ROANOKE TIMES
Copyright (c) 1997, Roanoke Times
DATE: Thursday, April 24, 1997 TAG: 9704240005
SECTION: CURRENT PAGE: NRV-2 EDITION: NEW RIVER VALLEY
COLUMN: HOEIN' & GROWIN'
SOURCE: DIANE RELF
According to Jay Stipes, a Virginia Tech plant pathologist, the biggest difference in wounds to plants and wounds to animals is that in animal wounds the damaged cells are replaced by new, healthy cells. In a plant wound, the damaged area is covered over by callus tissue. Or to put it
succinctly, animal wounds heal, plant wounds seal. Anything that reduces the size of the wound and facilitates the sealing will reduce the chance of disease organisms invading the plant.
For many years, people believed that the best way to prune trees was by cutting flush to the trunk or larger branch of the tree, and then coating the wound with tar or paint. After observing how trees recover from wounds in nature and conducting further research, Alex Shigo of New Hampshire proposed a new system of pruning that is now widely accepted. He showed that flush cuts actually remove the tree's natural defense system and provide an entry for decay, cracks and cankers.
As a tree recovers from flush cuts, callus tissue grows on both sides of the wound and gives the appearance of healing. However, by sawing the length of the tree along the wound, Shigo discovered that the callus growth did not occur at the top and bottom of the wound where pockets of decayed wood developed.
In nature, he had observed that sealing off of the wound occurred beyond a point out from the trunk or main branch that he called the branch collar. This collar is usually seen as a small swelling where the branch meets the trunk and provides a strong defensive barrier against attack by bacteria and fungi. By cutting outside the collar and leaving it intact, the recovery
from the wound is much faster.
Also, just as trees in nature recover without benefit of paints, Shigo's research showed no long-term value to painting a wound. In fact, some theorize that moisture can be trapped behind the paints and increase disease problems.
Many of the problems created by improper pruning can be avoided by selecting plants that will do well in the locations where they are planted, thus avoiding altogether the need to prune.
Dr. Diane Relf is a horticulture specialist with Virginia Cooperative Extension and is a member of the faculty of Virginia Tech.
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