Pruning is the most common tree maintenance procedure. Although forest trees grow quite well with only nature's pruning, landscape trees require a higher level of care to maintain their safety and aesthetics. Pruning should be done with an understanding of how the tree responds to each cut. Improper pruning can cause damage that will last for the life of the tree, or worse, shorten the tree's life.

A common mistake is to remove too much inner foliage an small branches. It is important to maintain an even distribution of foliage along large limbs and in the lower portion of the crown. Over-thinning reduces the tree's sugar production capacity and can create tip-heavy limbs that are prone to failure.

Mature trees should require little routine pruning. A widely accepted rule of thumb is never to remove more than one fourth of a tree's leaf bearing crown. In a mature tree, pruning even that much could have negative effects. Removing even a single, large-diameter limb can create a wound that the tree may not be able to close. The older and larger a tree becomes, the less energy it has in reserve to close wounds and defend against decay or insect attack. The pruning of large, mature trees is usually limited to the removal of dead or potentially hazardous limbs.

Wound Dressing

Wound dressings were once thought to accelerate wound closure, protect against insects and diseases, and reduce decay. However, research has shown that dressings do not reduce decay or speed closure, and rarely prevent insect or disease infestations. Most experts recommend that wound dressings not be used. If a dressing must be used for cosmetic purposes, then only a thin coating of a nontoxic material should be applied.

Q: My oaks have never been pruned. Do they need to be pruned, and if so, how?

A: Pruning can be used to manage tree safety, improve aesthetics, extend useful life span, manage certain pest problems and minimize conflicts with people, buildings, vehicles and power lines. When improperly done, it destroys natural shape and beauty, creates hazards, causes stress, reduces longevity and increases cost and reduces property value. Trees are routinely mis-pruned by well-intentioned people who don't understand how trees grow or respond to pruning. They are often motivated by fear of tree failure as trees grow in size or the misconception that trees need to be pruned to maintain health.

Pruning may not be desirable or beneficial. There is always some negative impact, at least temporarily. When done improperly, pruning can be destructive. Severe (hard) pruning can kill or seriously weaken trees by removing too much foliage. This diminishes food production (photosynthesis) causing stress, which may result in energy depletion, dieback, increased susceptibility to pests, or decline. Problems, however, seldom develop when pruning cuts are few in number, kept small and properly made. Damage, of course, depends on extent of pruning, size of wounds, time of year, and tree health prior to pruning. Pruning can also alter branch spacing and attachment strength, architecture (shape, foliage, density and distribution), and susceptibility to decay. Thus, trees should be pruned only as needed to accomplish the desired goal. Some goals, however, may be incompatible with tree health.

Important concepts to consider when pruning

• Pruning large trees is both dangerous and difficult; It is best left to professionals, preferably someone certified by the Western Chapter of the International Society of Arboriculture.
• Prune only as needed.
• Have a valid reason to prune each branch.
• Remove as little foliage as possible particularly for an older or declining oak.
• Prune with the health of the tree in mind.
• Remove dead branches if they pose a safety hazard to people, property or structures. Dead branches can be left in trees in naturalized settings for wildlife purposes.
• Keep cuts small and few in number. Don't stub unwanted tree branches; remove branch where they originate.
• Don't cut branches flush to the trunk or branch; leave the collar or swollen area at the branch base intact. Removal of the collar will encourage decay.
• Wound dressing does not prevent decay and should not be used on pruning cuts.
• Prune in late winter to early spring, or early to midsummer. Avoid pruning when the leaves are forming and in the fall when leaves are shedding.
• Dead and weak branches can be removed at any time.
• Correct extensive structural problems gradually.
• Avoid thinning interior foliage.
• Don't alter natural shape.
• Avoid excessive crown raising (removal of lower branches, some times called lion-tailing).
• Don't top oaks and other trees to reduce tree height or spread.

Destructive pruning, e.g., topping and over-thinning, diminishes health, safety, longevity, aesthetics, environmental benefits, and property value. Proper pruning minimizes foliage removal and avoids significant changes to the tree's natural shape, growth habit and size. This saves money, reduces impact on health and maintains maximum environmental benefits.