When pruning trees, is it desirable to make flush cuts?

FAQ
Question: 

When pruning trees, is it desirable to make flush cuts?

Answer: 

Do not make flush cuts when pruning trees.  Flush cuts are pruning cuts made as close as possible to the trunk or main branch.  They destroy the tree’s natural defense mechanisms that promote wound compartmentalization and callus formation.  When pruning trees, make the final cut just beyond the branch collar and branch bark ridge.  The branch collar is the swollen area at the base of the branch.  The branch bark ridge is the dark, rough bark ridge that separates the branch from the main branch or trunk.  Pruning just beyond the branch collar and branch bark ridge retains the tree’s natural defense mechanisms and promotes compartmentalization and callus formation.  

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