April, May and June are not good months to prune because deciduous trees need to use their stored energy to produce new leaves instead of healing wounds. Also, pruning during warmer spring months encourages the spread of diseases such as Oak Wilt and damage by pests such as Emerald Ash Borer.
Trees also heal more quickly when pruned before spring bud break. Avoid pruning in fall. Pruning cuts can stimulate new growth that, unfortunately, will be killed as temperatures drop to freezing.
Late summer or early fall pruning causes vigorous re-growth of tissue, which in some cases may not harden off by the time winter arrives, leading to possible cold damage to the new growth.
DON'T prune during fall.
This can weaken and damage the plant—especially if there's an early frost.
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.
If you want to prune in fall, wait until trees drop their leaves and are dormant—usually October or November. After leaf drop, you can see the tree's structure and identify disease and insect problems more easily.
Any gardening expert will tell you, (contrary to what you may believe) that pruning encourages new growth just when the plant is trying to go dormant and new growth doesn't have enough time to harden before the first frost and freezing temperatures hit. Pruning at this time of year will severely weaken the plants.
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.
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.
Summer-Blooming Trees and Shrubs
Prune them in winter while they're dormant, or in early spring just before they push out their new growth. You can even cut them all the way to the ground in late winter, and they'll still bloom that same summer.
Summer is an excellent time for restorative pruning. Summer pruning tasks may include removing dead, damaged or diseased branches. Many people think of pruning as a winter task, but there are a number of great reasons to prune your trees and shrubs in summer.
Municipalities and homeowners often remove the lower limbs for pedestrian and vehicular clearance or to let the sun shine in for grass to grow. Mature trees, especially evergreens, benefit when healthy lower branches are left intact. Removing large limbs can increase the risk of decay.
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.
ALL-AROUND BAD CUTS
Even if only one branch was removed, you can tell your trees have been poorly pruned if you see any of these bad pruning-cut types: Stub cuts leave a stub of branch that doesn't allow the tree to seal off disease. If you can hang something off the end of a branch that's been cut back, it's a stub.
The period between the full and new moon (third and fourth quarters) is best for harvesting, slowing growth, etc.
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.
Pruning in August improves plant form just ahead of fall's growth spurt. Light shaping now will look beautiful after new fall growth arrives in the weeks ahead. Note: Plants should be well watered in the days ahead of pruning. Those that are experiencing drought stress should not be pruned.
Don't Prune in Late Summer and Fall
Do not prune deciduous shrubs in late summer. Pruning shrubs in August or early September may encourage a late flush of growth. This new growth may not harden sufficiently before the arrival of cold weather and be susceptible to winter injury.
Perennials NOT to Cut Back
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.
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 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.
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. There is less impact on the surrounding landscape and our crew can easily see what they're doing while all the leaves are gone.