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In this blog post, I’m going to put my arborist hat on again.  I’m going to discuss something that I’m sure just about everyone has seen; a large hole in a tree.  Sometimes it is call a hollow.  In arborist language, we refer to these as a cavity.  In many cases, these are a result of some sort of damage, whether that damage is from lightning or high winds or animals.  Many times, trees in an urban or suburban setting may have a cavity or a wound due to something caused by people.  These injuries are usually inadvertent.  Someone strikes a tree with an automobile, construction equipment or a lawn mower.  It could also be as a result of improper pruning.


Trunk wounds that penetrate the bark will damage the cambium layer, a thin layer of vascular tissue, which is vital to movement of water and nutrients.  If less than 25% of the bark around the trunk has been damaged, the tree will probably recover.  When fresh wounds occur on the trunk, the damaged bark should be removed carefully, leaving healthy bark that is sound and tight to the wood.  A wound dressing (tree paint) is not necessary.  Sometimes I do use a spray “pruning tar” when I prune away larger branches.  Theoretically, it seals the area where the branch was removed.


For wounds, you will be able to observe the wound closing from the edges each year as the tree grows.  When an older wound is discovered, remove the dried and loose bark back to the area where the new wood can be seen along the edges of the wound.  Trunk wounds that are not addressed could potentially be a hazard in the future.


Once a wound occurs, decay-causing fungi can enter the heartwood and the decay process begins.  Trees have a unique defense.  The wood around the injury begins to produce special compounds in the wood cells that set up a wall or barrier to isolate the infected area.  This is called compartmentalization.  In a vigorous tree, new growth continues to form and add to the sound wood.  Once compartmentalized, discoloration and decay will spread no further unless one of the barriers is broken.  Storm-damaged branches should be properly pruned to expedite the healing process.  Avoid pruning directly against the trunk since flush cuts can lead to extensive decay.  Remember to prune away from the branch collar, as I said in an earlier post. (“Tree Care for Turfgrass Warriors”)  Prune hazardous branches immediately.


Years ago, filling cavities was an accepted practice.  The wound would be cleaned and scraped down to sound wood and filled with cement, mortar, or bricks.  These practices frequently penetrated the tree’s natural defensive barrier, allowing decay to spread.  Fortunately, this practice has decreased, along with flush cuts and tree wound paints.  If the tree ever had to be removed, having cement or bricks in a tree could be very dangerous for the arborist crew.

Your mission is to help the tree heal those cavities/wounds.  If we can't get them to heal, we can try to get the tree to compartmentalize those cavities.


The cavity in itself is not important unless it is a very large one (like half the diameter, or more, of the tree), but it is the breeding place which it affords for enemies such as insects and fungi that is highly important and worthy of the most serious consideration in the care of trees. 


The accumulation of moisture and the exclusion of light, which are characteristic of every cavity, are the ideal conditions which the spore of a fungus disease seeks.  You want to make sure this does not become a major health issue for the tree so it doesn't become weak, then come down on your house, automobile or loved ones in a storm.


Having cavities near the base is worse than having one up high.  If decay sets it in at the base, the tree could become unstable.  Here are some courses of action:


1.  You want to keep critters and other stuff out of that hollow area in that tree, get some screen material (metal or plastic screen is fine - like the material from a screen door or porch).  Cut it to size for the entire area.  Tack it up around the area.  Use short nails (like tacks, not thumb tacks, but those might work).  Do not use long nails that will penetrate through the bark into the cambium.  A screen will allow air flow.


2.  Do not spray more water into the openings to try to clean it.  You want this area to get as dry as possible. 


3.  Using your air hose attached to an air compressor to clear it out is fine.


4.  Try to clean out or pull away any loose material.  But don't pull away any "good/healthy" bark or wood.  Cut off any ragged bark edges with a sharp knife or saw.  You could even use a hammer and chisel.  If you can take the surface down to healthy material, that's fine.  Take care not to remove any healthy bark and expose more live tissue than necessary.  If possible, the wound should be shaped like an elongated oval, with the long axis running vertically along the trunk or limb.  All bark around the wound should be tight.


5.  If you are feeling really industrious, get some lime sulfur solution and spray it (lightly, with a pump sprayer, not something attached to your garden hose or a hose attachment) all over the inside of the cavity - after you manually clean it out.  Lime sulfur is very caustic.  It can mess you up.  Wear protective clothing, a mask and eye protection.  Lime sulfur also may be hard to find.  Or, use copper fungicide.  Same procedure.  Copper fungicide can be found at Home Depot or Lowe's.  Do this, then install the screen.


6.  As I said, in the old days (actually not too long ago), they used to fill tree cavities with concrete.  They thought it would strengthen the tree.  It is now known that it causes more injury.


7.  Once you have treated the cavity, and have covered it with the screen for a while, some say you can fill the hollow with spray insulation.  Not sure about that.  I think that could possibly still retain moisture, or trap moisture behind it.  The point is to get that cavity clean and as dry as possible.  The screen allows for airflow and keeps out any animals.


8.  This tree needs TLC now.  It is under stress.  Make this tree the top priority for tree trimming/pruning.  Get rid of the deadwood up top.  If possible, or if there is a bed around the tree already, get some good mulch around it. 


9.  If there are insects in the vicinity, it is okay to spray insect killer/pesticide around that area.  Just on the surface of the soil.  Do not get it on the tree.


10.  This tree will need SLOW watering if you get in a bad drought at some point. 


11.  Also think about fertilizing the tree.  Having a professional arborist fertilize it would be great.  If you can’t go with a professional, do it yourself.  Fertilizer stakes are okay.  Getting a root feeder and using that with your garden hose would be even better.  Remember to fertilize out along the drip edge. 


Below is a photo of a tree in my yard.  I’ve done what I could.  The screen material I used is something used to cover a gutter on a house.  It was attached with some very small and shallow screws which did not penetrate the bark.  The bad news is, this cavity is right at ground level, at the base.  Is does get wet.  But, the screen allows it to dry out well.  Guess we’ll have to see how this works.  Seems okay for now. 


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