Mechanical Damage to Trees

We talk a lot about insects and diseases that can wreak havoc to our landscape trees, but many times people are a tree’s worst enemy – especially people wielding lawn mowers and weed trimmers. And while a slight bump from a mower’s frame or a quick zip of trimmer line around a tree trunk may seem insignificant, it can create an injury that leads to disease or death.

mechanical damage

The site for most of these injuries is the root flare – that section where the tree trunk becomes tree roots, just above the soil line. This section of the tree is protected by bark which guards the plant’s nutrient and water transport system, called the phloem and xylem, respectively. Any damage to this transport system affects tree health. Young, smooth-barked trees (i.e. maples, birches) have a very thin layer of bark protecting this system, less than 1/16 of an inch. Therefore, even minor injury to the bark can impact the transport system and cause problems for the tree.

Even when a tree tries to recover from a wound, leaves and branches often decline and die back, because food and water pathways were destroyed. Although a large wound is generally more serious than a smaller one, repetitive wounding adds up to greater trouble for the tree. If the damage extends completely around the base of the tree (a situation called girdling), the tree ultimately dies.

All tree wounds are serious when it comes to tree health. No matter what size the wound is, the damage done is irreversible. The tree must devote a great deal of energy and many resources into trying to seal off the damaged area to prevent further complications. The wounded area is an opening for wood-rotting organisms and decay fungi to enter and cause further damage.

Injury to trees can be avoided easily and at very low cost if you follow one or more of these suggestions:

1. Physically remove turf or prevent grass and weeds from growing at the base of the tree using herbicides.

2. Add a 2˝ to 3˝ layer of mulch on the root zone of the tree.

3. Add trunk guards or similar devices to give the tree additional protection. White, expanding tree guards can help trees withstand equipment contact and also can help to reduce winter injury.

Submitted by Kelly Jackson, Agent for Horticulture, Christian Co. Cooperative Extension Service