Specific types of pruning may be necessary to maintain a mature tree in a healthy, safe, and attractive condition.
Cleaning is the removal of dead, dying, diseased, weakly attached, and low-vigor branches from the crown of a tree.
Thinning is selective branch removal to improve structure and to increase light penetration and air movement through the crown. Proper thinning opens the foliage of a tree, reduces weight on heavy limbs, and helps retain the tree’s natural shape.
Raising removes the lower branches from a tree to provide clearance for buildings, vehicles, pedestrians, and vistas.
Reduction reduces the size of a tree, often for utility line clearance. Reducing a tree’s height or spread is best accomplished by pruning back the leaders and branch terminals to secondary branches that are large enough to assume the terminal roles (at least one-third the diameter of the cut stem). Compared to topping, reduction helps maintain the form and structural integrity of the tree.
How Much Should Be Pruned?
The amount of live tissue that should be removed depends on the tree’s size, species, and age, as well as the pruning objectives. Younger trees tolerate the removal of a higher percentage of living tissue better than mature trees do. Generally, no more than 25% of the crown should be
removed at once, and less for mature trees.
Removing even a single, large-diameter limb can result in significant canopy loss and can create a wound that the tree may not be able to close. Care should be taken to achieve pruning objectives while minimizing live branch loss and wound size.
Research has shown that dressings do not reduce decay or speed wound closure, and rarely prevent insect or
disease infestations. Most experts recommend that wound dressings not be used.
Pruning Young Trees
Proper pruning is essential in developing a tree with a strong structure and desirable form. Trees that receive the appropriate pruning measures while they are young will require less corrective pruning as they mature.
Keep these few simple principles in mind before pruning a tree:
Always have a purpose in mind before making a cut. Each cut has the potential to change the growth of the tree.
- Poor pruning can cause damage that lasts for the life of the tree. Learn where and how to make the cuts before picking up the pruning tools.
- Trees do not heal the way people do. When a tree is wounded, it must grow over the damage. As a result, the wound is contained within the tree forever.
- Small cuts do less damage to the tree than large cuts. Correcting issues when a tree is young will reduce the need for more drastic pruning later.
Making the Cut
Pruning cut location is critical to a tree’s growth and wound closure response. Make pruning cuts just outside the branch collar to avoid damaging the trunk and compromising wound responses. Improper pruning cuts may lead to permanent internal decay.
If a large branch must be shortened, prune it back to a secondary branch or a bud. Cuts made between buds or branches may lead to stem decay, sprout production, and misdirected growth.
Small branches can be cut easily with hand pruners. Scissor-type or bypass-blade hand pruners are preferred over the anvil type as they make cleaner, more accurate cuts. Cuts larger than one-half inch (1.27 cm) in diameter should be made with lopping shears or a pruning saw.
Hedge shears should be used for shaping hedges only. Do not use shears to prune a tree. Whatever tool you use, make sure it is kept clean and sharp.
For most young trees, maintain a single dominant leader growing upward. Do not prune back the tip of this leader or allow secondary branches to outgrow the main leader. Sometimes, a tree will develop double leaders known as codominant stems. Codominant stems can lead to structural weaknesses, so it is best to remove or shorten one of the stems while the tree is young.
A tree’s secondary branches contribute to the development of a sturdy, well-tapered trunk. When numerous branches are being removed, it is preferable to retain some, at least temporarily, to promote trunk diameter growth.
Permanent Branch Selection
Most of the branches present on a young tree at planting will be pruned away at maturity to provide clearance for mowing, pedestrians, and / or vehicle traffic.
The height of the lowest permanent branch is determined by the tree’s intended function and location in the landscape. The road side of a street tree may be raised to 16 feet (5 m) to accommodate traffic. In most other situations, 8 feet (2.4 m) of clearance is sufficient. Trees used as screens or wind breaks, however, usually branch low to the ground.
Sufficient branch spacing and balance, both vertically and radially, is important. The space between permanent branches should be approximately 3 percent of the tree’s eventual height (for example, 1.5 feet [0.5 m] for a tree that can grow to be 50 feet [15 m] tall).
Beyond spacing, the strength of branch structure depends on the relative size of the branches and branch angles. Branches similar in diameter to the trunk or limb from which they arise are more prone to failure than those smaller in diameter.
Narrow angles of attachment or tight crotching can enclose bark within a branch union. Such growth is called included bark, a condition that weakens the branch attachment and may lead to failure when the tree matures. Branches with weak attachments should be pruned while still small. Balance should be considered by retaining some branches in each direction radially, spreading from the center outward. Make sure one scaffold branch is not allowed to grow directly above another.
When pruning, be sure not to remove too many branches. Leaves and their supporting branches are major sites of food production and storage. Eliminating too much of the canopy can “starve” the tree, reduce growth, and increase stress. No more than 25 percent of the crown should be removed in one pruning.
Newly Planted Trees
Pruning of newly planted trees should be limited to the removal of dead or broken branches. All other pruning should be withheld until the second or third year, when a tree has recovered from the stress of transplanting.
Despite any claims otherwise, research has shown that wood dressings do not reduce