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Winter trim

Why it is prudent to prune during a plant's dormant season.

Winter and very early spring are the best times of year to prune many kinds of trees and shrubs, much better than late spring or fall when many home gardeners traditionally dust off their pruning saws. Pruning plants when they are dormant causes them minimal stress. Trimming deciduous plants when they are leafless makes it easier to spot the problem areas. And plants that bloom on new wood benefit from an early spring bob to maximize flower production later in the season.

''The best time to prune is late in the plant's dormancy period, because that's when it has the most energy reserves," says John DelRosso, an arborist at Harvard University's Arnold Arboretum in Jamaica Plain.

In winter, fungus spores are not present, so disease is less likely to enter a pruning cut, adds Scott McPhee, president of the Massachusetts Arborists Association and a consulting arborist at Hartney Greymont Inc., a tree-care and landscaping company in Needham. McPhee and DelRosso agree that amateur gardeners headed for the garage in search of their loppers and saws should keep in mind the cardinal rule of the professionals: Prune only if necessary.

"There should be a reason to prune," says DelRosso, "and the reason should be obvious."

Pruning a tree or shrub that is storm damaged, diseased, or has branches that rub against one another would improve its health, says DelRosso. Young trees and shrubs should be pruned of misshapen growth to prevent problems in later years. Suckers that grow from the base of a tree should be pruned to prevent them from draining energy from the main plant. In addition, many mature plants are pruned to improve their shape or because they have grown too large for their spaces, he adds.

"People who take a class here often say their foundation plantings have gotten too big and they want to know how to control the size without shearing them," says DelRosso, who teaches classes on pruning and chain saw safety at the arboretum.

Yews, which so often are pruned into balls, rectangles, and other unnatural shapes, will tolerate severe pruning if they are healthy. A vigorous yew can be cut back to within 6 inches of the ground to encourage new growth. However, thinning out some of the main stems to allow air and light into the interior of yews and other shrubs is a less Draconian method that stimulates new growth of previously dormant buds. Gradual pruning works best if done over two or three years, according to DelRosso.

Exposing interior branches can have an aesthetic benefit as well as improving a plant's health, he notes. "Yews have beautiful flaking, cinnamon-colored bark."

Unpruned yews can reach 20 feet or more in height and width. Some varieties, such as Taxus media 'Natorp' are spreading meaning their mass and growth more horizontal than vertical. Others, T. cuspidata expansa and T. media 'Hatfieldii,' for example, are upright or vase shaped.

Pruning strategies should vary according to whether a species flowers on new growth or old growth. Plants that bloom on the current year's growth flower better if they are pruned before they begin to leaf out. Among them are caryopteris, St. John's wort, clethra, callicarpa, hydrangea, buddleia, and many roses.

Shrubs that bloom on the previous year's growth, such as forsythia, lilacs, and rhododendrons, can be pruned in winter without hurting the plant, but since flower buds have already formed, blossoming will be sacrificed. Such plants are generally pruned shortly after they have bloomed.

DelRosso recommends that homeowners looking for new landscape plants buy slow-growing varieties and give them plenty of space to avoid "a maintenance nightmare" when plants become overcrowded. He notes that it is essential to find out how large plants will be at maturity.

If a plant is in decline from age or disease, pruning will not rejuvenate it. Other remedies, such as soil improvement or feeding, may be the answer in such cases.

The worst time to prune, says DelRosso, is when a plant is in the process of leafing out. So much of the plant's energy is directed toward leaf growth that it has diminished ability to heal pruning cuts or fight diseases that may enter through exposed wood.

Once the decision has been made that pruning a plant is necessary, have a goal in mind before setting to work. "Know what you want it to look like when you are done," says DelRosso.

During the pruning process, periodically step away from the plant to check your progress. Look for balance, and if you find you did get carried away and pruned too much on one side, it won't be too late to trim another section so the finished plant looks even.

In all types of pruning, the proper method is important in order to minimize stress to the plant. A pruning cut is, after all, a wound, and the scar will remain for the life of the plant.

DelRosso recommends cutting a branch back to a junction with a branch that is at least one-third larger than the one being pruned. Cut just in front of the branch collar (the slight bulge where the branch attaches to another stem) and avoid cutting the bark ridge on the inside of a branch crotch. A cut too close to the main stem produces a larger wound - one that takes longer to heal - than one that leaves the branch collar intact. A cut too far out invites rot and infection to enter through the branch stub. Dressing a cut with a nontoxic paint is not necessary, but is sometime desirable for cosmetic purposes.

Heavy limbs should be removed with three cuts. Start on the underside of a branch, at least six or eight inches from the larger stem, and cut a third of the way through. This first cut will prevent bark peel when the branch falls. Next, cut through the branch beyond the first cut, sawing from above. Finally, saw the stub back to the branch collar.

DelRosso recommends using a pruning saw for larger branches and loppers or pruning shears for smaller jobs. He advises homeowners not to use pole saws, because it is difficult to see exactly what is being cut while standing on the ground.

Whatever the pruning tool, it needs to be kept sharp enough to ensure clean cuts. Leather gloves and eye protectors are also recommended.

As important as keeping tools in good working order, however, is knowing when it's time to put down the pruning saw and call in the professionals: If a pruning job requires climbing a tree or a ladder, especially with a chain saw, it's probably best to call an arborist.

Nearly every year, McPhee says, he has customers who have come to him after they have broken an arm or a leg falling from a ladder or tree while attempting to prune their landscaping.

"It is high risk for homeowners to get up on a ladder with a chain saw," says Virginia Wood, executive director of the Massachusetts Arborists Association. Tree work can be tough even for professionals who have proper training and the right equipment, she notes.

Trees with long branches are prone to storm damage, as are trees with a dense canopy. Thinning out branches can reduce the sail effect in thickly branched trees and can also let in more light and air, thus reducing the spread of disease.

Still, pruning should be conservative. Trim no more than a third of a plant's growth at a time. Cutting too many branches at one time can weaken the tree.

"You need to have enough leaves to support the life of the wood on a branch," says McPhee, adding that young plants can take heavier pruning than older ones.

Too much pruning also may expose bark to sunburn. Beeches are especially sensitive to solar burning. McPhee also warns against "lion-tailing," the practice of trimming off all the side branches except those at the end of a limb.

Avoid cutting off a tree's top to reduce its height, says McPhee, noting that such horizontal cutting of large branches destroys the natural shape of a tree, giving the impression "a helicopter crashed into it." Such so-called "topping" invites disease and the growth of numerous new shoots, or water sprouts, which can grow large and heavy.

A more effective way to reduce height is to cut individual limbs back to forks lower in the tree.

Ultimately, perhaps the best advice on pruning comes from the Arnold Arboretum's DelRosso, who says, "Plants are not always perfect."