Why Grass Dies Under Shade Trees

Customers come into our stores every fall and ask, “Why does my grass keep dying under my shade tree?” It’s always the same story, “My grass looked great after I overseeded last fall. It looked amazing in the spring but died out during the summer.” So the short answer is that trees are bullies.

It’s Not Just About Sunlight

What would seem to be the most apparent resource in short supply would be sunlight; however, the area under the tree can be shady, but it is rarely too shady to sustain turf. So what is happening under the tree is that the tree will out-compete the turf grasses for resources necessary for a healthy life. The most significant shortage is water. Imagine how much water it takes to support a full-grown tree having several acres of leaf surface from which water evaporates.

Trees are a Giant Drinking Straw

Trees draw up water from roots through a process of transpiration. Transpiration is water evaporation through the leaves. This evaporation creates negative water pressure and works much like a drinking straw using the tree’s xylem to draw water upward from the roots into the tree and, eventually, to the leaves.

The xylem, in simple terms, is the water highway for plants to transport water and nutrients from the roots to the leaves. Grass plants also contain xylem. Comparatively, the grass xylem is a one-lane country road compared to a tree as a one-way eight-lane super highway. Consequently, the trees will move water faster and in higher volume than grass plants.

Why Shady Areas Look Great in the Spring

Weather and the environment influence transpiration. The shorter daylight hours of spring, frequent rains, and cloudy weather reduce the transpiration process; thus, the tree uses less water, leaving plenty of moisture available for grass plants. As a result, the trees and grass are simpatico, and everyone gets along.

Why Shady Areas Burn Out In the Summer

Unlike spring, during summer, we have infrequent rainfall, constant southern winds, extreme temperatures, and longer daylight hours, which will intensify transpiration. The rise in transpiration means the trees are sucking more water from the soil, sometimes hundreds of gallons per day, to support themselves. This situation creates a battle for water between the tree and your grass, and it is magnified during drought seasons.

Maintaining Grass in Heavy Shade

Compensating for this unfair advantage the tree holds over the turf, we try to level the playing field in several ways. First, always water the shaded areas more frequently during the hot, dry summer months. Second, Relieving compaction by mechanical core aerating and filling the aeration holes with Primera Sports Field Conditioner will increase the water-holding capacity.

Importance of Healthy Soil Conditions

Less-than-ideal growing conditions for large trees can reduce the amount of groundwater available from the soil to support the tree and your grass. Tree roots need permeable soils to exchange air between the root zone and the atmosphere. This process is called respiration. Respiration is the opposite of photosynthesis. The tree roots take in carbon dioxide to produce sugars and oxygen as a food source.

See Related: Fertilizing for Healthier Trees

Root Bound Roots lead to Compaction

Tree feeder roots can extend two to three times the tree’s drip line. The largest concentration of tree roots will be in the top two to three feet of the surface. Imagine a massive pin oak tree growing in the middle of a tiny front lawn. You have a house and foundation along one side of your yard, your driveway on one side, possibly the neighbor’s driveway on the opposite side, and a sidewalk or street on another. Where do you think those tree roots are going to grow? The root system that supports that 40-foot tree is super condensed into this tiny lawn. Surrounded by concrete, the pin oak is essentially planted in a huge concrete planter.

Surface Roots Complicating Aeration?

Often this heavy root concentration leaves large surface roots within the critical root zone and dense, compacted feeder root zones, making it almost impossible to core aerate mechanically to loosen the soil and allow better water and nutrient penetration. An alternative solution to a mechanical core aerator is non-mechanical soil aeration using a granular application of Soilbuilder C20.

Offset The Lack of Beneficial Sunlight

Lack of beneficial sunlight also plays a part in grass performance. Pruning lower branches to raise the canopy will help to increase sun exposure from the sides and allow for better wind circulation to fight turf disease. However, little can be done to improve the quality of sunlight coming from directly above. A university study found that 95% of the most beneficial sunlight is absorbed by the tree’s leaves, leaving little to land upon the grass plants.

As grass plants mature, more sunlight is required to maintain health and vigor. In full sun, grass blades reach towards the sky to soak up healthy sunlight and flourish. The grass plant reacts the same way in the shade, searching for quality light. Unfortunately, having reduced light levels result in a thinner and less vigorous grass plant. Weakened grass plants are less tolerant of drought, heat, and disease. In addition, the lack of vigor will affect turf density allowing weeds to invade thin areas.

Lessons Learned From Mother Nature

Mother Nature designed her baby grass seeds for sprouting and tolerating lower light conditions under the shadow of established taller grasses. Re-seeding dense shade areas three times a year (April 15, August 15, and September 15)) will add new generations of youthful, vigorous, and low-light tolerant grass plants to the maturing turf stand.

Best Grass Seed for Under Shade Trees

Uncle’s Premium Shade grass seed is the ideal blend of elite grass seed varieties to introduce genetic diversity with lower water and nutrient requirements, improved shade tolerance, and disease resistance.

Trees Are Bullies!