Question: What is "topping"? Why are they bad for trees?
Dr. Alex Shigo: "Topping" is the drastic removal of the tops of vertical stems, where cuts are made between the nodes. Nodes are positions on stems where a leaf bud or stem arises. To cut between these nodes leaves a stub, which is food for organisms that start rot or cankers. The swollen branch collar is not a stub. It is important to know that the age and vitality of the tree determine the best approach to pruning and training a tree. A young tree is much easier to train than an older tree, and a young tree can more easily tolerate the removal of more branches. Older trees should be pruned only to remove dead, dying, diseased or hazardous branches; this will conserve their energy.