While the mahoganies were still in #5 pots (sometimes erroneously called “5-gallon” pots), we started pruning them to develop strong structure. We hadn’t worked with this species at the Zoo but noticed right away that the young trees had a tendency to develop “co-dominant stems”; instead of a strong central leader, the trees started to grow several competing leaders. In the forest, a young mahogany would typically be growing with a lot less space around it and probably much less light. It would tend to shoot up a long central leader, reaching for sunlight, rather than using its energy to push out growth in all different directions. Many times, when we use a tree in an urban setting, we give trees a lot of space and a lot of light. While co-dominance isn’t usually an issue with a small tree, if allowed to grow that way unchecked, problems usually develop later.
Co-dominant trunks tend to be weak at the crotch and will break apart easier than trees with a strong central leader. Once the tree is mature, rather than making small corrective cuts that close over easily, we might end up removing very large branches. These large cuts leave large wounds. The open wound leaves a tree vulnerable to all kinds of disease and pest infestation. And many times, removing a large limb leaves the tree looking lopsided, or with a large hole in the canopy. So, for the health of the tree, and for our sense of aesthetics, we know that it is always better to train a young tree to develop into a strong, mature specimen.
When we plant a tree, we try to project out perhaps 25 to 100 years. Many of the trees we plant will be around longer than those of us planting them. We ask ourselves lots of questions before we plant a tree: How big will this tree get? Will it be in the way of vehicles or pedestrians? How invasive will the roots become and what structures may be affected by the roots?
Trees in our communities and parks make us feel good. They create shade, screening, healthier air. They trap particulates, sequester carbon, and make our surroundings feel more natural and pleasing. They can also create problems when we don’t pay attention to what the tree will be like when it is mature. The first five years of pruning a tree takes discipline, but it can save countless hours of corrective pruning in the future and possibly even save us from the heartache of having to remove a beautiful, mature tree.
Keep an eye on those trees in Monkey Trails and all around the Zoo. These young trees will tower above us in no time, keeping us cool in the summer and making Monkey Trails become even more “the place where every monkey would love to live.”
Mychael McNeeley is a lead gardener and certified arborist at the San Diego Zoo.
Read Mychael’s previous blog, A Perfect Way to Start the Day.