Yesterday I wrote this on Bonsai Nut:
One would think that the question of wound sealing is something that can easily be solved by listening to professionals who trim trees for a living and to scientists who do the research on which the horticultural practices are based. As one can see it is not that simple. There are fights about this in garden clubs. There are fights about this on the local side of newspapers when trees are trimmed in a park. In bonsai circles this is like a religious war. How is it possible that something which should leave no doubts because of obvious simplicity causes such aggravation?
The main reason in my eyes is that folks in general have no or almost no clue about functioning of a plant but they have a general clue how a human functions. They know that if a person has a large wound which is not treated it is not so healthy. They know that it must be avoided to rot from within if you are a mammal. If KK hates me so much that he cuts my belly open and leaves me without treatment it would be just but not healthy. If I told him that any would treatment would be detrimental and it should be left as it is he would call me nuts or an ignorant; and right so. And the same should be true with trees - the earth is flat. Wound sealants were and still are based on this huge misunderstanding. Then along came Dr. Shigo and he revolutionized the horticultural world. His research found something that is called codit (because most folks could never spell or pronounce the word compartmentalization). In a nutshell it says that trees do NOT rot from within as common sense tells as. They form compartments with chemical and physical boundaries. These compartments are started in the center of a trunk and there are several in rings to the outside. While the center compartment will rot at some time the rot will not continue into the next compartment easily. It will eventually, but on the outside the tree will grow and form new compartments. Even if all inner compartments rot there will still be an outer one where the living root will keep growing. This process normal is so slow that a tree will grow away from the rot successfully for many hundred years, even thousand years. Almost every very old conifer in nature rots from within. But due to compartmentalization the don't die.
Dr. Shigo found that humans who 'help' trees more or less professionally make two mistakes:
1) They cut branches off flat to the trunk and by doing so they make big open wounds across several compartments. Thus they create the danger that otherwise healthy compartments will be easily invaded by all sorts of critters, mainly fungi and to eventually rot.
2) Wound sealants as applied conventionally are meant to prevent rotting of the inner parts of the tree. He found that exactly the opposite is the consequence of this. First out in nature there is no way to prevent some sort of fungi or bacteria getting onto a fresh wound. If some innocent landscaper thinks he can prevent this he simply does not know enough. If you seal it you are creating an ideal environment for these critters to do their work. They need warmth and moisture which the get much better with a sealant than without one. So Dr. Shigo revolutionized horticulture by stating that wound sealants are superfluous and can even be dangerous.
Dr. Shigo is considered an eternal landmark in horticultural science for his findings. A student of horticulture who has never heard of him is an ignorant. A person who think he can fight decades of scientific study of the best capacities in the world with just common sense is a silly ignorant. A landscaper who pretends to not have heard of Dr. Shigo and cannot pronounce compartmentalization is at best a very simple person or a charlatan.
But the world has not changed too much. It is accepted mainstream horticultural practice by now to follow Dr. Shigo, but many don't. Well, fact is that one gets away with it because it takes years before the results show and then one can make money with ignoring Dr. Shigo. How that? Well, cut off big branches in a public park an do not treat them with something and you will see that people jump all over you. They 'know' they you are doing something wrong. Folks ask you to treat their trees in the garden and you live big open wounds. They will think you don't know what you are doing. So there are many charlatans who do put wound sealants onto big wounds to make money and to make people shut up. These charlatans know that they should not really doing this, but it is good for them and not all that bad for trees; a least not immediately. And then there are some who have not bothered to continue learning after basic horticultural education. They are doing things the way we used to do them thirty years ago. The problem is that the public would rather listen to them than to science.
What has this all to do with bonsai?
Well we do create big wound on trees all the time. Should not all the findings of Dr. Shigo apply.? My take is that they certainly do.
So far to science. Now to practice. What do I personally do after decades of study?
I know that I am endangering compartments if I do cuts as they are the norm in bonsai; which means close to the trunk and even with a concave cutter which is even worse as Bill V. has pointed out. I choose to still do this because it will not kill the tree. It will create rotting compartments but it will NOT KILL the tree. Trees can be all hollow and still be very much alive. Large trees in nature can be endangered because the loose stability. With bonsai this is not an issue. When I do big cuts I usually make sure that the wound is NOT regular like it is recommended in bonsai. There will be callus and with very big wounds there will always be hole. With smaller wounds there will always be a scar. I want to make sure that the hollow or the scar look natural, meaning never round like done with a knob cutter. I make crude indentations on the edges of a would on purpose.
As far as sealant goes there is a huge difference between conifers and non-conifers. Conifers in general will not rot easily because they have the best protestant already, which is raisin. Deciduous trees will rot much easier, but I know holes will rot even more when 'protected' Well, much to the surprise of many I generally want my hole to rot. I want big natural looking hollows in my bonsai normally. So I could well use sealant to make the holes rot. But since I know that sealant is superfluous I just don't use any. IN GENERAL.
I do use sealant on some cuts where I see the danger of drying out quickly. This is on some tender maples sometimes. Sometimes I brake branchlets accidentally and think that they still have a chance to heal. Then I put some sealant to the wound to prevent dehydration. I use sealant that will disappear by itself after a few months and that is inexpensive and not very visible: COW MILKING FAT. I have a farm supply store very near by where I get this fat for almost nothing. (Yes, it is fat that you put on your hands when you stroke the tits of the cow). It is almost invisible, works like Vaseline and stays on for about a year before it disappears. and is certainly not poisonous.
And I do hide obviously fresh wounds sometimes by putting some camouflage onto them.
What do I tell people who insist in still sealing every wound on a bonsai? Well, go ahead if it makes you feel good but be aware that it is more for YOUR soul than for the tree. And do not believe that the one who is telling you that he does use sealants is the better person.