Summer flowering shrubs should be pruned in late winter or early spring. This still gives them time to set flowers for the following year. Deciduous shrubs that aren't grown for flowers can be trimmed back at the same time.
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.
Late winter is a good time to prune most plants. Because plants and their pests are dormant this time of year, pruning cuts can be made without the risk of pests and pathogens entering the tree through the wound.
As mentioned earlier, watering plants several hours before a freeze is the first defense against damage. During a temporary cold snap, cover entire shrubs with burlap, sheets or blankets for insulation. For the best protection, use a frame to prevent the covering from touching the plants.
Know Your Plants' Cold Tolerance
Some frost-tender plants need to be covered as soon as the temperature hits 32°F, while others can handle lower temperatures and/or longer freezing periods. You'll need to look up each of your plants to verify its cold tolerance.
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.
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.
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.
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.
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.
DON'T prune during fall.
This can weaken and damage the plant—especially if there's an early frost.
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.
Pruning in Winter Causes Less Stress for Trees
Research shows that pruning before buds open in spring leads to “optimum wound closure.” Trees are able to heal from pruning cuts before warmer weather brings out destructive insects and pathogens.
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.
Prune most broadleaf evergreens now: abelia, boxwood, cleyera, elaeagnus, nandina, privet, and holly. Wait until after flowering for spring bloomers. Viburnums (both evergreen and deciduous) can be cut back hard to reduce size, but you will sacrifice flowers for the season.
Cover Plants – Protect plants from all but the hardest freeze (28°F for five hours) by covering them with sheets, towels, blankets, cardboard or a tarp. You can also invert baskets, coolers or any container with a solid bottom over plants. Cover plants before dark to trap warmer air.
You may wonder how to protect plants from frost when they are planted in the ground? One method – which is useful for larger garden plants and shrubs – is to cover them with horticultural fleece. You could use blankets or bubble wrap, too, to create a protective cover.
A fabric covering is best because it will allow moisture to escape while still protecting your plants from frost. Fabric coverings will prevent the freezing air from coming into direct contact with the moisture on the plant while also capturing the heat that is radiating from the ground.
Cover with a burlap sack: After wrapping a shrub with twine, encase it in a burlap sack or commercial shrub wrap if cold, high winds are a threat. The burlap weave is wide enough that the plant can breathe, but it keeps the worst of the biting wind at bay. This covering also protects the shrub from browsing deer.
Once temperatures drop below 40°F, you may want to start covering your plants with a frost blanket depending on what you're growing, which we'll get into in just a bit. Freeze – A freeze is when the air temperature is 32°F or below.
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.
Fall isn't the right time to prune trees and shrubs. There are good reasons to wait till late winter or after the plants' leaves have fallen. Pruning too soon can harm your trees and shrubs.So, when it comes to fall pruning, procrastination is the way to go.