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Cavity in Your Tree – Internal Decay

by Keith Brown
July 11, 2010

Ever wondered how a tree that is hollow on the inside can be alive, or have not fallen when oak_tree_hollow.jpgthe wood is hollow in the middle? Internal decay is a common phenomenon in trees. I wouldn’t call it good, but it is normal (i.e. very common). These cavities are caused by mother nature’s decay fungi. Their job in the grand scheme of things is decompose woody materials back into the soil.

The decay process usually gets started from some large wound. Could have been from a lightening strike, broken limb or even a damaged root underground. Any large wound usually leads to decay. Wounds larger than about 3-4″ diameter (can vary depending on tree species and health) take the tree so long to callous over and close up the wound that rotting fungi get established and start breaking down the inner column of wood in the tree.

It is a good idea to avoid making large wounds to trees. If you can achieve your goals of pruning by removing multiple small limbs instead of one big limb, that is ideal. If you have to trench or cut roots near a tree, stay as far from the trunk as possible. Or, use an airspade to dig without cutting roots.

Many trees, such as live oaks do a good job of compartmentalizing the decay. This means that the fungi only extensive-hackberry-decay.jpg rot away the dead wood in the center of the tree and the newer growth rings are protected. Therefore, internal decay is usually more of a structural integrity issue and less of a health and vigor issue for trees. Some trees do a very poor job of compartmentalizing decay and should be evaluated carefully. Hackberry and Ash are two good examples of trees that do a poor job of compartmentalizing decay.

Compartmentalizing decay is one way that trees deal with decay. There is another factor that affects a trees ability to tolerate wood rotting away. I like to explain this by comparing to a steel fence post. A steel fence post is hollow in the center, but its shape gives it strength. The same goes for a tree. The tree can tolerate loss of wood in the middle so long as the tree puts on new growth rings on the outside. Each new set of rings adds to the tree’s strength.

“Rams-horns” also help trees deal with massive trunk wounds. If there is a very large wound on the side of the tree trunk, this type of wood development will usually occur. rams-horns.jpg A “rams-horn” is when the new callous tissue growing on the edge of the wound culs inward to create a columnar growth rather than growing flat across the wound. Growing flat across the wound would heal the wound faster, but rams horns provide better structural improvement. By growing these two columnar growths on each side of the wound the tree has create two smaller “fence posts” on each side of the wound.

Decay is a tricky subject; even for the most experienced and skilled arborists. Tree species, tree health, weather conditions and the location of the tree all play a role in quantifying the risk factor of a tree with rot. Most importantly, you should consider potential targets. If the tree is in a field you can live with a high percentage chance of it falling over. If it is growing over your kids play area, you can’t tolerate much risk at all.

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Be sure to post a photo if you want some advice on your tree.

  • Stephen says:

    One of our live oaks is pretty hollowed out to about 8 ft high, leaving a C-shaped base of the trunk rather than an “O”. The trunk then continues up and slighty angles off part of “C” base, ending with a 20-25ft tree that otherwise looks healthy. Is it necessary to do something to stop/slow the internal decay? Also, is there something structurally that should be done to the trunk?


    • Keith says:

      There isn’t really anything you need to do at this point. A wound that severe is going to affect the trees life span substantially, but for now the tree is doing it’s normal thing to cope. As the tree grows in size there will be some structural pruning needed, but that is years down the road.

  • Stephen says:

    Thanks very much! So no need to fill with cement or place a steel bolt.

    Thanks again!

    • Keith says:

      Definitely no cement. I have seen scenarios where a bolt would help, but it is not a standard application. Can you post some pictures of your tree?

  • Stephen R says:

    I liked the article and was wondering if I could get a bit of advice. I have a large oak tree near my house. There is a large hole in it — maybe 2 feet tall and 1.5 feet wide, and a large hollow within the tree.

    The first thing I noticed was that the hollow went lower than the surface hole, so it would hold water when it rained. This appeared to be causing damage to the wood at the bottom of the hollow, so a year or so ago I drilled a small drain hole from the hollow to the outside of the tree. Result? The bark is now split away from the tree below the drain hole — apparently the water draining out the hole was running behind the bark and damaging it from beneath.

    I’ve had one tree guy say leave it (before the drain hole damage, though) and another suggest I could “fill it with chicken wire and Great Stuff” (foam). Because of the drainage issue I decided I would fill it with wire and foam as suggested, but as I looked closer, I realized that the hollow goes quite a ways UP into the tree. I can reach my arm up inside the tree until my elbow hits the TOP of the surface hole — the hollow goes up and tapers to a rough point. Does this mean water is coming down from above somewhere? If so I would think filling it with foam would be a very bad idea.

    Any thoughts? Should I try to clean the rotten wood and fill it in, or leave it alone?

    (BTW, I would submit some photos, but I can’t find the “Browse” button !)

    • Keith says:

      The problem with filling cavities is that you cannot create a water tight seal. That means water IS going to get in when it rains. And, if you have attempted to fill the hole, there will be very minimal air circulation so the cavity will never dry out.

      It is very possible that water is entering the tree through wounds up in the canopy and draining through to the base.

      • Stephen R says:

        Good information. So my guess was right — filling it would be a bad idea because water would still get in, and have a much harder time drying out.

        Thanks for your time and expertise.

  • Meagan says:

    I have a maple, pecan and oak tree all close together in my front yard, and for the most part leaning away from our home. Last night a huge limb fell off the maple tree, and the base where the limb fell appears to be black, possibly rotting? Other than than that the leaves appear normal. Should we be concerned with the health of the tree?

    • Keith says:

      You shouldn’t be concerned about the health of the tree, but you should be worried about the structural integrity. Health and strength are two different things. Maples don’t deal well with decay around here; you may need to remove the tree. Definitely have someone look at it.

  • Clayton Armistead says:

    I have Peacon trees . Two of my 30 year old trees have big limbs broken from wind a few years ago. There are holes left that are not healing over that allowes water to get in.
    What can be used to fill these holes?
    Thanks for your suggestions.

    • Keith says:

      It is best to leave the wounds open. You cannot make a water tight seal, and if you attempt to, you will block air circulation. That would prevent water from getting out once it gets in. If the wounds aren’t healing well maybe you should address the health of the tree to see if you can speed that process up. Check out my tree fertilizing article and read about the tree fertilizer programs we offer.

  • brian lecleir says:

    is there something i can do to save my cherry tree there is a big hole in it

    • Keith says:

      Typically, I recommend you take no action on cavities. The tree may be fine. However, cherry trees don’t grow well in Austin so I don’t have much experience with them.

  • Herb says:

    I have a young apple tree, that at planting three or four years ago, I placed a plastic guard around the trunk to prevent damage from small gnawing animals When I removed it recently I found a wound that appeared to be wet with sap, various insects and a large worm in a hole. I have decided to leave the guard off , clean out the rotting wood as best possible. What else can I do?

  • Joel says:

    We have quite a few oaks (white or red) with with hollows where trunks or branches broke off years ago forming “cups” that hold water long after each rain, and others where a pool was formed at the base where 3 or 4 trunks merge together. I appreciate your comment that it’s not possible to completely seal these hollows, but leaving them is problematic because they are breeding grounds for mosquitos. I’ve seen it suggested to fill these hollows with sand but that doesn’t seem such a good idea either. What do you think about filling them with something like peat moss or vermiculite or some other material that can absorb water & conduct it to the surface to allow it to evaporate? (Just thinking about it; I haven’t tried this yet. ) Can you think of a better material to use for this purpose?

  • sylvia says:

    I have a large Chinese maple tree on my property. It has ganoderma applanatum mushrooms at 5”; 1′ and 2′ from it’s base.
    Is there anything I can do to save the tree which is as tall as a two story dwelling .
    Are my other trees compromised? I live in a wooded area in the north east

  • reid piper says:

    i have a very large (30″ diameter) pecan tree and very old. i have done all the wrong things to repairing damage to the trunk. there is a large hole about 8 feet off the ground through which the water drained to the base (internally) causing a large amount of rot to occur.

    the soft wood was full of insects of all kinds which was difficult to control from the outside.

    SO i filled the hole with concrete, caulk the edges and painted the concrete with a concrete paint hoping to stop the flood into the trunk, then i took the chain saw and cut out all (most ) of the dry wood , insects and all t;his left a cavity of 15″ across by three feet high by six inches deep
    i am down to good solid wood , have hopefully stopped the leak, have removed the soggy soft wood and can better control the insects. the cavity is only a small part of this huge tree.

    what i wwould like to do is coat the open wound with something protective to allow the tree to heal over.
    is there any type of sealer?
    could it be painted with a water base exterior paint? pecan colored ?


    • peg says:

      I have a bradford pear that had a large branch break off from about two ft up bottom several yrs ago. An area of decay has been happening and getting deep enough I am getting very worried I might lose the tree if I don’t do something. Soft decay can be flaked out. I have cleaned it out before and sprayed with tree coat, but it is continuing. Would cementing it after cleaning soft decay out be good since water is not pouring in from an upper area? Help, this is such a great shade tree. My son topped it out after the large limb broke away and it quickly filled out again, but with lighter weighed limbs which has helped with preventing further breakage.

    • ARNOLD says:

      I have a live oak that the previous owner cut a 8 inch limb off and did not coat cut. It has rot started in the center of cut and is about 6 inches deep. How should I stop the rot and seal?

    • clayton nelson says:

      it appears the general rule is to eave the cavity alone. but, i can remember 200 year old oaks in new orleans city park where very large cavities were filled with concrete and the limbs were supported by cables and turnbuckles?

  • Norman says:

    we have a beautiful 60ft red oak, 5ft from our home in New Jersey.
    about 10 years ago we were cooking barbeque with the grill too close to the tree and it killed the bark in that area (3′ W X 2’H). the bark in that area came off and I thought the tree was healing quite well.
    I just discovered only the outside of the tree was healing and started picking at the center of the wound only to find the core filled with termites. The cavity is almost 2ft across and I don’t know how far up or down. I have some large limbs (12″ dia) about 15-20ft high.
    will I lose the tree? should I trim the large branches and fill the hole? Please help, we have enjoyed the shade for many years.

  • peg says:

    Can a magnolia tree be topped out?

  • Megan says:

    My flowering Cherry tree is about 20 years old and 20-25ft tall, two years ago, it started to sap during the spring and this year the bark around the trunk has started to crak and deeply split in several places. The tree is very close to our house and and an area were our grandchildren usually play. Can we help this tree? We would like to save it, but worry about the children.

  • Dale says:

    I own a Farm in NJ with a Willow Oak that is about 60 years old and the trunk is close to four foot in Diameter. About ten years ago it was hit by a tractor.The tree won the battle but it was wounded and now the wound has opened to see the inside hard wood chewed up and rotting.Since the hole is only about one foot by one foot, I want to close it up to keep pests out because an army of black ants was moving their larvae around inside until I sprayed. The wood inside the tree is still very hard but looked like termites and/or ants were making mazes in it. Can I seal this hole with something if it appears the tree is healthy with no other way for moisture to get it? Thanks!

  • terry stein says:

    is there any way to help a large rams horns heal my case is a poplar and a wild cherry right in front of my new cabin in ohio

  • Amber Sheppard says:

    HELP! What do I do (if anything) and why is this happening? Approx. 60 year old Red Oak and the bark has started splitting. Removed dead limbs and branches about a year ago and treated for Iron Deficiency. Is it internal rot and if so, roughly how long before I should have tree removed? Should I plant something in front of this now or wait til old tree is out?

  • Nancy says:

    I have a 80 year old Maple that lost a limb and the tree now has a whole that catches rain. The tree is rotting inside and fungi is growing outside.
    I am concerned that the tree will die…..I lost a 120 year old tree last year.
    What can we do to improve the trees health?

  • Brian Backstrom says:

    We have a big hard maple in our front yard. It developed a big split and we had to have several limbs removed and the tree cabled and bolted. There’s a big cavity where it split. I climbed up in it and tried to clean it out, but was unable to reach all the way to the bottom of the cavity. I’ve heard many contradicting things about what to do. Fill it with a tar based prodict like Black Jack, with expanding foam (Great Stuff), not fill it at all. My main concern is that with leaves, dirt etc. inside that will trap in moisture and the rotting will continue unabated. On the other hand sine I can’t completely clean it out filling it will trap what is in there in.
    Any suggestions? Should I fill it? If so, with what?
    Brian Backstrom

    • Tom Brown says:

      I just read your question about what to do about the hole in your tree. I have the same problem with sugar maple that seems to be in good shape otherwise and would like to know the responses you received about what to do.

      Thanks, Tom

  • Tom says:

    I am an Arborist and would like some advice please.

    The picture is of an Ash tree about 80-90 feet high.
    Has major decay in the bottom. Tree forks at about 20 feet with both forks reaching the same height.

    Is this still safe to climb and dismantle, lowering stuff down off of itself?

    Any help or advice would be very much appreciated please.

    Thanks, Tom

  • apollo says:

    I have a maple tree that has about half of the trunk’s inner core is rotten with mushroom growth inside as shown by the photos. The tree is about 50 feet high and 2.5 ft diameter at the base of trunk. Is there anything I can do to save it or from being hazardous?

  • Ann May says:

    My 67yr old oak tree 30 ft. from my house is huge. We noticed a large hole going down middle of trunk. A tree man drilled tiny holes in a few place to drain water. Scooped out the wet bar etc., then filled the tree close to the top of the hole with sand a a tar mix. Tree is green & has been dropping lots of acorns. Should I be concerned about it dying? It seems pretty healthy otherwise. Appreciate your opinion. I would hate to lose this tree. Thanks, Ann

  • ALAN CODY says:


  • C Mckeown says:

    A large old ash tree has been felled it is black in the middle does this mean it was dying with decay it was about 200 years old it had green leaves on the branches it did not look sick

  • Rachel Sheppard says:

    We removed ground cover off the lower section of our huge magnolia to discover the trunk has a hollow area. We don’t know much about this tree as we bought the property only 9 months ago. Can you tell us what the future holds for this big tree?

  • Rob Hutchins says:

    I have a large White Oak tree with a injury from a limb breaking off. The tree has a circumference of 156″ and the wound is 2 1/2′ wide, about 5′ off the ground. Looking at the wound it seems full of dead wood. The tree is healing but the wound is too large.
    My question is: Should I cut out all dead wood? and if so should I do anything to the surface of the cavity?
    The tree is deep in the woods, only hunters walk near it.

    This is by far the largest tree on my woodlot. I only want to do this if it would give a little more life to the tree.

    I also have a small Black Walnut tree with about a 5″ diameter that I have been cutting the lower limbs on. It has a small wound a couple feet up the tree. The bark is missing in one spot. Looks like maybe a limb was there at one time. All I can see is an opening. My question on this is:
    Should I carve the bark around the wound so to see inside? Or should I leave it alone. I saw this a year ago and thought it would heal, which it did not.

  • Catherine says:

    I am looking at a house for sale in So. Cal. near the mountains of Pasadena. The neighbor’s huge sycamore tree has huge branches hanging over the house I am looking at. In the middle of the tree near the ground a branch must have broken off years ago causing a gapping hole. The tree looks healthy but I am concearned about those branches hanging over the house. It’s super dry there but I can’t find much info about sycamore trees.

  • Anthony Vega says:

    Wut can I do with fungus (rams horn) on the bottom of a pecan tree is there a way to cure it or slow it down from decaying anymore? tree is about 40ft 2ft diameter.need help asap Thanks, A.Vega

  • delshad says:

    Hi Dear, can you tell me if there was a problem with the tree in the picture that caused it to fell down? In the middle of the trunk is dark it is natural?

    • Keith Brown says:

      The dark color in the middle is decay. I would need to see more of the canopy to see what weight distribution and lean looks like. Also, I can’t tell what kind of tree it is from the pictures; often, general tree habits can insightful. What part of the country is this?

  • Anna Mae Becker says:

    I have a flowering Apple tree on my front bank in front of my townhouse that has a large cavity on the lower side near the base of the tree. A land- scapper said that he could take care of it by filling the cavity with branches of trees and covering it\Please comment.

    • Keith Brown says:

      That solution sounds bizarre to me. I have never heard of covering cavities with tree branches. I cannot imagine what benefit it would offer.

  • steve smith says:

    I have a ribbon leaf Maple it is approximately 15 years old. And one of limbs was trimmed and now from the point of the cut downward. The tree has began to rot.

    Please help me do the right thing.

    I have removed all of the loose spongy wood my cuts are approximately 2 inches into a 4 1/2 inch ranch. This is a vertical branch and because it runs into the main trunk I would like to save it.

    I simply want to make sure that I am using the very best method.
    Over the last few years I have used printing sealer and appears that there has been more water getting inside.

    That, one point I did actually use enamel clear spray sealer which is typically used as a clear coating over any project that you would paint.

    Many thanks for your help and your ideas.

    This tree is important and it is one of a pair in my front yard.

    Thank you in advance for your comment,


    • Keith Brown says:

      Trees actually manage decay better with no pruning seal. Would you put paint or tar on yourself to treat a cut? Really, the best thing is to address the overall health of the tree: composting, mulching, fertilizing.

  • Bobby Rash says:

    Hello Keith. I have a mature pecan tree that is developing about a 12″ diameter rot hole where a large limb had been removed from the trunk about 10 years ago. Should I clean the rot out and fill with an asphalt base sealant or possibly concrete to prevent further decay??? Thanks

  • Heidi dickins says:

    We have a cherry tree in our garden, which is 50+years old. Recently we have added an out house to our garden, and had to dig up a root of the tree. And unfortunately had to remove a section of the root.
    Whilst digging we removed a section, and then dug further down and replaced the root under the soil.
    In spring the tree came to life with leAves and blossom. However now the tree appears to be loosing all the leaves and has produced no fruit.
    Is there anything I can do to help keep it alive?

  • kim says:

    Had a tree guy cut a few limbs mind you he wanted to remove tree, when he was done he left a gaping hole in one of the huge limbs claims it is decayed, now is telling me tree needs to be removed. my question is can I save the maple tree?

    • Keith Brown says:

      Decay exists in almost all trees to some degree. Assessing decay extent is very touchy and requires a LOT of experience. Without seeing the tree for myself, it’s impossible to say. If you post some photos here I’ll try to give what feedback I can.

  • Wendy says:

    Regarding the subject matter and a Magnolia tree I love… what about wood destroying insects? Ants, termites (?)… should we leave them be? do they serve a purpose of eating the decaying wood? or should we exterminate them? I hosed out three holes on this tree and flushed out thousands of them, including a larva looking “thing” (queen?) as big as my thumb. If we exterminate, is there anything natural to use? Thank you for your blog!!!

  • L. Lipscomb says:

    I have a red maple approx. 12 yrs old. Some of the limbs have broken due to a hollowing in the center of the woody portion. This just occurred for the first time this spring. What could be the cause and treatment?