Green Scene

Pruning Shrubs

To avoid damaging emerging bulbs and other perennials, clean up the garden as soon as possible, then determine what needs pruning.
To avoid damaging emerging bulbs and other perennials, clean up the garden as soon as possible, then determine what needs pruning. (Photos By Sandra Leavitt Lerner For The Washington Post)
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By Joel M. Lerner
Saturday, February 28, 2009

There's still some time left to wrap up winter gardening chores before spring -- cleaning up, renewal pruning, composting and some mulching should be done before the sap starts flowing.

The most important aspect of renewal pruning of broadleaf evergreens is to do it before growth begins, by the end of February or beginning of March in the Washington region.

To avoid damaging emerging bulbs and other perennials, rake leaves and clean up the garden as soon as possible. After you rake the leaf debris, assess whether trees and shrubs need pruning. Renewal or hard pruning of broadleaf evergreens is usually considered only when plants are overgrown or lose ornamental value. Such severe pruning might be needed to renew overly large hollies, camellias, azaleas, photinias, mahonias, nandinas, osmanthus or boxwoods. If this type of pruning is done now, some faster-renewing plants can be cut back to bare sticks.

These aren't the pruning guidelines you see in most manuals, which typically recommend pruning after flowering. Denuding plants is not your everyday practice for evergreens. This is reserved for what azalea growers call the "mighty whack" (cutting back to the ground) or what holly growers refer to as "hat racking" (pruning to bare wood), done only at this time of the year.

For example, if hollies have pyramidal habits, renew by topping trunk and pruning side branches back to bare sticks, while shaping branches into pyramidal forms. The bush will have a hat rack look until July or August. American, Chinese or English hollies will be covered with new foliage within a year. It is crucial to do this now, before growth begins. Hollies and azaleas grow new leaves by forming buds under the bark as growth begins in spring, as well as renewing from suckers growing directly from roots.

Sometimes azaleas are cut back hard, from six or seven feet to 10- to 12-inch bare stems. They will leaf out and grow into full small plants from the bare stems that were preserved. I've been successful with this type of pruning, but there is a chance of losing a plant. It is an expedient way to deal with overgrown azaleas, although it will sacrifice this year's flowers.

Some broadleaf evergreens require hard renewal pruning only every five to seven years; most years, a light shearing is enough for them. Otto Luyken, 'Schipkaensis' cherry laurels and osmanthus are good examples of plants that can be maintained with shears for several years. Cherry laurels can be touched up right after they bloom in spring. Lightly prune osmanthus about the time evergreen azaleas are flowering in spring. Osmanthus blooms in fall on new growth.

Japanese hollies are not as forgiving as larger-leaved hollies. Prune them, as well as boxwoods or blue hollies, by reducing them a maximum of one-third annually over a period of three years. They respond best when some greenery is left on stems. It helps maintain a natural growth habit. Now is the best time to do this work.

If shrubs have been sheared into shapes of rectangles and gumdrops, re-creating their natural growth habit can require patience. Cut tufts of branches out of tightly sheared plants by reaching inside the shrub, cutting back to a branching stem or above a leaf. This will create openings for sunlight and air circulation, so the plant can renew slowly and develop new foliage throughout the inside.

To reduce size of plants, cut the longest stems. Prune back to a branching stem, or ground level. In the case of boxwoods make "holes" throughout the entire shrub, cutting back to branching stems and thus encouraging growth on the inside of shrub. Rhododendrons should be pruned to a lower branch or whorl of leaves, maintaining a natural-looking habit. As foliage regenerates, plants will look fresher and fuller.

The shape of mahonias and nandinas is dependent on how the canes (the colony of upright stems) are managed. Cut the tallest canes to the ground unless there is another branch to prune back to. In extreme shade, shrubs renew more slowly. Prune only where necessary. With each cut, look at and assess the balance and fullness of the shrub to determine if more pruning is needed.

If you prune wilted or dead sections of plants with diseased stems, sterilize the blade with bleach before pruning healthy wood. Bleach is corrosive, so the pruner should then be lubricated with a little light oil, like WD-40.

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