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Mower Blight and other Man-Made Injuries to Trees

Maureen Sexsmith-West

ISA Certified Arborist, PR4600A

A tree is made up of a complex system that includes several layers:

  1. Bark (outer layer of hard dead cells) which protect the living cells from injury, sun damage, hail damage, etc.

  2. The layer of ‘green’ actively growing cells that include the xylem and phloem (these transport water and nutrients throughout the tree)

  3. Heartwood – non-living wood cell

When the protective outer bark layer is damaged, several things happen. The flow of sap from roots to shoots is interrupted. If enough of the bark is damaged, the tree can no longer transport sap and the tree will die. Young woody plants are particularly vulnerable to this type of damage because of the thin nature of the bark, but some varieties of older trees with thin, smooth bark are also susceptible (Birch, Mountain Ash, Linden).

Trees are much more easily damaged in the spring when the tree is putting on new growth.

Trees can be seriously damaged by small wounds, especially with repeated injury.

Damaged trees can grow poorly and branches may sucker and / or die.

Wounds provide entry points for pathogens and decay.

In the worst cases, the fungal decay nay become severe enough to compromise the long-term health or structural integrity of the tree.

Mechanical damage occurs when a tree is injured by human activity or a non-natural injury occurs. Damage to vegetation from equipment can be simple carelessness or incorrect use of the equipment.

Mower/Trimmer Blight

Typical mechanical injuries include:

  1. being repeatedly struck by a lawn mower (trunk or limbs)

  2. weed trimmers strip bark away from the trunk

  3. excavation, stockpiling (grade changes)

  4. girdling from tags, labels, guy lines or stringed lighting

Other examples:

  1. carving of initials

  2. attaching nails and screws, signs

  3. pruning cuts

  4. attaching chains, extension cords, etc.

“Mover Blight” This is a very common cause of tree injury. Damage to trees and shrubs occurs when weed whippers and lawn mowers break the cambial tissues. Sadly, this type of injury is often repeated routinely, with cumulative damage. Typical injuries to trees and shrubs include gouges and cuts through the bark into the cambium, where cells actively divide. These injuries are often close to the ground and easily invaded by insects and disease causing agents. Monitor lawn care activities (self or others) – see Prevention Strategies.

Remove Crossing and Rubbing Limbs

Pruning Yes, pruning is type of mechanical damage. When limbs are removed using saws, hedge trimmers and chainsaws, we create an injury. If pruning cuts are made properly, you can minimize the damage and allow the tree to compartmentalize and protect itself. Never TOP your tree.

Crossed limbs can rub and cause damage – better to prune them out – they cause more damage than your loppers. Keep in mind season specific trees such as Birch, Maple and Elm. Know when is the best time to prune. Heavy equipment and compaction, Damage from excavations See our earlier post on Construction Damage and Trees. Prevention Strategies Keep grass and weeds from growing at the base by creating a tree well of bark mulch. You can’t damage the tree with lawn care equipment if there is no grass to mow there. Remove weeds or grass by hand when necessary. Removal ALL labels and tags at the time of planting. Remove tree stakes within 18 months following planting to prevent girdling.

Note original green wire guy line in centre of 25 year old tree. Photographed following limb failure.

If you must use a trunk guard, inspect it annually. Repair damage as soon as it occurs. Prune broken limbs to the branch collar.



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