A pruning primer
By Adrian Higgins,
The first half of March offers a golden moment for pruning woody plants, when the days grow a little milder and longer, and the trees and shrubs are still without leaf.
Every garden has plants that would benefit from pruning, but only if it is done competently. So much of the chopping you see is so ill-considered that the better course would be to leave the plants alone.
The hallmark of bad pruning? Branches that are chopped arbitrarily. This is driven by the view that if a tree no longer fits its allotted space, make it conform, no matter where the plant wants to branch or project its latent buds, no matter if this destroys its natural beauty. Art and science are crushed under the heel of utility.
It would be one thing if this were done by a well-meaning but misguided homeowner. To see it done by a crew of “professional” landscapers is disturbing. Apart from the butchery involved, the practice suggests to the neighborhood that this maiming is horticulturally correct and something to be aped.
You see this a lot with hybrid crape myrtles that want to grow to 30 feet. People think this lovely plant should stop at, say, 10 feet, and they chop away accordingly. In the first year, a cluster of aerial flowering shoots erupts below the cut. But with the natural lines of the tree destroyed, the owner has to cut back the fresh stems each winter before the next season’s growth. The result is an ugly knee at the end of the branches and continual rank growth.
If you want a choice little tree, pick a dogwood, a small crape myrtle variety, a deciduous azalea, a Winter King hawthorn or something named Hamilton’s spindletree.
This might seem obvious, but experience proves otherwise. The smaller the garden space, the greater the care that must go into plant selection. If I lived in an 18-foot-wide rowhouse, I would pick, say, a native fringetree or amelanchier before a large crape myrtle. And even when the choice is right for its space, you must prune for a plant’s health and beauty.
How do you prune a large crape myrtle? You should remove entire branches, but conservatively and over several winters. The object is to open up the tree to lighten its mass without losing its natural beauty or overall balance (as opposed to strict symmetry). The same principle applies to most other ornamental trees: redbuds, dogwoods, Japanese maples, stewartias, crabapples and Japanese cherries, for example.
The best time to establish the basic structure of an ornamental tree is in its youth, when pruning cuts are small and will heal quickly. The first candidates for removal are branches that are rubbing and growing inward. You’ll need to revisit the tree every winter, thinning congested branches in the developing, ascending canopy. If you go overboard with pruning, the tree will respond by sending up undesirable water sprouts.
Another basic and correct form of pruning is called heading back, which can be done now or during the growing season until about August. This technique involves shortening a stem to just above a dormant bud or side branch. If done now, the bud will produce a little branch this spring that will hide the cut. By selecting a cut above an outward-facing bud, you can direct the future growth.
Broadleaf evergreens lend themselves to this, such things as azaleas, camellias, viburnums, boxwood, cherry laurels, hollies and pieris. If you prune spring-flowering plants after June, you will remove flower buds for the following season. If you care.
As with thinning, there is a knack to heading back stems. In addition to making the cut just above a lateral bud or branch, you should vary the length of pruned stems to avoid turning your azalea, etc., into a blob. The aim is to leave a shrub that does not look as if it has been pruned.
The third basic type of trimming is called renewal pruning and is suited to multi-stemmed and suckering shrubs that grow large and too congested and twiggy. Examples include flowering quince, forsythia, lilacs and weigela. The oldest, thickest stems can be removed entirely, but take out a third of them each year over three winters to minimize stress and suckering. Established forsythia and winter jasmine bushes can be chopped to the ground and will spring back to flower next year.
Some woody plants have their own peculiar pruning needs.
Nandinas, which used to suffer dieback of top growth when winters were more severe, can be reduced in height a little or a lot, all the way to the ground. Again, cut to just above a lateral branch or bud, and try to keep the overall effect natural. A mature stand will soon regrow.
Mophead and lacecap hydrangeas should not be cut back now. Wait until after the last frost and for new growth (late April is a good time), and trim back any stems with deadwood to healthy lateral shoots. With an old, twiggy hydrangea, you can remove the oldest stems now, taking care not to damage the dormant buds on other stems. If in doubt, don’t touch it.
In a mild winter like this, hydrangeas can produce new growth over the next month that is then killed by frost, ruining the floral display in June. Keep an eye on the hydrangea and the weather forecast, and cover the shrub with a sheet as needed.
Lavender should not receive a winter prune, however bedraggled it looks. You can groom a plant by removing dead stems after new growth develops in late April, but the fresh shoots should be left to bloom in June. As the flowers fade by early July, remove the long stalks and a few inches of leaf vegetation. This will promote bushiness and prevent the type of unattractive, open, woody growth you see on old, neglected lavenders.
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Step-by-step graphic Check out our guide to pruning ornamental trees, plus a list of the tools you’ll need .