If you need to build a fence anywhere near a tree, please keep in mind one simple fact: trees grow. Not just taller. Not just fluffier with new leaves and branches. Every year, trees add a layer of new growth all around their trunks, just beneath the bark. This layer might be skinny or thick but, whatever its dimension, the new growth expands the diameter of the trunk outward in every direction for the entire height of the tree, and even at the base flare. So, what?
This subject may seem almost too obvious for consideration. However, all the photos in this post were taken during a fifteen-minute stroll through one Cambridge (MA) neighborhood, revealing in this tiny sample just how widespread is our society’s lack of understanding about how trees grow. Even professionals who should know better seem to forget or ignore this basic reality.
When a fence is built too near a tree, the tree's thickening trunk sooner or later will bump up against it. The bark then will swell around that contact point. If the structure is not soon removed, it will eventually penetrate through the bark into the living interior (the cambium) of the trunk.
This in itself is not enough to totally kill a tree. For that, you’d have to cut through the cambium all the way around the trunk (called "girdling"), fully blocking the transport of water and nutrients from the soil up into the leaves and branches of the tree. If only a small portion of the cambium is cut, the harm may be minimal, and only the branches directly above (and fed via) that portion of the cambium will struggle or die. In some species quite a lot of the cambium can be damaged and the tree may still be able to grow.
However. Although a poorly-placed fence might not outright kill a tree, it will eventually damage either the tree or the fence, or both. The tree may become deformed, need special care, have to be replaced, or die and no longer provide beauty, shade and habitat. If the fence is damaged, it will surely need repair or replacement. Both consequences are not just unfortunate and disappointing, but also an unnecessary expense of energy, resources and money.
What to do instead
When deciding where and how to build a fence (or actually any kind of structure), the best thing is to plan ahead in full recognition of how all trees expand in girth as well as height, and position things so your trees can grow outward as well as upward without bumping into any sort of solid, unmoving structure.
You could choose any or all of these options:
- Figure out a way to build the fence so it is not physically attached to any tree.
- Decide to build a fence far enough away from an existing tree that the trunk can thicken a lot before hitting the fence.
- Plant new trees far enough away from an existing fence or structure so they can grow a long time and never reach those structures.
This can be a challenging design problem, especially in urban situations where space is at a premium. But to minimize later regret, you, and your trees, deserve this kind of planning and good judgment.
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