We would generally not advise to prune in high heat in the middle of the summer beyond any broken or dying branches. It also depends on the plants and if it is the right time of year due to blooming etc.
During hot, dry weather
Robbing them of their reserves of moisture by pruning away foliage can cause death. Instead of reaching for the shears, spend your time moving the sprinkler onto your plants. Excessively high temperatures put undue stress on your plants too.
Active cells are liable to be injured by cold weather, which is a reason to avoid pruning in late summer or autumn except in climates with mild winters or with plants that are very hardy to cold.
Do not prune heavily in late-summer or early-fall, because this may stimulate an unwanted spurt of growth that will be tender, and may suffer from winter injury. Late-Fall Pruning helps to prevent damage from heavy snowfalls. Fall pruning also helps eliminate unwanted insect and disease problems.
PRUNING: Generally it is safe to prune most trees in mid-summer, from roughly mid-July to mid-August to be more specific. However, you may want to pause pruning during an extreme heat event, including right before and right after an extreme event, so consult the weather forecast as part of your pruning plan.
Remember: pruning trees in spring can leave them more vulnerable to insect infestation and diseases. That's why you don't want to prune these trees in spring, summer, or early fall: Oak trees to reduce the chance of oak wilt (if oak wilt is in your area) Elm trees to reduce the chance of Dutch elm disease.
Most plants are pruned in winter, when dormant, but there are exceptions. Cherry and plum trees, for example, are susceptible to silver leaf disease, and should therefore be pruned in summer when the risk of infection is reduced. Likewise, most spring-flowering shrubs should be pruned immediately after flowering.
The 1/3 rule for pruning shrubs refers to the practice of cutting about 1/3 of wood or a branch. This approach is applied to fully established shrubs and small trees. This is done at this stage since the established shrubs and plants are past their transplant shock.
The best time to prune is between mid-February and early May. Trees pruned at this time in early spring develop a callous around the cut much more rapidly than those pruned at other times. However, there are a few exceptions to this rule.
Trees that Benefit from Summer Pruning
Some people prefer a summertime prune for these trees because they typically ooze sap when pruned in winter and early spring. Trees with dead branches: If you spot dead, broken, or dying limbs, you can remove those at any time.
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. Best time to trim hedges woule be on a cludu day to avoid excessive leaf burn.
Summer prune timing
This is normally around the last week of July or early August. If done too early, the branches regrow leaving soft unripe wood that won't survive too many frosts. If pruned too late, the pruning wound has less time to 'heal' again resulting in winter die-back.
However, you can prune shrubs year-round. Summer is as good a time as any as long as you pay attention to the particular needs of the species and keep your landscape goals in mind. Sometimes, weather or other priorities make pruning challenging during the winter.
The general answer is around 90 degrees F, with some exceptions to the rule. This means that when temps rise above 90 and remain there for a lengthy spell: Leaves wilt. Water evaporates into the atmosphere more quickly in high heat, draining a plant's reserves.
If you notice that your trees aren't growing as well as they should be or are losing foliage, it's possible that you've pruned them too much. Additionally, an increase in interior sprouting indicates over-pruning, which is when they grow sprouts internally rather than on the tips of limbs.
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.
If you learn nothing else about pruning, remember the May Rule. If the shrub blooms BEFORE May, then prune the plant immediately after (or while it's blooming to bring the blossoms inside for an arrangement) the shrub has bloomed.
Late Summer Pruning (August-September) Few plants are pruned this time of year because pruning encourages new growth that does not have time to harden off before winter. Late-season pruning typically does more harm than good.
After pruning, give your trees a thorough watering to help them begin their recovery quickly.
LaRene Bautner from Millcreek Gardens demonstrates a trimming tutorial with focus on the “3 D's for pruning” : Dead, Diseased, Damaged.
Make pruning cuts just outside the branch collar to avoid damaging the trunk and compromising wound responses. Improper pruning cuts may lead to permanent internal decay.
When making pruning decisions, keep in mind that you can safely remove up to one-third of the plant's growth at any one time. There may be times when you prune more, such as when you are rejuvenating an overgrown shrub, but generally speaking, the “one-third rule” is the best guideline to follow.
Always prune just above a node on a stem.
In this way, the dormant buds in the node itself will grow out into new stems. If you cut below a node, you leave a section of stem (the internode) that cannot grow new stems. This section will be prone to rotting and becoming susceptible to diseases that can kill your plant.