Trees and Turf - Can We Have It All?
Trees and Turf: Are They Compatible?
The simple answer is, not always. While we have trees growing in our lawns, parks and golf courses, it is easy to assume they coexist well together. The truth is that it really depends on the situation we have placed them in. Dr. Alex Shigo, known as the father of modern-day arboriculture, once said that when we plant a tree in the urban environment, we then become responsible for it. The urban environment often contains disturbed soils producing site constraints, which are not to a trees liking when compared to forest or woodland soils. My bias here, as you will notice, is what’s best for the trees health.
Trees evolved in a forested situation where their root systems find favorable growing conditions. Turfgrass evolved in open prairies with full sun where they develop dense highly competitive root systems. Trees and turf rarely thrive together. When pioneering tree species like the Bur oak began to co-mingle with turf, it was because they were able to adapt and grow into the tall grass prairies we call savannas. Not all tree species were capable of this transition.
Forest soils are dark, nutrient rich and contain sufficient organic matter giving them great texture and structure. They have a favorable pH allowing roots to absorb the many nutrients required for growth. The soil is teaming with beneficial microbes including fungi, bacteria, arthropods, earthworms and other life forms. Forest soil is covered with dead leaves and other decomposing organic matter. That duff layer keeps the soil cool, moist and full of oxygen. Trees growing primarily in a forested situation form a symbiotic relationship with fungi in the soil. This mutually beneficial relationship allows tree roots and fungal strands to connect. Through those fungal connections, trees can share resources and communicate with one another. Trees growing in the open develop a different crown shape, adapting to light availability. Its crown is lower and much wider than forest trees.
Trees and turfgrass in the urban environment compete very heavily with one another in the upper 6 inches or so of topsoil where water, oxygen and nutrients are most available. In most cases, the turf root systems out-compete the trees. Turfgrass species have been developed to grow in soils that are often compacted, low in organic matter and tolerate many other urban adversities. Turfgrass species like Kentucky bluegrass and perennial ryegrass prefer soils with a higher pH range than trees. They prefer growing and forming relationships with bacteria in the soil whereas trees prefer fungi.
All that being said, the direct effects of turf on tree roots and tree viability may be minimal especially for trees that have matured. Newly planted trees, during the establishment period, will find it more difficult.
You will notice that our turf maintenance practices can lead to these problems. Here are some ways in which turfgrass competition can cause harm to trees:
We tend to cut our grass short to eliminate more frequent cuttings and to produce a manicured look. As the closely mowed turf dries out, the soil heats up damaging tree roots.
Frequent foot traffic and lawn mowers compact the soil, restricting root growth, water movement through the soil and oxygen penetration.
Chronic drought stress can harm trees, whereas some of the cool season grasses we grow here go dormant, protecting them from short-term drought.
Grass clippings decay quickly and contribute little to soil rebuilding over time. Some people collect grass clippings and fallen leaves disposing of them offsite, making problems worse.
Mowers and string trimmers damage tree trunks.
Because many of us desire lush green lawns, we tend to over water during dry spells. Tree root systems suffer oxygen deprivation in nearly saturated soil, while the turfgrass thrives. Tree root systems do not require that much water. Irrigation systems, improperly set, have stressed and killed many trees.
Our lawn fertilization practices favor turfgrass health. These high nitrogen-containing fertilizers are formulated to meet turfgrass needs, often raising the soil’s pH.
Herbicide applications affecting broad-leaved plants can be taken up by tree roots affecting the tree.
Trees in the urban environment are frequently stressed. Stressed trees become more susceptible to disease and opportunistic insect pests.
For trees growing in turf-covered landscapes, Arborists highly recommend mulch rings that extend out to a trees drip line 3-4 inches thick. The objective is to mimic the forest floor. Done right, we can have both.