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There are a lot of articles on the web ranked highly in searches that advocate the use of tar/tar based products to seal tree wounds such as pruning cuts. For example — "Homemade tree wounds". However, this is really an outdated practice and the use of such products only seals in the bacteria and fungus, further worsening the situation and accelerating the decay.

Are there concrete sources of information (published literature/university program articles, etc.) that go into further detail on the benefits/hazards of using tar based products for such purposes? I'd appreciate detailed answers with links so that this post can quell any disinformation floating around on the internet.

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up vote 5 down vote accepted

Alex Shigo pioneered the research which has shown that anything that covers a pruning cut works against the natural habits trees have to wall off injury. His works (Shigo, A. L., 1982 Tree Health in the Journal of Arboriculture 8 (12) and a New Tree Biology 1986 helped to show arborists a more effective way to prune.

The basic idea is that trees seal off injuries, they do not heal like we do. If there is damage they build a wall of tissue around it and carry on growing. When you apply tar, or aloe gel or pectin to a tree wound you are sealing in moisture and preventing the wall of tissue from forming. Some of the ingredients can serve as a food source for pathogens and none of them will stop rot.

More details can be seen in these articles here and here. Trees have been around for around 360 million years and managed quite well without our "help". If you see an urban tree chances are it needs a big drink of water more than anything.

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Sometimes I coat upward facing cuts that are very wide(twelve inches or more) with a sterilizing agent and then quickly cover it in a thick layer of tar. It really does keep the rot out, although it slows the healing process. The cut would have rotted out by the time it was half covered if I hadn't intervened. I've seen it happen. –  jmusser Mar 21 '12 at 1:11
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