Typically caused by storm damage or poor pruning practices, we've all seen them: those gaping holes in tree trunks, perhaps home to a family of birds, a pesky squirrel, or a buzzing beehive. Maybe you think they give a tree character, you consider them an eyesore, or you're concerned the cavity will lead to decay and eventually a fallen tree... However you feel about tree cavities, we beg of you, please don't fill them with concrete!
Should you be worried about cavities in your trees?
The short answer is: probably not.
Cavities used to be considered a danger to trees and their surroundings because they open trees up to moisture from rain, snow or sprinklers. In the past, cavities were filled with concrete or more holes were drilled beneath cavities in hopes the water would drain out of the tree and the risk would be minimized. The reasons these solutions are terrible for the tree are, respectively:
Concrete is incapable of moving naturally with the tree, meaning it will rub the inside of the tree and create even more wounds. Not to mention, it'll cost you big bucks if you ever decide to have the tree removed. (Arborists usually don't travel with a jackhammer.)
Drilling into the tree causes wounds in new, healthy tissue, thereby causing more damage instead of fixing it.
In fact, water alone is not what harms the tree. Even though rain may cause decay and rot inside trees, trees are unlike teeth in one major and magical way: they compartmentalize. Compartmentalization means that new tree growth forms around the decay and shuts it out from the rest of the tree. Have a look at the picture below for an example. You can see the decay in the trunk easily enough, but it's been completely contained and--given enough time--the tree would have even grown around it so it wouldn't be visible from the outside at all.
So if rain was all you had to worry about, cavities would be no problem at all. The real issue with cavities is that the moist environment creates a prime breeding ground for pathogens and pests. Despite their best efforts, trees can't shut out fast-spreading fungus or burrowing insects, and these things can hollow a tree and make it a real risk to you and your property.
That being said, if your tree looks healthy and has lots of live growth, it's probably fine. But you can always ask an arborist if you're unsure. It's pretty easy to find an ISA Certified Arborist who's happy to give an opinion at no cost to you.
What can you do about the cavities in your trees?
Unless your tree's cavity opens straight up so water can't even trickle out after a rain (in which case, we encourage you to consider the solution below), there's really no reason to mess with the cavity at all. But if you're still concerned about your tree's cavities or you just don't like the look of them, we've got you covered. Though there are still many people who may recommend filling cavities with spray foam--which, though more flexible than concrete, can absorb rainwater and create even more moisture inside the tree--our favorite solution is simply to cover the hole.
When called to patch a tree cavity, Blue Ox's go-to materials are tin sheeting (because it resists corrosion), aluminum nails (because they're non-toxic to the tree), and just a little bit of silicone sealant (because we're nothing if not thorough). Here's a full run-down on our recommended method:
Drain or dry up excess water in the cavity.
Using your hand and no tools, remove any soft and rotted wood which comes out easily. It's important to take care not to remove anything that might damage the barrier zone that's protecting the tree from the spread of further decay.
Cut a large enough piece of tin to cover the entire hole with some overlap.
Line the tin with a thick bead of sealant all the way around.
Tack the tin in place with a few aluminum nails.
Paint the tin to match the tree if desired.
Voilà! Once your tree is properly patched, you should eventually begin to see the bark grow around the patch until it's like nothing was ever wrong. And that admirable tenacity and adaptability is the real beauty of trees.