Pruning can be deep and dangerous water for new gardeners. Walking around the garden in spring we grow anxious to “tidy up” trees and shrubs when in fact the best medicine for the plant may be to do nothing at all. And as every “mature” gardener learns, leaving it alone is sometimes the best choice.
But there are definitely times when pruning is necessary. Neglected, overgrown shrubs that pose hazards to landscaping equipment or passersby need to be tamed; diseased or damaged limbs on trees should be cut off for safety reasons and to preserve the health of the tree; some trees and shrubs need to be pruned annually to produce better fruit, more flowers and prevent disease. How to prune and when to prune depends on the species of the tree or shrub, the time of year, and the reason for pruning.
“Pruning, like any other skill, requires knowing what you are doing to achieve success. The old idea that anyone with a chain saw or a pruning saw can be a landscape pruner is far from the truth. More trees are killed or ruined each year from improper pruning than by pests. Remember that pruning is the removal or reduction of certain plant parts that are not required, that are no longer effective, or that are of no use to the plant. It is done to supply additional energy for the development of flowers, fruits, and limbs that remain on the plant.”
Follow Proper Pruning Techniques, Texas A&M AgriLife Extension
Why the timing of pruning matters
Each spring, trees and shrubs start the annual process of maturation. New growth emerges on the plant, which is referred to as “softwood” because it is indeed soft. The new wood is very pliable and to a degree, can be bent without breaking. Softwood is not yet mature and cannot withstand freezing – this is why perennials are sometimes damaged by a late frost.
Throughout summer, the softwood is converted into hardwood and becomes stiffer. The bark thickens and the color changes from green to brown, or something close to it. The season’s growth, now hardwood, is able to withstand nibbles from animals and damage from wicked weather.
It seems counterintuitive, but pruning encourages the plant to produce new growth to replace what is removed, and the new growth can appear anywhere on the plant. When a plant is pruned in spring, the softwood has time to mature into hardwood before winter. But if pruned too late in the season, the new wood may not have time to mature and may die off over winter.
Equipment needed for pruning
- Long-handled Lopping Shears (aka “loppers”) for cutting branches up to 1-inch in diameter
- Hedge Shears for cutting shrubs like boxwoods and yews (never use them on trees)
- Hand-held Bypass (scissor action) Pruners for cutting twigs and stems and other small, unwanted growth.
- Bow Saw for cutting branches larger than one inch in diameter
- Pruning Saw is also for cutting branches where the bow saw won’t fit
Late fall or early winter is an excellent time to give the blades on pruning tools a tune-up by sharpening and oiling them to prevent rust. Blades of pruning equipment should be kept as sharp as possible to ensure clean cuts and should be disinfected with bleach after each use to prevent spreading diseases.
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When to prune trees and shrubs
Pruning is essential for maintaining the health, size, and form of trees and shrubs. During the first few seasons after planting a tree, pruning helps it develop a scaffold of strong, well-spaced branches with wide angles of attachment to the trunk. In many cases, trees that break and fall during storms are those that were never pruned correctly (or at all) to develop a strong structure.
When to prune shrubs depends on the plant’s flowering habit. Some shrubs produce flowers on “new growth” – stems emerge in spring and summer, and flower buds are set and bloom later in the season. On these plants, pruning needs to take place before flower buds are set. Other shrubs produce flowers on the former year’s growth, aka “old growth”. These shrubs typically flower March through May and the flower buds are produced the previous fall. As new growth appears, the flowers bloom (forsythias, lilacs, and azaleas for example). These shrubs should be pruned 4-6 weeks after flowering. If you prune these shrubs in late winter, there’s an excellent chance you’ll cut out branches that contain flower buds. On the other hand, shrubs like Dogwoods, grown for their foliage, should be pruned in early spring before new foliage appears. If a shrub is non-flowering, it’s treated like one that blooms on new growth.
Most deciduous trees should be pruned in late winter or very early spring prior to bud break when the tree is dormant. Dormancy is the time in the plant’s seasonal cycle when it has ceased all new growth and has prepared itself to survive winter. No leaves are present during dormancy, offering the best view of shape, damage, disease, and growth problems. A tree grows rapidly in the spring and pruning before new growth begins allows pruning wounds to heal quickly, reducing the chance of damage from pests and disease. Fruit trees should always be pruned before they break bud (leaf out) in spring.
TIP: Hydrangeas can be a little confusing when it comes to pruning, as their stems remain brown until just before leafing out. This makes it very difficult to tell what is old wood or new wood in early spring. Most gardeners grow Bigleaf Hydrangeas. If you prune this variety too early in spring, or in late winter, you’ll cut out the growth that contains the current year’s flower buds. To maximize blooms, wait until new foliage begins to grow in spring and only then prune the dead wood with no green flower buds or foliage. But (and there’s always a but in gardening), when to prune also depends on the species of hydrangea you’re growing. Check out this excellent primer from the University of New Hampshire on when to prune different varieties of hydrangeas.
Prune these shrubs after they flower:
- Flowering Cherry
- Flowering Plum
- Mock Orange
Prune these shrubs in early spring before buds break and new foliage appears:
- Alpine currant
- Burning bush
- Bush Honeysuckle
- Clematis (prune back to live wood)
- PeeGee Hydrangea
- Purpleleaf sandcherry
- Shrub Roses and repeat blooming Roses (prune back to live wood)
- Smooth Hydrangea
- Spirea (summer blooming)
To learn when to prune your favorite shrub, see this calendar from Virginia Tech, or download it here.
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“The two main types of cuts made during a pruning operation are thinning cuts and heading cuts. In most cases, both thinning and heading are used to encourage reasonably vigorous growth in the right directions. Thinning cuts remove entire branches back to the trunk, to main branches or to the ground. This opens a plant up to allow better light penetration and air circulation, while maintaining the overall form. Heading cuts remove branch tips back to lateral buds or small side branches. Generally, two to four new branches arise from the buds or branches just below the heading cut, increasing branch density (Figure 1). Often, when thinning cuts are made, the remaining branches will respond by growing long and leggy. Heading some branches after thinning will often reduce this legginess. Also, when some upper branches show the potential to compete with the terminal leader for dominance, heading them back will encourage the development of a strong leader.”
Pruning Ornamental Trees and Shrubs, University Of Missouri Extension
When to prune hedges
Pruning hedges can be done throughout the season. Once the hedge is formally pruned to the desired size and shape, it should be cut back whenever it adds 6-8 inches of growth. This usually occurs twice in the season, in spring and mid-summer. But the hedge should only be pruned back to within 2 inches of the previous pruning, to maintain healthy growth and avoid bare spots. Overgrown hedges should be renewed by removing up to one-third of the oldest, thickest stems or trunks, down to the ground. This encourages new growth from the roots. Hedges can tolerate this kind of aggressive pruning once per year. A hard cut like this can also be used to rejuvenate most overgrown deciduous shrubs in early spring like forsythia, spirea, and cane-growth viburnums. Note that this kind of hard cut may sacrifice that season’s flowers, but you’ll have a more manageable, attractive shrub.
Evergreens like Arborvitae, Pines, Firs, and Spruces rarely need pruning except to cut out growth defects, and damaged or diseased growth. Yews and Junipers should be pruned in early spring before new growth begins and should never be pruned in fall. Fall pruning of evergreens invites winter injury.
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Tip: To make Arborvitaes look fuller, remove the Apical buds at the very top of the plant in early spring. This forces the plant to produce lateral shoots further down the stem which gives the plant have a bushier appearance. This technique should not be used on other evergreens.
Types of pruning cuts
Pinching, shearing, and topping are all different types of heading cuts. With a heading cut, the growing tip of the stem or branch is removed, which stimulates the lower buds to begin growing. When used carefully, heading cuts encourage branching. But using heading cuts like topping to reduce the size of trees and shrubs can permanently harm their structure and health. If heading cuts like topping are necessary for a plant to fit in a location, it may be wiser to replace it with a more appropriate size plant.
Pinching – removing the active growing tips of stems with your fingers (hence “pinching”) – stimulates the growth of lower buds on the stem. This should be done early in spring. This pruning technique is often used on the soft new growth of annuals and herbaceous perennials to encourage more branching and flowers.
Shearing is used on hedges, where many dozens of heading cuts are made to make shrubs appear thicker than they would be naturally.
Thinning cuts (reduction or drop-crotch cuts) reduce the length of a branch back to a lateral branch. This type of pruning redirects the plant’s energy to stimulate stronger growth of the existing shoot rather than in forming new branches. Thinning cuts are used to increase light penetration, improve structure, or decrease height.
Removal cuts are thinning cuts that remove branches all the way back to the main stem or trunk.
Pruning tips and techniques
- Prune out dead, damaged, or diseased branches first, then remove any branches that are rubbing together and those that are growing inward toward the center of the plant.
- When pruning small branches and twigs, make the cut about ¼ inch above a bud, facing the outside of the plant. The new branch will grow in that direction.
- When pruning large branches, make three or four cuts with a saw to avoid tearing the bark near the final cut close to the trunk.
- Make the first cut starting on the underside of the branch about 18 inches from the trunk, stopping halfway through.
- Make a second cut one inch from the first one, but further out on top. Cut until the branch breaks free.
- Make the third cut close but not against the trunk.
- The fourth cut should be close to the trunk, being careful not to cut into the branch collar (the “shoulder” between the branch and trunk). The branch collar should be left intact with no branch stub to help the wound seal quickly.
- Do not use pruning paints – research shows that they are not necessary and the tree wound heals safely and faster on its own.
- When pruning hedges, shear the sides so the top is narrower than the base. This allows the entire plant to get light and doesn’t shade the base.
- When renewal pruning overgrown shrubs, cut 1/3rd of the oldest stems or trunks down to the ground. This encourages the growth of new stems from the roots. If the shrub is completely out of control, cut the entire shrub down to the ground in early spring. The shrub will not flower that season but will return to a manageable shape that year.
- Remove unwanted sprouts (suckers) growing from the base of the tree or shrub. In general, vertical shoots (aka water sprouts) growing from the roots or from horizontal branches serve no purpose.
- Make all pruning cuts clean and smooth. This encourages the rapid healing of wounds. Keep saw blades and pruning blades sharp and free of rust.
- Do not leave stubs since this is usually where dieback occurs.
Here is an excellent video from North Dakota State University on how to prune trees.
Sources: University of Minnesota Extension, Iowa State University, Penn State University Extension, Kansas State Research and Extension, University of Missouri Extension, Texas A&M Agrilife Extension, University of New Hampshire Extension, University of Georgia Extension.