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.
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.
ALWAYS prune back to or just above a growing point (branch or bud) or to the soil line. NEVER leave a stem or branch stub. NEVER top a tree to “rejuvenate” growth.
Maples. Along with birches, elms, and other so-called "bleeder" trees, pruning maples in winter can lead to a great deal of sap. Wait until summer to avoid a possible mess.
Helps Control Disease
Some trees, like oaks, should only be pruned during the dormant season because they are especially susceptible to oak wilt disease, which is known to spread extremely fast in the spring and summer.
Tips on Summer Tree Trimming
The warmer months are a great time to do some structural cleanup. Remove suckers, water-sprouts and branches that are crossed or rubbing each other. Take out “the three Ds” where you see them, too, (aka dead, diseased and damaged wood).
Summer is an excellent time for restorative pruning. You can correct problems that have resulted from over-pruning or poor pruning. You can also clean up damage from winter and spring storms. Proper pruning will begin to restore most plants' natural shape.
To avoid certain diseases, pruning in the late winter is especially great. Avoid Oak Wilt disease by pruning oaks between November and March.
Here's when it's time to put down the pruners: In the fall. As your tree starts dropping its leaves and gets ready for winter, the last thing you want to do is prune. This only wakes the plant back up and tells it to start growing again.
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.
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.
Differences Between Pruning and Trimming
Webb said pruning typically involves removing dead or diseased wood and thinning out stems and branches to improve the overall health and appearance of a plant. On the other hand, trimming typically involves cutting back plant material for reasons other than health concerns.
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.
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.
If your tree has a less severe problem, one of the best times to remove trees is during the dormant season, between late winter and early spring. Here's why. Dormant trees are leafless and lighter, so it's much easier for a certified arborist to cut and handle the branches.
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.
It's a common misconception that tree maintenance cannot be done during the winter months, or that tree care companies don't operate during this time of the year. The reality is that winter is a good time for pruning and tree removal services.
In February, harmful tree diseases are at a minimum and deciduous trees have already “hardened off”, meaning they have prepared for winter dormancy. When the growing season begins, the wounds will be sealed, and the healing process will begin. July and August are the second-best times to prune.
February and March are statistically the cheaper months for tree removal—as they are otherwise known as “dormant season” for trees. During the winter the leaves have fallen from the trees making it easier for a tree service to cut down the tree safely and quickly.
There is never a bad time to remove dead, damaged or diseased branches. But most trees benefit from pruning in mid to late winter. Pruning during dormancy encourages new growth as soon as the weather begins to warm.
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.
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.
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.