A Pruning Primer: Tools, Techniques and Timing
I’m not a perfect gardener. In fact, I’m glad that most people reading this don’t know where I live. My landscaping, while improving, doesn’t yet match up to the practices I write about. But after observing poor pruning practices by several landscape professionals, I felt less guilty and thought that an article discussing good pruning practices could help many of us. In the first instance, VDOT indiscriminately topped and side sheared the white pines that border our neighborhood entrance, scarring an otherwise appealing roadside border. Then, a landscaper squared off a neighbor’s pretty forsythia hedge that had previously featured the naturally sweeping shape of the shrub’s canes. Finally, a tree service pruned a neighbor’s oaks in August, despite numerous oaks in the area dying of oak wilt and apparently not understanding that open wounds during active growth periods add susceptibility to fungal disease. So, if the “pros” are using such poor practices, maybe a brief summary of good technique is in order. This article will outline pruning’s importance, summarize basic techniques, provide calendars for pruning specific plants, and cite references for readers to go deeper as needed.
There are several reasons to prune trees and shrubs:
- To improve structure: for example, removing branches that are structurally weak or interfere with other branches or plants.
- To control size or manage space: Not a substitute for putting appropriate size plants in appropriate spaces, but trimming to maintain a healthy and esthetic fit
- Repair or prevent damage or disease: This should be done promptly when an issue is identified
- Create effects: This is specialty pruning and won’t be dealt with in this article, but topiary, espalier, etc., are special pruning practices that create specific appearances or functions.
Pruning is generally broken down into three classes of plants: shrubs, deciduous trees and evergreen trees. While techniques vary, the tools used overlap.
The illustration above shows a cross section of basic pruning hand tools. A hand pruner, lopping shear, pruning saw, and combination saw comprise a nice starting point. Hand pruners are good for branches to about ¾” in diameter. Scissor action designs are preferred since they shear both sides with less crushing than anvil or snap cut designs. Ratchet action pruners are also available to provide a mechanical advantage that reduces hand force needed for a given cut. Lopping shears have more leverage and are generally the preferred tool for cuts up to 1.5” in diameter. Above this size a saw is necessary. Bow saws cut well but their shape can make it difficult to get into tight spaces, so a lower profile pruning saw is a necessity. Combination pole saw pruners are great for reaching high branches, offering both clipper and saw. Chain saws are also used for large work by those with the inclination. Hedge shears, like the hand shears shown above, or powered, are probably the most abused tools in the kit. Generally, they should be used sparingly, mainly for gentle esthetic improvements, without submitting to the temptation to shear shrubs into uniformity.
Fundamental Pruning Techniques
There are two basic pruning cuts that are used across all classes of plants to remove healthy growth:
- Heading cuts trim the leader off a branch or stem. They should be made just above a bud that is growing in a desirable direction. Heading cuts stimulate growth in the vicinity of the cut by redistributing the hormones that drive the growth of the terminal bud.
- Thinning cuts remove branches at an attachment point, rather than adjacent to a bud. The effect is to reduce density without stimulating new growth.
In either case, the cut should be made close to the bud or attachment point to minimize stubs which die back and can provide entry points for pests or diseases.
When cutting back shoots or branches that are dead or diseased, make the cut at the branch collar, the thickened section where the branch joins the trunk, or on healthy tissue. This assures that diseased parts are completely removed and prevents further dieback. Clean and disinfect the cutting tool between cuts with Lysol, Listerine or rubbing alcohol. Pine- Sol and bleach are not recommended because they are corrosive to tools.
Pruning cuts should be made at an angle of about 45 degrees to the branch. Also, when cutting back to a lateral branch, as in the thinning cut illustration above, the lateral branch should be at least half the diameter of the branch being removed.
In general, understanding and encouraging the natural growth habit of a shrub should guide pruning practice. Shrubs can have mounding (e.g. evergreen azalea, spirea), cane (e.g. forsythia, nandina, winter jasmine) or tree-like (e.g. witch hazel, rhododendron, crape myrtle) growth habits.
The important thing to understand is that heading cuts, where a terminal bud is cut off, generates thick new growth at the new end of the branch. When a shrub or hedge is sheared, removing many terminal buds, new growth flourishes at the outer perimeter of the shrub, allowing less light to reach the interior, resulting in a hollowing of the interior.
Thinning cuts are usually the preferred technique. For mounding and tree-like plants this means cutting at a lateral bud (a above), lateral branch (b above) or branch collar (c above). Don’t leave stubs. For caning varieties it means cutting older interior canes at ground level, allowing younger growth to flourish.
Shearing can be tempting to even out perimeter growth, but it is a short-term solution that leads to longer term issues. Better to use thinning cuts on older growth from the interior. Shrubs may not look as neat immediately after pruning is completed, but they will maintain a natural shape and be healthier in the long run.
For hedges, where formal shaping is desired, shearing is most difficult to resist. Follow the above principles to encourage balanced growth from the ground to perimeter of the shrub or shrubs. Shear minimally. And most importantly, the base should be wider than the top to allow access to sunlight and avoid ending up with hollow, leggy looking bushes.
Renewal pruning is recommended for older and overgrown shrubs. This means cutting plants back to 6-12 inches above the ground in early spring when new growth starts. By mid-summer, when new growth is 6-12 inches long, trim the tips at a lateral bud, as in (a) in the illustration above, to encourage new lateral growth and a compact shape.
Good deciduous tree-pruning practice includes a variety of measures:
- Remove suckers that grow from roots alongside the main trunk.
- Ditto for weak vertical shoots or water sprouts growing on branch interiors.
- Cut away dead, diseased, rubbing, or internally crossing branches.
- Manage branch spacing to promote a desirable visual effect and prevent one branch from shading another.
- Remove branches with weak attachments. Branch angles of 45-60 degrees to the vertical are desirable for structural strength.
- If young trees have multiple leaders, remove all but one leader and cut back laterals that are higher than the leader.
Two techniques used primarily for deciduous trees deserve mention:
If a branch is being removed, cut it at the collar, the slightly swollen area at the connection point of branch to trunk. Don’t cut flush with the trunk. Don’t leave a stub. A proper cut at the collar will heal and form a protective callus quickly, without further treatment or sealers.
Heavier branches, that will be unsupported during the cut, should be cut in three moves as illustrated above:
- Cut about half way into the branch from the bottom, a few inches from the collar
- Cut through the branch at a point just past the initial cut.
- Remove the stub using a collar cut.
This three-move technique prevents peeling off bark or tearing wood that may leave the tree susceptible to pests or disease.
Evergreens are grouped by branch arrangement:
- Whorled branches: growth forms a circular pattern around the growing tip. Examples include spruces, firs, and white pines.
- Random branches: grow in a random pattern, like yew, arborvitae, cedar and juniper.
Evergreen trees tend to have a single strong leader and typically require little pruning:
- Primarily, pruning is to remove dead, diseased or damaged branches.
- Avoid pruning branches in the inactive branch sections between green growth and the trunk of the tree, because whorled branch evergreens will not form new growth on the exposed stubs.
- New growth on whorled-branch conifers can be made denser by pinching or cutting off the tips of new growth (candles) in the middle of the growth as illustrated below.
- Random-branched conifers are pruned using similar practices as described for deciduous trees. General pruning is best done in early spring when new growth will cover pruning wounds. Maintenance pruning to control size can be done in early to mid-summer.
Timing: When to Prune
It is important to understand a few basic rules concerning when to prune:
- Flowering trees and shrubs fall into two groups:
- Those that flower in the spring that bloom on “old wood”, aka last year’s growth. These should be pruned after flowering in late spring/early summer. Common examples include azalea, dogwood, forsythia, redbud and rhododendron.
- Those that flower later in the year on “new wood”, meaning on this spring’s growth. These can be pruned during winter dormancy or in early spring before buds begin to swell. Crape myrtle and abelia offer examples.
- There are, of course, exceptions. Oak leaf hydrangea flowers in the summer on buds that form during the prior season.
- Dead, damaged and diseased material can be removed at any time.
- Late summer/fall pruning is a bad idea because new growth risks damage when cold weather arrives.
- Fall pruning is also a bad idea because fungal activity is high and plant susceptibility to disease is heightened when fresh wounds are exposed.
The following links provide pruning calendars for many specific plants, so if you’re wondering about when to prune a particular shrub or tree, you’ll find these very helpful:
A mindful task
Pruning can be viewed as just another gardening task. Or, it can be seen as a thoughtful, even artistic exercise. Pruning for plant health, beauty and function can be very satisfying gardening work for those of us who enjoy the esthetics of appealing landscapes. I hope that the principles and guidance offered in this article makes pruning a more effective and satisfying activity for all who read it.
The Virginia Master Gardener Handbook, Chapter 11, 2015 edition.