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Are You Killing Your Trees?

Mulch, mowers and weed wackers can be murder on trees

     I was recently called to diagnose the cause of death of some large dogwood trees. While visiting the site I also noticed that several maple trees and an ash tree were exhibiting dieback of branches. Closer examination of the stems near the ground indicated the bark had been destroyed.
     When I asked the owner if he used a lawnmower to trim the grass around the plant, he assured me that he has never bumped the stem with the mower. He preferred using his Echo weed wacker. He would not accept the fact that a weed wacker could do that type of damage. Only after I demonstrated that his weed wacker could strip the bark from three-inch-diameter seedlings growing in a nearby wooded area did he accept he was the villain.
     He was also concerned about a green ash. There was no sign of bark damage at the base, but upon examining the bark on the stem, I saw evidence of borer entry. Using my pocket knife, I dug into several holes before I uncovered an emerald ash borer. 
     The emerald ash borer will eventually kill all ash trees. This insect can only be controlled by using a systemic restricted-use insecticide applied through the soil. It is unlikely that it will ever be used on a mass scale.
     As I was about to depart, I noticed high mounds of bark mulch around the base of all the trees in front of the house. I cautioned him that he should remove that mulch before it causes problems.
     He informed me that he has been mounding mulch around the base of those trees for 10 years. I asked to pull back a mound of mulch surrounding a red maple tree with a trunk diameter approximately six inches. After removing the mulch, I measured the diameter of the trunk above the mulch and compared it to the diameter of the same stem measured six inches above grade. The diameter of the stem buried under the mulch was two-and-a-half inches smaller than the stem above the mulch. I also showed where the stem had generated roots. I predicted that unless he removed those mounds of mulch from around the trees, they would either die or break off at the base when exposed to strong winds.
     Another problem caused by deep mounds of mulch around smooth-bark trees is susceptibility to clear-winged borers. High mounds of bark or wood mulch keep the bark and wood beneath soft and easily penetrated by borers. Clear-winged borer adults lay their eggs in the warm, moist mulch. After the eggs hatch, the larvae are able to penetrate the soft bark and tunnel into the wood. The tunnels created by the borers will interrupt the flow of water and nutrients up the stem, resulting in dieback of branches and eventually the stem.
     Trees don’t die suddenly. Because they have a large storage capacity for water, nutrients and energy, depending on age and size, trees may not exhibit symptoms of distress for three to four years.