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To do the least damage, prune trees at the right time

June 28, 2017

We all have weak, gullible friends who are easily taken advantage of, people who we may somewhat unkindly refer to as saps. Sap in this sense is from "sapwood," the weak, soft wood between the heartwood and the inner bark of a tree. But the more familiar sap is the sugary, nutrient-filled juice of trees that is indeed their lifeblood. 

Just as our circulatory system moves blood around our body, a tree’s system moves sap throughout its trunk and limbs. 

When you cut into a tree, that life-giving sap oozes out. 

The amount of sap in the tree varies based on the time of year. The sweet sap of sugar maples flows heavily in early spring, then tapers off later in the season. 

Many trees tend to bleed sap when cut. Some trees have higher sap levels in early spring. If you drain the sap or lifeblood out of a tree, you may end up hurting it or even killing it. When we need to cut or prune trees, we should try to lose the least sap possible. 

To do the least damage to trees, prune them at the right time for that variety of tree. Trees that bleed sap include Elms (Ulmus), Lindens (Tilia), Beeches (Fagus), Birches (Betula), Maples (Acer), Mulberries (Morus), Poplars (Populus) and Walnuts (Juglans). Prune these trees in midsummer for best results and least damage. 

We prune trees for many reasons - to remove damaged, dead, or diseased branches; to thin out thick, shady limbs to let more air circulate and to let in sunlight; and often to increase flowering and fruit production.  

Prune out any crisscross branches so they don't rub against each other. Eliminate weak crotches that can split apart when the tree grows older. You should always cut off suckers or water sprouts that grow up from the base of the tree. Sometimes trees will send up two or more main trunks or leaders, and you need to choose the stronger and cut off the weaker one, so the tree has only a single dominant main trunk. 

Keep in mind that trees seal their cuts by growing a protective callus much like a scab to keep out disease. This is helped by leaving a little protruding bit of the branch attached to the tree so the cuts are not flush against the bark, or the cuts will not callus over. 

To increase flowering, prune spring-blooming trees after flowers fade. Wait for winter or very early spring to prune shrubs and trees that bloom in mid- to late summer. Try to avoid ripping in the fall, because trees might not have time to grow a protective callus before winter. 

If you cut or prune a tree and it bleeds sap, don't paint or in any way cover the wound. Painting over the cut can actually hurt the tree by preventing the formation of a protective callus. The only exception is some elm or oak trees that might bleed lots of sap, so they can be painted over to keep out Dutch elm disease and oak wilt. 

Prune most trees and shrubs in midsummer and you will be fine. Wait until fall and you can kill the tree, which will make you, not the tree, a sap.

  • Paul Barbano writes about gardening from his home in Rehoboth Beach. Contact him by writing to P. O. Box 213, Lewes, DE 19958.