Pruning is one of the most fundamental tasks in gardening, and knowing when to trim is as essential as knowing how.
Pruning woody plants at the wrong time can interrupt bloom cycles and damage stressed plants.
“It's time to prune whenever your pruners are sharp” is an old adage that applies only when removing diseased, dead or dying branches – the “three D's” of pruning that can be done at any time.
Otherwise, certain times of the year are better for pruning than others, based on plant life cycles, said Ryan Pankau, an Extension horticulturist with the University of Illinois.
“It's pretty safe to say that most woody plants are best pruned in winter, during their dormant period,” Pankau said. “At other times of the year, such as leaf out, leaf drop or during flowering, pruning can have a very negative impact on plant health.”
Plants expend a lot of energy during those stages, and pruning can place undue stress on trees and shrubs because it causes so much of that vital energy to be lost, he said.
Spring-flowering plants often bloom on flower buds produced the previous year. Pruning before spring flowering removes many of those buds, reducing the number of blooms that would have emerged. Pruning them shortly after they flower gives them time to regenerate.
“(But) if your plant is already stressed and it has to be pruned, it may be best to sacrifice some of the coming year's flower buds and prune during winter, since pruning during dormancy has the least impact on plant health,” Pankau said.
Why prune at all? Many reasons. Pruning eliminates the threat of property damage, controls plant growth, improves aesthetics, boosts crop yields, shapes and rejuvenates, reduces disease threats and prevents insect infestations.
Deciduous trees should be pruned in the spring, before they leaf, said Leonard Perry, Extension professor emeritus with the University of Vermont. Summer-flowering shrubs, like hydrangea, should be pruned in early spring before they bud. Fruit trees should be pruned in late winter while they're still dormant but before the buds begin to swell, Perry said.
Evergreen trees usually don't require pruning except for shaping. Pruning evergreen shrubs may be required in mid-summer to keep vigorously growing plants at the desired size, he said.
“Wait until after flowering or, if the plant produces berries, after the berries fall,” Perry said in a fact sheet.
Pruning is never the answer for maintaining a plant too large for its space, Pankau said.
“Consider the mature size of the plant before deciding where to plant it,” he said. “By selecting the right plant for the right place in your landscaping, you can avoid a lot of maintenance issues and you will have a much healthier plant in the long run.”
Disinfect pruning equipment with alcohol before using it on another tree or shrub, Perry said. “Disinfecting tools helps reduce the spread of disease,” he said.
As a general rule, wound dressings or pruning paints are unnecessary, said Bob Polomski, an Extension horticulturist with Clemson University. He cited studies showing “that these topical applications impair the natural wound closing process,” Polomski said. “In some cases, these paints trap moisture, which fosters fungal infections.”
– Dean Fosdick, Associated Press