PPDL Picture of the Week for
June 8, 2015
Janna Beckerman, Associate Professor, Botany & Plant Pathology Dept, Purdue University
Topping, also called ‘heading’ or ‘hat-racking’, often occurs when a tree is considered too big for the space it occupies (Fig 1). Topping reduces the size of the tree—and it also reduces its vigor, vitality and likelihood to survive. By removing the crown where most of the foliage is produced, topping is like putting a tree on a very sudden crash diet. This results in carbohydrates (food) in the buds and roots, going to create more leaves, instead of being saved for next year. The tree responds by trying to produce new shoots with new leaves to obtain more food. These ‘desperation shoots’ develop from buds near the surface of the old branches that were ‘back up buds’. These resulting branches often have poor structure and are prone to breaking in windy conditions. In the case of pears (Fig. 2), these new shoots are even more susceptible to fire blight.
Even if the tree escapes disease, or other decay problems, it often doesn’t have enough energy to survive the winter, or leaf out in the spring. With reduced shoot growth, there is less food for roots. With reduced root growth, there is less water for leaves. This negative feedback loop sends the tree on a downward spiral, and it will begin to decline, and eventually die.