By Dr. David Gardner Managing turfgrass that is growing under the shade cast by either a tree or a structure, such as a home or a stadium, can be difficult. Turfgrasses are adapted to, and grown best in, full open sunlight.
Trees reflect yellow, green, and orange light and absorb red and blue light for photosynthesis. Since much of the sunlight is either reflected or used by the tree for photosynthesis, the amount of light reaching the ground under the tree can be as little as 5 percent of full sunlight. The quality of light is also different under tree shade. Far-red light passes through the leaf canopy to the surface below. As a result, there is a higher ratio of far-red to red light than what appears in full sunlight.
Turfgrass can sense changes in both light quality and quantity. This results in many harmful changes to the plant as it alters its morphology and physiology in an attempt to find more light. In addition, under shade there is an increase in relative humidity, a decrease in wind movement, increased carbon dioxide levels and competition from tree roots.
One option if the turfgrass is growing under trees is to attempt to improve the light environment in order to promote turfgrass growth. Both the intensity and duration of sunlight are important. The length of time the plant receives sunlight is more important than the time of day. Be aware that light intensity changes with season, latitude and time of day. Be aware also that as a tree grows, the size of the shade canopy and the density of the shade will increase.
Based on research conducted at The Ohio State University, you should selectively prune trees to increase the amount of time of exposure to full sunlight, not to decrease the level of shade. Canopy thinning alone will not solve most turfgrass growth problems.
Several management strategies can favor growth of turfgrass in shade. First and foremost is to select a species that is more tolerant of shaded conditions. Consult results from the National Turfgrass Evaluation Program
(NTEP) testing site nearest your location. NTEP contains a wealth of informtion relating to the strengths and weaknesses of all common species and cultivars of turfgrass.
Several cultural practices can help shaded turfgrass:
1. Increase mowing height
to the maximum recommended for the species. Longer leaves mean more leaf area for light interception.
2. Reduce fertilizer rate
to half of normal in order to try to reduce excess leaf production. Leaves of shaded turfgrass naturally grow thinner and longer and the addition of excess nitrogen will exacerbate this.
3. Irrigate heavily but infrequently
to reduce time that foliage is wet. You should also, when possible, avoid traffic on shaded turfgrass. The longer, thinner leaves of shaded turf are less tolerant of traffic and wear.
4. Closely monitor weed, insect and disease pressure.
The turf is already under stress due to the shading and is more susceptible to pest pressures, particularly diseases. This is because the increased relative humidity and decreased wind movement act to increase the amount of time that the surface of the leaf is wet.
Managing shaded turfgrass is a challenge, because there are many harmful physiological and morphological changes that occur in the plant as it attempts to adapt to shade and gather more light. However, selection of proper turfgrass species and cultivars and slight modifications to management practices can significantly improve the quality of shaded turfgrass.
Most research concludes that, regarding cool-season grasses, either tall fescue or fine fescues are better adapted to shade than are Kentucky bluegrass or perennial ryegrass.
The author is Associate Professor of Horticulture and Crop Science at The Ohio State University, and technical advisor to Turf