
Topping is the senseless practice of indiscriminately removing a majority of a tree’s branches. Topping violates acceptable pruning practices, including National Tree Care Standards (ANSI A-300).
Many people think that topping will help their tree or make them smaller and safer, but in reality it is one of the worst things you can do to your tree! Topping does not make your tree safer. Topping actually creates a more dangerous tree because it opens branches up to decay and rot fungi, slowly weakening the tree internally, shortening its life, and causes future branch breakage and storm damage. Before you prune your trees, get some advice from the local Shade Tree Commission or Penn State Cooperative Extension office about proper pruning and how to select a qualified arborist, such as an ISA Certified Arborist. 
The following site has a list of University and Industry websites that explain the problems that arise when you top your tree 
Penn State University’s Don’t Top Trees
Penn State University’s Pruning Landscape Trees publication
Tree Topping – Tree Care or Tree Abuse?
TLC for Trees Homepage – a partnership between University of Tennessee and USDA Forest Service – good video clips
Plant Amnesty’s Bad Pruning Gallery – very funny images
Urban Tree Foundation ‘s Tree Topping Website

National Arbor Day Foundation’s Don’t Top Trees
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