The key to pruning mature trees, according to a community forestry expert in Penn State's College of Agricultural Sciences, is remembering that older trees, much like aging people, are slower to recover from injury, illness or structural adjustments.
Mature trees are defined as trees that have reached a mature age or size for a species, according to Bill Elmendorf, instructor and urban and community forestry program coordinator in the School of Forest Resources.
"Most homeowners can determine if a tree is mature by researching its size and age range," Elmendorf says. "If a species can grow to 80 feet and your tree is 60 or 70 feet, it's a mature tree. If the species can live 100 years and your tree is 70 years old, it's mature."
Mature landscape trees must be pruned carefully because haphazard or ill-considered pruning causes injury, decline or even death in a plant that has established its system for energy production, storage and use. Fruit trees, he warns, have very different pruning guidelines than other tree species.
"A tree gets its energy through its leaves by photosynthesis," Elmendorf explains. "A tree uses energy for growth and maintenance of living tissue, reproduction and resisting decay. In addition, some energy is stored for future use. While younger trees use most of their energy for growth, older trees use most of their energy for maintaining live tissue, which means not as much energy is available for growth and defense against decay.
"If you remove too much green, the mature tree will respond by using its stored energy to promote new growth," Elmendorf adds. "You also create many wounds susceptible to decay, which the tree must defend against. With repeated or large injuries, you reduce the tree's energy generation and storage capability, so the tree will use its stored energy again and again until it goes into a spiral of decline or death."
Elmendorf says mature trees should be pruned only if:
- Trees have crossed branches, weak crotches or defects o branches are dead, decayed, dying or hazardous
- Lower branches interfere with people or vehicles, or obscure visibility
- Branches are growing into buildings or utility wires
- Limbs are broken by storms
- Trees are too large and present a hazard to people and property
Elmendorf says homeowners should not try to prune mature trees in most cases. "Pruning should always be done by a qualified arborist who understands and uses current safety and pruning guidelines," he says.
Homeowners should be familiar with different types of pruning cuts to ensure proper tree care.
Thinning cut. This method removes a branch to its point of origin on the trunk, or shortens the length of a branch back to a side branch that is large enough to assume growth of the pruned limb. "On large trees, arborists generally prune back to a side branch that is one-third the diameter of the pruned branch," Elmendorf explains. "For example, if a limb is 10 inches in diameter, prune it back to a connecting branch that is at least 3 inches in diameter." - more
Drop-crotch cut. Used on large branches more than 15 inches in diameter, this technique uses the same method as a thinning cut. "It's a large thinning cut used mostly for reducing a tree's size," Elmendorf says.
Heading cut. This cut should not be used unless the entire tree is being removed. The branch is trimmed back to a bud or to a very small branch that cannot assume the growth of the pruned branch.
Stub cut. This cut is made indiscriminately on a branch where no bud or adjoining branch exists. "This 'topping cut' should only be used when you are removing a tree," he says. "Topping is the ultimate in destructive pruning practices."
Cutting a branch correctly is another important element in pruning. "Branches should never be cut flush to the trunk or left as a branch stub," Elmendorf says. "Leave the branch collar -- the area of swelling at the base of a branch. This natural target pruning helps the tree form wound wood to heal the cut."
To ensure tree health, Elmendorf says homeowners should make sure they are pruning at the right time of year. "Deciduous trees should never be pruned when they are growing new leaves in the spring or when they are losing leaves in the fall," Elmendorf says. "The best time to prune a tree is in winter, when it is dormant."
Elms and white oaks should not be pruned in the summer, because beetles carrying the pathogens for Dutch elm disease and oak wilt are active in the summer and are attracted to fresh pruning cuts. Dogwoods and flowering fruit trees should be pruned right after their flowers fade in late spring or early summer to protect the next year's flower buds.
Elmendorf lists other pruning do's and don'ts:
- Do keep pruning cuts as small as possible.
- Don't use tree wound dressing. "Dressing seals in moisture and decay spores and prevents wound wood from forming," Elmendorf says.
- Don't remove more than 30 percent of the tree's leaf area in any year.
- Do remove all dead wood first. "You may find you don't need to continue pruning," Elmendorf says.
- Do step back and look at what you are doing. "Pruning cuts should be distributed equally," he says. "Don't cut all interior branches or all branches in the lower half of the tree."
- Don't attempt to climb a tree, use ropes to climb a tree or use a ladder to prune. "Only people trained in those methods have any business in a tree," he warns.
- Don't take a chainsaw into a tree or operate it above your head. "Homeowners should prune using only a hand pole saw while standing on the ground," he says. "There have been many horrific injuries to homeowners using chainsaws to prune."
- Do hire a qualified arborist to do any major pruning on a mature tree.
"The key to reducing pruning needs in a mature tree is to buy and plant a healthy tree with good structure," Elmendorf says. "Once the tree is planted properly, prune the young tree using hand shears so you can reduce pruning and the size of wounds in the tree's later years."
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