Tree Health Alert - Mowers and Tree Roots

— Written by Steve Pettis | September 28, 2023 (Updated: Sept. 29, 2023, 10 a.m.)

Lawns are great. They provide people a nice, soft, verdant area to recreate outdoors. Trees are equally great. They provide shade and beauty for gardeners. However, the management that trees need and the care that grass needs can sometimes be at odds.

Protect Tree Roots

Arborists consider the 'critical root zone of a mature tree to be about 30' from the trunk. That means we want to protect this area and avoid soil compaction and root damage. The best way to do this is not have a lawn under large trees and mulch the critical root zone of trees.

In the image above you can see how far the roots extend from the trunk of this maple. Imagine there are roots through much of this lawn. Trees and lawns are not good for one another. Grasses typically do not grow in the woods and trees generally do not grow in grassy meadows. Tree roots can't compete with the thick fibrous root system of a robust healthy lawn. Lawns need the sun that is often blocked by tree canopies. Both grasses and trees need soil nutrients and water and are in competition for both in home landscapes.

Tree Roots Are Not Deep

Tree roots need to be in the upper few inches of soil so that they can take up oxygen. Roots do not go deep underground due to low oxygen in soil. Most topsoil in the southeastern US is only a few inches deep. Below that thin layer of topsoil is acidic clay where roots can't grow.

beech roots — The 'Medusa Tree' in Greenville, SC demonstrates clearly how tree roots form just under the soil. The soil over this root system has been lost over the years exposing the intricate network of roots.

Equipment Can Compact Soils

Soil compaction can be caused by equipment driving over a tree's root zone. This compaction squishes the air spaces between soil particles out of the soil. These air spaces are where oxygen is obtained by tree roots and where soil water is stored. When soils are compacted they do not hold oxygen and water as well.

Tree roots seem to rise up out of the ground because today's heavy and powerful mowers compact the soil and blow loose soil particles away.Today's zero-turn mowers can weigh between 500 and 1000 pounds. In the image below you can see that these trees appear to be on raised islands after years of soil compaction and soil loss due to mowing over the root zone of the trees. It would have been far better for the health of the trees to have cultivated a mulched area under the trees to at least the drip line of the tree.

According to NC State experts, "Ideally, the distribution of solid (soil particles) and pore space (space between soil particles) will be 50% mineral and organic matter, 25% air, and 25% water. Compacted soils will have a higher percentage of solids and less pore space for air and water. The negative impacts of soil compaction include poor drainage, decreased root growth, decreased nutrient and water uptake, and overall reduced (growth)."

mower tree — Years of mowing which blows away loose soil and compacts the remaining soil has left these trees on raised islands. Mature trees usually decline and eventually die due to this sort of root damage.

Preventing Tree Root Damage By Mowers

Landscapers and gardeners should give trees more space. Try not to grow lawns over the 30' critical root zone. Stay at least thirty feet away from trees. Mulch this critical root zone. Lawns do not like shade and trees do not like the competition from the grass roots. Keep the two separated.

Merge mulch areas to prevent driving mowers between trees and groups of trees. As you can see in the image below, the area under these three trees could easily be joined together by mulch. this would eliminate the need to drive over the critical tree root zone.

When mowing change the mowing pattern and direction frequently. Slow down so mowers do not bounce. Try not to disturb the soil by spinning tires and wheels. If you see tree roots, avoid them with the mower. Raise the mowing height to avoid scalping lawns and exposed roots. Lower mower tire pressure to spread out the weight of the equipment.