the best time for major pruning is late winter to early spring because wounds close faster. Pruning in late summer and early fall may also stimulate new growth, which has little time to harden before cold weather comes.
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.
By not cutting back the statuesque achilleas, eryngiums, perennial astilbes, sedums, alliums and many ornamental grasses, you can enjoy their structure against a winter sky. The sight of their stems silvered with frost adds a whole new level of interest to the garden.
With fall garden cleanup in full swing, maybe it's all the raking and mulching that has people going bananas. But before you start hacking at your trees and bushes, take a tip from a seasoned gardening expert. "The rules of fall pruning are simple: Prune nothing in the fall!
Choose the Right Time
Likewise, prune in the early morning or late afternoon to avoid the newly cut ends of branches becoming scorched by the hot sun. Wait 48 hours after rain to prune, to allow plants to dry out and avoid fungal diseases. When pruning, make cuts just above leaf nodes, or buds.
Avoid pruning in fall.
Pruning cuts can stimulate new growth that, unfortunately, will be killed as temperatures drop to freezing. Trees and shrubs reduce their energy production as the growing season ends, so new growth in autumn will use a plant's stored energy reserves.
"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.
The 1/3 rule involves cutting about 1/3 of wood during any pruning activity. This moderate pruning practice is a balanced approach to thinning out shrubs to stimulate new growth. The plant loses a good amount of stems, usually on the top section, allowing more light and air to enter the inside of the plant.
Watering is very important during the couple of weeks or more that a plant is recovering from root pruning. Right after pruning, I give the whole pot a thorough soaking. As new growth begins, I make sure to thoroughly wet all the soil at each watering.
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.
Deciduous Trees
Most trees should be pruned during the late dormant season (February through March). February through March is generally regarded as the best time to prune most deciduous trees.
If it's raining – don't prune! Wait until you have a few days of dry weather. Rain can spread disease. Get the right tools.
Spring flowering shrubs and trees.
Shrubs and trees like lilacs, azaleas, and others will grow best when they are pruned right after they flower in the spring. Avoid pruning them in the winter as it may stunt their growth.
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.
A rough rule of thumb is to prune spring-blooming shrubs soon after they finish flowering because most bloom on old wood, while those that bloom in summer and fall usually bloom on new wood and can be pruned in late winter or very early spring.