Topping is the indiscriminate cutting of tree branches to stubs or lateral branches that are not large enough to assume the terminal role. Topping is perhaps the most harmful tree pruning practice known. It is usually done to reduce the size of a tree. Often, homeowners top their trees because they feel that they have become too large for their property and are a hazard. However, topping will make a tree more hazardous in the long term and is very harmful to the health of a tree.
Information credits: International Society of Arboriculture and Trees Are Good
