Pruning to Promote Strong Structure
Developing a preventive pruning program requires that managers be familiar with the techniques of structural pruning. Structural pruning should be practiced for the first 15 to 25 years of a tree’s life. This is the amount of time required to establish strong structure in the canopy and will help to make the tree more resistant to storm damage (Figure 8). In structural pruning, reduction and removal cuts are used to slow the growth of large or rapidly growing branches that compete with the leader. This encourages the one stem you chose as the leader to grow faster.
The next step in developing a preventive pruning program is to determine the pruning cycle and pruning dose. A pruning cycle is the interval of time between each pruning event. The interval is affected by many factors. For instance, trees coming from a nursery with sound pruning practices will have a better structure to start out with than trees coming from a nursery with poor pruning practices. These low quality trees may require more pruning at a higher interval than the high quality trees. Pruning cycles are also affected by growth rate, climate and species. In warm climates where trees grow faster, the intervals between pruning events should be shorter. Species that are prone to decay should also be pruned more often so that the need to make large cuts can be avoided. A typical pruning cycle for an active, preventive urban forestry pruning program in Florida is about three years. If the pruning cycle is too long, defects may become more severe. This results in having to make large pruning cuts, which can initiate pockets of decay in the trunk and branches. A pruning cycle of 3-5 years will require a higher pruning dose to achieve pruning objectives. Conversely, a pruning cycle of 1-2 years will require a smaller dose.
Regular tree maintenance can mean a world of difference when it comes to tree strength during a storm. Preventative pruning to thin the top of the tree – the canopy – reduces wind resistance and in turn, the force that can damage branches or even the trunk of the tree. In addition, pruning removes dead branches that can break easily, causing damage to the tree and the surrounding landscapes. Strong branches are better able to withstand high winds. Both the size of a branch and the size of its attachment to the rest of the tree determine its strength, so careful evaluation is needed to determine which branches may need to be removed or reinforced by cabling and bracing.
Protecting the root system of the tree is important, too. A wide mulch ring around the base of the tree removes the need to use lawn mowing equipment close to the roots of the tree, minimizing damage, and also allows water and nutrients to drain down through the soil to the roots.
In addition to the excessive forces that come with storms in the form of high winds, lightning is also a concern with summer storms. Lightning will strike anything that provides a good path for the electrical charge to travel from the storm cloud to the ground, including trees. Installing lightning protection systems in valuable trees can help prevent damage.
Your arborist may recommend a lightning protection system for your landscape trees that directs electrical forces down a series of wires and into the ground, away from the tree itself.
Watch for Cavities
An open cavity in a tree’s trunk, just like in a person’s mouth, creates a weak spot in the entire structure. A tree with strong, healthy wood is more likely to survive destructive, stormy weather.