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 during fall.
This can weaken and damage the plant—especially if there's an early frost.
Don't Prune in Late Summer and Fall
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.
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.
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.
Late spring, early fall, or winter (when the hedges are fully dormant) are the best times for pruning hedges. Don't prune hedges during very hot, dry weather, and prune well before your first frost date.
Pruning at this time of year will severely weaken the plants. This is disastrous for the plants and all the hard work you did during the year to make your landscape beautiful.
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.
Common Mistakes
Prune just above a healthy bud, with the bud pointing in the direction you want the plant to grow. Cut at a 45-degree angle, with the low point of the cut opposite the bud. Don't cut too far from or too close to the bud you want to encourage. Leave a “collar” when cutting off a branch.
The inner limbs need air and sunlight, so cut back along the branch just above new growth or at the plant base; avoid electric trimmers for this. Trimmers just top the shrub, encouraging new growth near the cut. Cut farther back to encourage growth along the limb.
During excessively cold temperatures
It's minor, but it could impact the look of a plant or create a haven for insects and diseases later. Don't prune when the temperature falls below 25°F.
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.
You probably already know just how fast shrubs grow and start to look messy. If you wait too long, your shrubs will start to become an eyesore. Pruning less of the plant but more frequently is by far the best for the overall health of it. We recommend every other month, which comes out to be five times per year.
The period between the full and new moon (third and fourth quarters) is best for harvesting, slowing growth, etc.
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.
You should trim your hedges at the same time of year you trim most of your trees — late winter to early spring. If your hedge has gotten out of hand, power hedge shears may be the best way to begin. Follow up by hand with freshly sharpened hedge trimming shears to clip away all the stray leaves or branches.
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 to Cut Back Hostas. As a general rule, hostas should be cut back in the late fall. Start with leaves that have wilted or turned brown. Healthy leaves can stay a bit longer to help the roots store needed energy.
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.
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.