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First published on garden.org on February 12, 2009, by Suzanne DeJohn
It's pruning time in the apple orchards along Lake Champlain, a sign that home gardeners should be sharpening their shears, too. At first sight it would appear that apple trees grow naturally in their characteristic spreading shape with low-hanging, horizontal branches. But appearances are deceiving: Orchardists methodically prune their trees every winter to encourage that growth pattern.
Prune With a Goal
Pruning is a way of manipulating a plant's growth for a specific purpose. For an apple grower, the goal is maximizing fruit production. In your landscape, you might want to limit a plant's size, promote flowering or fruiting, create a more pleasing shape, rejuvenate an overgrown plant, or remove branches to allow more sunlight to pass through. How you prune will depend on the type of plant and what you are trying to accomplish.
Many gardeners are reluctant to prune, fearing they'll harm their plants. But pruning a plant correctly can improve its vigor and attractiveness. The keys to pruning are having a goal in mind, then doing your homework so you understand the best technique for each type of plant. You should have a reason for every cut you make. Some plants, such as buddleia, forsythia, and red-twig dogwood, are so vigorous you can cut them down to the ground and they'll regrow. But most other plants prefer a lighter touch.
When to Prune
Many woody plants are best pruned in late winter or early spring, just as buds are beginning to swell. Summer-blooming shrubs fall into this category, including butterfly bush (Buddleia davidii), rose of Sharon (Hibiscus syriacus), and summersweet (Clethra alnifolia). The plants produce flowers on the current season's wood ("new wood") so by pruning now you'll encourage lots of new growth. Most fruit-bearing trees and shrubs should be pruned in late winter, too, if you're growing them for fruit production rather than their spring flowers.
Spring-blooming shrubs, such as azalea, lilac (Syringa spp.), and mock orange (Philadelphus spp.) are best pruned immediately after flowering. These plants set their flower buds during the previous growing season ("old wood"), so pruning them now will remove the flower buds and you'll sacrifice some of this spring's show. Instead, prune immediately after flowering, so the plant can produce new growth this summer -- and new flower buds -- in time for next year's bloom.
Some plants benefit from a second light pruning in midsummer. Orchardists often prune upright shoots on apple trees, for example, because these "waterspouts" don't produce fruit. Horizontal branches tend to produce more fruit than upright ones. The growers want to limit vegetative growth and redirect the plant's energy to the fruiting wood. Your summer pruning might consist of pruning errant branches on a shrub to keep the plant tidy.
In most cases, it's a bad idea to prune in autumn because this encourages new growth that may be damaged by the winter cold. Fall pruning can also interfere with a plant's hardening off process.
Follow the Natural Shape
In general, it's best to prune in a way that follows the plant's natural shape. So, instead of removing the tips from all the branches on a shrub to make a perfect sphere, it's better to cut some branches further back and allow some to remain as they are. As the plant leafs out you can re-evaluate and make further cuts to give it a pleasing form.
Formal hedges, such as boxwood, are often sheared to geometric shapes. However, sheared hedges will form most of their foliage on the branch tips, leaving the interior of the shrub leafless. If you want to reduce the size of the shrub or encourage a more natural shape, you'll have to do it over several seasons, cutting some branches back to the main trunk to allow light to penetrate and to encourage its natural growth pattern.
Here are a few pruning guidelines:
1. All plants benefit from having diseased, broken, damaged, and rubbing branches removed, and this can be done at any time of year.
2. Prune back to another branch or outward-facing bud.
3. When cutting large branches, cut halfway through from the top, then make a cut underneath, then finish the cut from the top. The bottom cut prevents the bark from stripping when the branch falls.
4. A general rule of thumb is to remove no more than one third of the growth in any one season. If you need to remove more, wait a year to avoid stressing the plant too much. (As mentioned above, some plants can be cut right back to the ground, but they're the exception.)
5. Use sharp tools for clean cuts.
6. Avoid sealers unless there is a specific reason to use one, such as to minimize attack by borers. In general, pruning cuts heal faster when left unsealed.
7. Walk around the plant as you prune, to keep the plant reasonably symmetrical.
8. Stay safe. If you're not up to a big pruning job, hire a professional.
I love driving by the orchards and seeing all the pruned branches on the ground; it's a signal that warm weather is just around the corner. So take your cue from the apple growers and head out to prune on a sunny, early spring day.