One of the Myth's of Gardening... The research showed that there was no difference from using many different kinds of dressing and using nothing at all, except some made the wound worse by harboring this or that pathogen or interfereing with the tree's own chemistry. To wit:
MYTH 5 Tree wounds need dressing
Countless generations of gardeners have painted tar or paint on wounds after lopping off branches. It was believed that, without protection, trees would be vulnerable to pests and disease. That myth was conclusively debunked in the 1970s and 80s by Dr
Alex Shigo of the US Forest Service. Shigo was a passionate tree specialist whose theory of compartmentalisation of tree decay changed the way trees are pruned. He showed that, when trees are injured, they respond with chemical and physical changes, forming barriers that stop or slow the spread of disease and decay to the rest of the plant.
In 1983 in the
Journal of Arboriculture, Shigo published results of 13 years of
research on wound dressings. Applying tar does nothing to aid this process but can provide a home and protective layer for pests and fungi, he showed. It also inhibits the process of compartmentalisation.
In spite of my being in possession of this great and incontrovertible evidence, I still paint shut all JM wounds greater than 1 1/2" in my yard. And in some cases it doesn't help at all. Stupid is as stupid does?