Every year, landscape gardeners should step back, look at their plantings and formulate a pruning plan that will keep ornamentals looking their best.
"Pruning should start as soon as you have the plants in the ground," says Robert Nuss, professor of ornamental horticulture at Penn State's College of Agricultural Sciences. "You can shorten long stems and encourage branching to increase the density and volume of new plants."
Nuss says most landscape plants should be lightly pruned every few years. "If you prune hard every year to keep a plant to a certain size in your landscape, it's better to replace that plant with one that has growth characteristics better suited to the site," he says.
All gardeners, Nuss says, should understand the difference between pruning and shearing.
Pruning. Pruning means individually selecting and cutting specific branches or twigs. Larger pieces are removed at specific locations on the plant.
Shearing. Shearing removes one to two inches of growth from the entire plant by indiscriminately clipping all twig ends.
"Before pruning, consider the properties of the plant," Nuss explains. "Look at its natural form, growth habits, growth rate, height, spread and flowering time."
Gardeners should prune if:
- There are any dead, diseased, damaged or insect-injured parts.
- There is a need to make the plant less dense, or open the center for light and air flow.
- The plant needs rejuvenation.
- A special shape is desired, as with hedges or topiaries.
- Dead flower clusters and seed pods must be removed.
Savvy gardeners should use one of three pruning methods.
Rejuvenation. This is the most severe pruning method. Used on older plants that have grown too large or woody, this approach removes the oldest branches at or near ground level, leaving only young stems. "Pruning old wood on shrubs will stimulate the growth of new wood," Nuss says. "The newer wood will have better flowers and form.
"If there aren't many young stems, remove about one-third of the older wood each season over three years," Nuss explains. "New growth will have to be pruned to encourage some branching and to retain the quality and density of the plant."
Thinning. These pruning cuts are done by removing entire twigs or branches where they attach to the main stem. It is the least conspicuous pruning plan. "By cutting the inward-growing twigs, the remaining growth will fill in the outside of the plant," Nuss says. "This method is best used on very dense plants."
Heading back. This method reduces the height or size of the plant. Branches or twigs are cut back to a bud or emerging side branch. "The shape of the plant is controlled by the location of the bud at the end of the cut," Nuss explains. "An inward-pointing bud will make the plant denser; an outward-pointing bud will do the opposite."
Nuss warns that heading back stimulates the development of smaller shoots and dense growth, particularly if every branch is headed back. Nuss recommends heading back just 30 percent of a plant's longest branches in a growing season.
Shrubs that flower in the spring should be pruned after they bloom. Plants that flower in mid- and late summer should be pruned in the spring before growth starts. "Any fall pruning should be done after the plant is dormant," Nuss says. "But keep pruning at this time of year to a minimum.
"If flowers aren't important to the plant, as in the case of needle evergreens, it's better to prune when the plant is just breaking dormancy in spring," he adds. "Without its foliage, the problem parts of the plant are easily seen."
Single copies of Penn State's Special Circular 235 "Pruning Ornamental Plants" are available free of charge by contacting your county Penn State Cooperative Extension office, or by calling the College of Agricultural Sciences Publications Distribution Center at (814) 865-6713.
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