Local Lifestyle

Friday, Dec 13 2013 05:30 PM

RICHARD SHIELL: Pruning trees can make them healthier

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    By Photo courtesy of Richard Shiell

    This professionally trimmed oak tree has a bright open canopy.

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    By Photo courtesy of Richard Shiell

    This fruiting mulberry pruned after harvest seals its cut with sap.

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By RICHARD SHIELL, Contributing columnist

Pruning not only shapes plants, it can make them healthier. In nature, trees survive bitter winters that reduce the canopy size by freezing the most exposed parts, and adapt by growing lower and bushier. Heavy infestations of caterpillars strip the leaves from woody plants, which not only survive but grow vigorously after new foliage emerges.

Plants growing in favorable conditions are actually pretty hard to kill. Pruning takes advantage of the resilience of plants to control their size, make them more attractive, and reduce problems of disease and pests.

Related Info

The University of California Cooperative Extension's local office will host its annual fruit tree pruning demonstrations at noon on Monday and Tuesday at the orchard of the UCCE office, 1031 S. Mount Vernon Ave.

Mario Viveros, of the University of California Cooperative Extension, will lead the demonstration on trees including apple, apricot, cherry and almond. Viveros will also show how to prune grapevines.

To reach the UCCE office and orchard, take Highway 58 and exit at Mount Vernon, then proceed south for about three-quarters of a mile. Publications on pruning, fertilizer for fruit trees and fruit tree varieties for the valley portion of Kern County will be available. There is no charge for attendance, nor is preregistration required.

-- John Karlik, adviser with the UC Cooperative Extension in Bakersfield.

Not all garden plants are pruned in winter. Most fruit trees benefit from thinning of the branches after the harvest, when growth is still active so cuts can heal for the remainder of the growing season.

Pruning is an art based on a few simple principles. Understand these and little mystery remains. These can be summed up as: time it right; only the trunk should aim straight up; make room for light penetration and air circulation; and avoid leaving dead stubs.

Most woody plants develop bare twigs within the canopy, as the upper foliage shades the lower parts. Removing dead or weak twigs improves air circulation. This reduces opportunities for fungi to infect the plant, and removes hiding places that encourage pests.

Self-shading

A multi-trunked tree will produce side branches from each trunk, some of which will cross over each other, depriving the lower ones of light and reducing air movement. Most shrubs also have issues with crossing branches. Thinning the canopy isn't an exact science, and mistakes are rarely important, so long as pruning isn't skipped altogether. Examine the pruning done by experienced rose growers and you see individual styles and choices, no two prune exactly the same way. Yet all their roses grow well. The same applies to the managers of blocks of fruit trees; all prune differently, and all harvest abundantly.

To thin the canopy, first remove thin branches midway up the plant that shade the trunk. Then look for so-called "water sprouts." These are vertical shoots that come straight up off the lateral branches. Some plants are more prone to water sprouts than others, especially fruit trees and flowering ornamental deciduous trees such as mimosa, flowering crabapple, and chitalpa. All vertical twigs should be removed early, as they produce problems of self-shading.

Structural pruning isn't required on most plants, that is, removal of large branches so they don't weaken and split on their own. What to look out for is a divided trunk, like a capital letter "Y," or with three or more trunks emerging from the same spot. The bark of each of those upper trunks will have no place to grow as the trunks thicken, so they will, in effect, try to force each other apart. This produces a "weak crotch" the can split off and fall in a wind storm. The thing to watch for on a tree with weak crotches is bark pushing up between the divided trunks. Such a condition merits structural pruning, selecting one to remove.

Life is hydraulic

Another fundamental guideline for pruning comes from a basic understanding of biological processes. Life is hydraulic, dependent on the movement of fluids. Animals and plants have circulatory systems. Plants depend on a siphoning action to move fluids, as water released from the leaves creates a vacuum-like effect that pulls moisture up from the roots.

Most woody plants require that some thin twigs remain along pruned branches, all the way up to the pruning cuts. If a branch is denuded below the cut, the result can be a dead stub. Few woody plants want to be pruned as hard as a hybrid tea rose bush or mulberry tree. Those exceptional plants thrive on being cut back hard. Experienced arborists are selective about which twigs remain untouched, being sure that each shortened lateral branch as enough twigs to keep the fluids moving. Dead stubs invite borers or termites.

The timing of pruning depends on the variety of plant. Evergreens, be they conifers or broadleaved, take only thinning, unless structural changes are required. The timing depends on the fruiting season, and generally when the fruit matures, or the seed pods ripen, is a good time to trim. This applies to plants with inedible fruit too, like India hawthorn and viburnum.

Some deciduous plants bloom on new wood, the soft new twigs of springtime bearing tender flower buds, as on blackberry. Others set flower buds on mature twigs before winter arrives, and most such plants produce their bloom from naked twigs before the foliage appears.

The latter, including stone fruit and pomegranate, should be pruned early in the fall, after the summer heat has faded. They may be thinned in winter too, if necessary. Those that bloom on new wood, like roses, are best pruned from late November to early February.

Many garden plants are grafted, growing on the rootstock of another variety. Sometimes the rootstock produces its own shoots from below the graft, or even from underground. This is how red flowers appear on a hybrid tea rose of another color; the most common rootstock on roses grown in the USA is called Dr. Huey, a red climbing rose that blooms only in springtime.

This is also how citrus trees produce extremely thorny branches with lumpy sour fruit; the rootstock grew up. Suckers, branches from below the canopy, should be removed, the earlier the better.

 

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