General Responses To Pruning
Pruning is an invigorating process. By removing the apex, pruning temporarily destroys apical dominance and stimulates the growth of lateral buds into shoots. Pruning also reduces the size of the above-ground portion of the plant in relation to the root system, and as a result, the undisturbed root system services a smaller number of shoots and buds. The relative uptake of water and nutrients by the remaining shoots and buds increases, and regrowth occurs. Generally, the more severe the pruning the greater the regrowth. In essence, the plant is growing in an attempt to restore a balance between the top and the root system.
Pruning generally stimulates regrowth near the cut. Vigorous shoot growth will usually occur in 6 to 8 inches of the pruning cut. This is particularly true for vertical limbs that have been pruned. However, growth on limbs having a 45° to 60° angle from the vertical will develop farther away from the cut. Pruning can also indirectly stimulate growth of lateral shoots by allowing more light to penetrate the canopy of the plant. Pruning a young plant will stimulate vigorous shoot growth and will delay the development of flowers and fruit. The length of the delay will depend on the species pruned and the severity of the pruning.
Types Of Pruning Cuts
There are two basic types of pruning cuts, heading and thinning. Each results in a different growth response and has specific uses. Heading removes the terminal portion of shoots or limbs. By removing apical dominance, heading stimulates growth near the cut. It also is the most invigorating type of pruning cut, resulting in thick compact growth and a loss of natural form, as in the case of a formally pruned hedge. Sometimes ornamental shrubs along a foundation overgrow their planting space and are rejuvenated by heading to within 12 inches of ground level. Many broadleaf shrubs such as burford holly, ligustrum, abelia and crape myrtle tolerate this type of pruning. Other types of heading are topping, dehorning, hedging and clipping. Thinning, on the other hand, removes an entire shoot or limb to its point of origin from the main branch or lateral. Some shoot tips are left undistributed, so apical dominance is maintained. As a result, new growth occurs at the undisturbed shoot tips while lateral bud development and regrowth is suppressed.
Thinning is generally the least invigorating type of pruning cut and provides a more natural growth form of plants. Important in maintenance pruning, thinning cuts are used to shorten limbs, to improve light penetration into plants and to direct the growth of shoots or limbs. Drop-crotching, a form of thinning used to reduce the size of large trees, involves the removal of a main branch by cutting it back to a large, lateral branch. The cut through the main branch is made parallel to the angle of the remaining lateral. When removing large tree limbs, a series of three cuts are recommended in order to avoid tearing the bark along the main truck and severely wounding the tree. One undesirable form of thinning is the bench cut, where a vigorous upright limb is thinned to horizontal limb. Vigorous, upright shoot growth result from the “bench” area because of the absence of apical dominance in the horizontal limb. The correct method is to make the thinning cuts to limbs that are similar in angle to the limb being removed but not greater than 45° to 60° from vertical. Shoots or limbs having narrow-angled crotches are weaker than those having wide crotch angles. The bark of the adjoining branches becomes tightly compressed, preventing normal wood development. Winter ice, trapped down in crotches, often causes narrow-angled branches to split.
Healing Response To Pruning
Healing naturally follows pruning or wounding. It starts in the cambium, a thin layer of cells between the wood and bark. Two areas of the cambium, the bark ridge at the junction of two limbs, and the branch collar, a ring of slightly raised tissue where the lateral branch joins the main limb, function to close off the wound between the plant and the pruning cut. For fastest healing, prune close to the main branch without injuring the bark ridge or branch collar areas. Leaving a stub will slow healing and invite decay. Wound dressings or pruning paint are cosmetic and do little to promote healing of the pruned area.
Time For Pruning
Time of pruning varies with plant species. Prune at times that best complement the growth characteristics, flowering, and other objectives you desire. Many woody ornamentals are pruned according to their date of flowering. Pruning spring-flowering shrubs during the dormant season will remove flower buds formed the previous fall. Summer-flowering plants generally are pruned during the dormant winter season. If plants are not grown for their flowers, the best time for pruning is during the dormant winter season before new growth begins in the spring. Avoid heavy pruning during the late summer and fall because regrowth may occur and make the plants more susceptible to cold injury. Some plants bleed heavily after pruning, which isn’t usually harmful. Trees subject to bleeding should be pruned in the late spring or early summer when leaves are on the tree. Actively growing leaves tend to reduce the amount of bleeding from pruning cuts and allow the cuts to heal more quickly. Plants that bleed readily include willows, birches, maples, beeches and dogwoods.