You can cover a bad haircut with a hat, but a poorly trimmed shrub has no place to hide. “Although whacking the top off a shrub is the easiest way to keep it small, it’s definitely not the best method,” says Jerry Goodspeed, Utah State University Extension horticulturist. “Giving them a cheap haircut may control the plant’s size, but it also embarrasses the heck out of shrubs, not to mention the damage it inflicts on the plants.”
If pruning shrubs is on your "to do" list this winter, be careful how and which plants you cut. Only certain shrubs can be thinned this time of the year without sacrificing their spring blooms or causing them harm, Goodspeed says. The plants to prune now are late blooming shrubs or those that are grown for their foliage rather than for their flowers.
Late blooming shrubs are those that typically bloom after Memorial Day, he explains. This includes many viburnums, sweet mock orange, some spirea, butterfly bush, hydrangea, hibiscus and pyracantha. Later blooming shrubs produce many of their flowers on the current year’s growth. Pruning them this time of year encourages the plant to grow new foliage and flower buds, insuring an abundance of blooms later in the year.
“Pruning earlier flowering shrubs, those that bloom before Memorial Day, during the winter can prevent them from blooming,” Goodspeed says. “These shrubs are blooming on last year’s growth. Pruning them before they bloom removes many of their flowers. And, after all, what good is a lilac bush if it produces few, if any flowers?”
The most important thing to remember when pruning any shrub is to maintain its natural shape and beauty, he emphasizes. This is accomplished best by simply removing a few of the older canes at the base when the plant starts to get out of control. Shrubs that are only a year or two old probably do not need pruning because their canes are young and normally have not outgrown their welcome.
Pruning out the older canes keeps the plant smaller and less dense, he says. Older canes are normally the tallest and removal also keeps the plant’s size under control. This opens up the middle of the plant to the light, encouraging new growth, and keeps it looking young and natural. Blossoms will then grow throughout the plant, and not just on the top few branches.
“This pruning method also helps keep the plant from getting top heavy,” Goodspeed says. “Shrubs that are sheared or topped have a tendency to branch and thicken higher up in the shrub. This shades the lower part of the plant, reducing lower leaf and flower formation.”
If a shrub has been pruned so it is top heavy in the past, it isn't too late to correct the situation, he says. Pruned properly and given time, shrubs can return to a more natural form. Begin by removing the worst offenders at the ground level. This stimulates new growth from the base, eventually replacing the top-heavy, funny-looking growth.
“Do not remove more than a third of the plant each year,” Goodspeed cautions. “After three years of properly pruning the old embarrassing growth, the confidence of the shrub (and pruner) will begin to once again soar.”
Most shrubs survive pruning, he says Be sure to remove any damaged, sick, or really sick or dead wood first. Remember not to remove more than a third of the wood in any given year and try to keep the plant as natural looking as possible.
For more information, contact your local USU County Extension office.
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Issued in furtherance of Cooperative Extension work, Acts of May 9 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Robert L. Gilliland, Vice-President and Director, Cooperative Extension Service, Utah State University, Logan, Utah. (EP/02-99/DF)