This winter reminded us that the hazards of storm seasons aren’t only summer headaches. Minnesotans understand that responding to and preparing for storms is necessary all year long. As arborists, we can mitigate storm risks with strategic pruning methods. Specifically, we use a technique we call reduction pruning to prepare for storms.
Arborists use reduction pruning to improve the structural integrity of branches and to address weak points. We do this by shortening over-extended limbs and suppressing growth in problem areas. Specifically, we remove the outermost sections by cutting back to another lateral branch. To ensure the remaining branch can sustain future growth, we prune back to branches that are at least one-third of the diameter of the original branch. Though, this can change depending on the tree and its age.
There are a variety of reasons why reduction pruning might make sense for a tree. It may be close to a power line, or maybe the canopy encroaches on your neighbor’s roofline. As we look back at heavy snowfall and prepare for the impending storm season, we use reduction pruning to manage the forces that a tree goes through.
As arborists, we often think of trees as large levers. For instance, when snow collects on the end of a limb, it exerts a force both on the end and along the length of the limb. When the branch remains attached to the tree, it’s because there is an equal force acting in the opposite direction: a basic principle of physics.
But this force is not simply equal to the force of the snow on the end. When we travel from the snow towards the branch union, this distance from tip to trunk creates a lever, and the force is magnified. This concept is simple, but when we look at the numbers involved, we see how consequential the mechanics are.
As arborists, we are always playing with forces at the end of levers. When we climb trees, we literally go out on a limb. Understanding how much we’re applying to a branch union is very important.
To approximate this force along the limb, we multiply the force applied at the source by the distance from that source. For example, if the force of snow on the tips is 500 pounds, and the distance to the branch union is 10 feet, then the force at the branch union is 5,000-foot pounds. This is an approximation, as other factors affect the force, but it shows how the force increases across distance. Reduction pruning shortens the length of the levers to lower the force exerted on key areas.
Many of the trees we encounter in our urban landscape evolved in a forested environment. In the forest, trees direct their resources to grow as tall as possible to access limited light. In an urban landscape, trees spread their canopies and grow many lead branches.
When these limbs grow outward, it can lead to stress and breakage at branch unions as the forces increase. Also, certain trees are susceptible to decay, while others tend to form weaker branch unions. Reduction pruning can lower the severity of these issues during storm season.
Pruning is necessary to maintain our urban forest. But cuts do wound the tree, so arborists follow specific practices to avoid unnecessary harm. These practices affect how we perform even the smallest cuts. With these in mind, there are some substandard pruning practices. These may have the same aim as reduction pruning but actually create more risk of damage.
A heading cut is any cut that leaves behind something too small to sustain future growth. This might occur when someone cuts back to a branch less than one third the size of the removed branch. Or someone might cut somewhere between branches leaving behind a branchless stub. Heading cuts create a higher risk for decay and cause a tree to grow sprouts with weaker attachments. This kind of cut might reduce the leverage on a limb but may introduce other structural issues. It is, however, appropriate on very young trees and shrubs.
A specific type of heading cut we come across is a topping cut. Topping a tree entails removing its top, cutting back to a smaller limb or a midpoint between limbs. Neither of these options are desirable under normal circumstances. Topped trees have a higher likelihood of trunk decay, which increases the risk of structural failure. The tree will send out waterspouts to compensate for the cut, which will need maintenance to avoid later problems. Sometimes, a severe storm will break off a tree’s top. In that case, a trained arborist can restore the site of that break and minimize later damage.
Lion’s tailing, also known as thinning, is a process that looks like it sounds. When someone prunes from the interior of the canopy, they leave bare limbs ending in leafy clusters. These limbs can look a lot like a lion’s tail.
The logic behind lion’s tailing is that thinning out interior branches allows wind to move through the canopy. This is actually a misconception. Interior branches and leaves create a dampening effect on the movement of a limb. To over-simplify, these leaves act like small counterweights pulling in different directions. When someone removes these, the leverage on the branch increases. Interior branch growth also helps the branch grow a more tapered shape. A tapered branch will have a stronger union, fortifying it against strong forces.
A common response to a high-risk limb is to remove it. Removing a limb ensures it won’t fall on anything, and sometimes this is the course of action we recommend. But, in some cases, removing an entire branch will create a significant wound at the trunk. Many mature trees especially have a hard time closing off larger wounds to decay. A large, unsealed wound can lead to structural problems in the trunk and sometimes death. But reduction pruning entails making many small cuts, of which the tree will have an easier time sealing. If possible, we recommend reduction pruning to lower the risk of limb failure and avoid problematic cuts.
We love trees and want to preserve them. But we understand the risks a tree can pose to personal property. For trees at risk, removing them may be the only option that eliminates the risk of storm damage. We often recommend low impact pruning or treatment if it makes sense, but sometimes it won’t reduce the risk. If removal is the best option, Vineland’s team of experts can safely remove your tree as well.
Not every tree needs reduction pruning, and it’s not a fail-safe strategy against storm damage. But this is one of many strategic pruning practices that can help you prepare for a stormy summer. To assess whether your tree could use a reduction prune, contact our expert arborists.
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