Branches are attached to the tree trunk by interlocking branch and trunk tissue. A new layer of interlocking tissue is produced each year over the previous layer. A branch collar, produced by the trunk, holds the branch base. When the branch/trunk union has a narrow angle, the branch and trunk bark become overgrown and is referred to as included bark (Figure 1). This results in a weak union that is likely to split.
Tree Wound Response
Following an injury, Wall 1 is formed by plugging the vertical vessels of the vascular system above and below the injury. This is the weakest wall but can slow the vertical spread of decay. Wall 2 is formed at the outer edge of a growth ring. It is a stronger barrier and provides some resistance to inward spread of decay. Each growth ring is subdivided into compartments by the rays (Wall 3). Rays provide resistance to lateral spread by way of a maze of physical obstacles and a chemical barrier. Wall 4 is formed by cambium growth after an injury. It is the strongest of the four walls. Internally, it separates the wood present at the time of injury from new wood formed after the injury. Externally, wood develops around the injury and should eventually cover it by growing over the dead wood at the site of the injury.
A tree branch has a branch
bark ridge, which denotes where the upper side of the branch meets the
tree trunk (Figure 3). The bark collar is the swelling located at the
base of a branch where the lower side of the branch joins the trunk.
The natural decay of a dead branch usually does not spread beyond the
collar. When pruning a dead branch, do not create a new wound by cutting
into the collar of live wood that forms around the dead branch.
When to Prune
Some trees, such as birch, dogwood, elm, honey locust, maple, and walnut exude excessive sap from the wound when pruned in late winter or early spring. Sap flow does not hurt the tree. Prune these trees in late spring, summer, or fall to minimize sap flow.
The time of pruning should take into account the life cycle of insects and diseases. Plants in the genus Prunus (flowering cherry, cherry laurel) are prone to develop bacterial cankers. The spores for the diseases, which are released in fall and early winter, can enter plants through fresh pruning cuts and wounds. Prunus trees do not initiate new cankers during late spring or summer. Dogwood borers are most active in May, June, and July. Thus, dogwoods should not be pruned during these months.
Pruning Young Trees
Scaffold branches (primary branches that will make up the tree's framework) emerging from the trunk should have a 45 degree angle of attachment (Figure 4). Generally, branches with less than 45 degree angles becomes weaker as they grow longer and increase in diameter. Branch angles of less than 30 degrees result in a high percentage of limb breakage while those between 60 and 70 degrees have a small breakage rate.
Scaffold branches should look like ascending spokes around a central axle (Figure 5). This will provide a structurally strong tree that is attractive, balanced, and allows sunlight to penetrate and wind to pass through the canopy. Major scaffold branches should have at least 8 inches and preferably 20 inches of vertical separation. Closely spaced scaffolds will produce less lateral branches than widely spaced branches. The result will be long, thin branches with poor structural strength. Good radial spacing prevents one limb from overshadowing another. Branch arrangement and spacing is more critical for large shade trees than for small flowering trees.
Laterals that have grown taller than the terminal leader or beyond the canopy of the tree should be headed back. Laterals that have grown inward towards the center of the tree should be removed back to their origin. Branches that are less than half the diameter of the trunk have a stronger branch/trunk union than those that grow larger than half the trunk diameter. Water sprouts that result from extensive pruning should be removed because they are structurally weak and can lead to overly dense growth in the interior of the tree.
Pruning Large Trees
Dead, broken, weak, or diseased branches should be removed. Then, selectively remove limbs from the perimeter of the canopy, especially those growing close together or beyond the desired canopy size. Also, remove branches with narrow angles of attachment. Branches should be taken back to their point of origin or to laterals that are at least one-third the diameter of the limb being removed. Trees vary in the amount of thinning they can tolerate without creating undesirable effects. An over thinned tree will respond by producing numerous watersprouts and suckers. Sunscald can occur on trees with thin bark. Never remove more than 25 percent of the total foliage at one time.
Heading back - will reduce the overall size of a tree. Cut back to good lateral branches and possible head the tips of the laterals (Figure 7). Its best to cut back over several years than to attempt dramatic pruning in one year. When cutting back to an intersecting lateral branch, choose a branch that forms an angle of no more than 45 degrees with the branch to be removed. Also, the branch that is cut back should have a diameter at least half that of the branch to be removed.
When cutting branches over 1 inch in diameter, use the three-cut method (Figure 8) to avoid tearing the bark of the trunk. Make the first cut on the underside of the branch, 1 to 2 feet out from the trunk and about half-way through the branch. The second cut is made on the top of the branch, about 3 inches further out from the first cut. As this cut is made, the weight of the branch will cause it to break between the two cuts. If there is danger of the branch damaging other limbs below or objects on the ground, it must be properly roped and supported, then carefully lowered to the ground after the second cut. The remaining stub can then be cut back to the branch bark ridge.
slanting cuts when removing limbs that grow upward; this prevents water
from collecting in the cut and speeds closure. Large branches should
be removed just outside the collar ---- not flush with the trunk.
Prepared by: Erv Evans, Consumer Horticulturist, NC State University
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