Edward F. Gilman, Professor, Environmental Horticulture Department, University of Florida
Proper pruning and training of young, large-maturing trees has a significant impact on their future growth. Arborists and urban foresters know that young trees properly trained in the nursery with a dominant leader survive storms better and are likely to live longer in the landscape than trees that are not trained, or those that are improperly pruned. In addition, trees with one dominant leader can be limbed up easily to create clearance for vehicles; whereas those with multiple leaders are difficult to prune. The nursery industry recognized this with the publication American Standard for Nursery Stock (ANSI Z60.1 1990) which calls for a dominant leader in large-maturing shade trees. Some states have adopted more detailed standards for nursery trees that also call for a dominant leader (Grades and Standards for Nursery Plants, Florida Dept. of Agric.1998). For a number of years, leaders in the nursery industry have trained their large maturing shade trees to one trunk. There is now consensus among green industry leaders that a dominant leader is the most appropriate method of growing shade trees.
Second to placing a tree in an appropriate location and planting it correctly, pruning has the biggest impact on longevity. Landscape managers should know that planting well structured nursery trees makes it easier for them to complete the job begun in the nursery of developing structurally sound trees.
The main objective of pruning in the nursery and landscape is to create strong structure by guiding the tree’s architecture. You minimize the growth retarding effects of pruning by removing the smallest amount of living tissue at any one pruning, while producing a strong structured, healthy tree with a functional and pleasing form. For most trees, this is accomplished by developing and maintaining a leader early in the life of the tree (one year old is not too soon), and pruning at regular intervals when the tree is young. A regular pruning program prevents branches from growing too big too fast and outpacing the leader. Branches that grow too big too fast often develop into codominant stems, sometimes with included bark. These weakly attached stems can split from the tree as it grows older. In addition, low codominant stems often have to be removed in the landscape as they droop too close to the ground. This leaves a huge pruning wound that can initiate trunk decay. Nursery and landscape managers can discourage lower branches from developing into codominant stems by pruning on these branches to slow their growth rate. Strive to keep branches less than about half the trunk diameter.
There are three types of pruning cuts designed to develop a strong structure in trees. A reduction cut removes an upright stem back to a live lateral branch no smaller than about half the diameter of the cut stem. A removal cut removes a side branch back to the collar on the trunk. Cutting into the collar could slow growth and slows closure over the wound. Correctly executed removal cuts close in a circle, not an oval. The third type, a heading cut, is made back to a bud or between branches and, along with the reduction cut, is used to slow growth on the cut branch. Heading cuts also initiate sprout growth from behind the cut. Heading cuts are not appropriate for trees in the landscape unless there are few other options such as during restoration pruning following storms.
It is never too early to begin pruning to subordinate or suppress growth on codominant stems. Each time you subordinate a stem or aggressive branch, growth rate is slowed on the cut stem or branch. This allows the stem that was not cut to grow faster and dominate. More light reaches branches that were previously shaded above the cut, and these will grow to fill in the void created by the removed branches. It is important to remember that removal of small branch parts at a young age rarely leaves unsightly voids or gaps in the tree canopy. If you are leaving large gaps you waited too long to prune or you are removing too much of the stem or branch. In many cases, the reduction cut is the best choice for creating sound structure in young trees, although heading cuts are sometimes appropriate on young trees in the nursery. It is important to learn this technique and to spend enough time with the pruning crews to ensure it is carried through. I find that it is easy for workers to revert back to old habits in a matter of hours following an initial instruction session. Once you have learned the technique, have your employees watch you, then have them prune trees while you are present before sending them on their own.