Managing Turf In Shade

Managing Turf in Shade


The trees are finally beginning to leaf out – a welcome sign of spring for most of us. But when trying to manage turfgrass, excessive shade means trouble. Turfgrass needs light, and shade reduces both quality and quantity of light reaching the turfgrass plants. Understanding some attributes of light along with how turfgrasses grow, will help in understanding the effects of shade on turfgrass, and will guide us in adjusting cultural practices to compensate for those effects.

Here’s the crux of the matter: Turfgrass (and all plants) must make carbohydrates their food by photosynthesis. Each individual plant needs a certain level of carbohydrates just to survive. The plant converts these carbohydrates into the energy required to maintain basic life processes. Beyond that, more carbohydrates are needed to initiate new growth and to recover from wear, disease, or insect injury. If the demand for carbohydrates exceeds the supply, the plant starves, eventually resulting in death.

The light for photosynthesis comes, of course, from the sun. Sunlight contains light of varying wavelengths, which correspond to the different color of the spectrum. A rainbow is the most familiar example of the light spectrum, the visible part of which includes the colors red, orange, yellow, and green, blue, violet. Red and blue light is what the plant needs for photosynthesis. Contrary to what you might first think, green light is basically inactive for plant growth and development. In fact, plant tissues appear green because they reflect the green light, rather than absorb it.

By the time sunlight has filtered through the canopy of trees, most (if not all) of the red and blue light has been absorbed by the tree leaves, leaving little photo synthetically-active light for grass. Thus, turfgrass in shade is often unable to make the necessary carbohydrates for healthy growth. The severity of the problem depends on the level of shade; turfgrass under very heavy shade may not persist, leaving nothing but bare ground and perhaps some shade-tolerant weeds. Under more moderate shade the turfgrass may survive, but it often lacks in this condition will not be able to withstand traffic or pest injury, because the reserve carbohydrates needed for recovery growth are not there.

Light that does get through the tree canopy is skewed towards the longer wavelengths. This low-quality light causes the grass to grow more upright, with thinner, longer leaves. Other effects of reduced light include reducing tillering, shallow rooting, and thin cuticles. Furthermore, turf growing in shade may be subjected to competition from tree-roots for water and nutrients in the soil. As if all that’s not enough, the higher humidity and stagnant air often found in heavily shaded areas increases the chances for disease.

What can be done? First, choose the right grass species. Turfgrasses differ in their capacity to grow in shade. Among Kansas turfgrasses, tall fescue is usually the best choice. Although the fine fescues (i.e. creeping red, chewings, hard and sheep) have better shade tolerance per se, they lack heat tolerance and typically decline during the hot summer. The warm-season grasses have the poorest shade tolerance, although zoysia does better than Bermuda and buffalo.

Where fescue is not an option (such as on a fairway), or where shade is still too heavy for fescue, there are other course of action. The most obvious is to either remove trees, or to prune up limbs and thin the tree canopies. Grass will do better under openly spaced trees than under closely spaced trees because some diffuse blue and red light can still hit the turf from directions other than directly overhead. The quantity of light will be low, but at least its light the plant can use for photosynthesis. Pruned up limbs and thinned canopies will allow more sunlight to directly reach turfgrass.

If possible, raise the mowing height in the shade to compensate for the more upright growth of the leaves, and to provide more leaf area for photosynthesis. The thin, weak turf in shade may tempt you to fertilize more. Don’t do it! The nitrogen rate for shaded grass should be cut back to at least half of that for grass in full sun. Too much nitrogen causes the plant to grow faster; because the photosynthesis rate can’t keep up with demand for growth, the carbohydrate supply is further depleted, and starvation may be a result. Late fall fertilization. After tree leaves have fallen, is especially important for shaded cool-season turfgrasses. Irrigate infrequently but deeply. Light, frequent irrigation may encourage tree feeder-roots to stay near the surface, which increases competition between the trees and the turf. Restrict traffic in shade, Remember, carbohydrates are needed to recover from wear injury, and shaded turf usually has a minimal supply of reserves.

Finally, fungicides may be required for disease control on high-value shade turf. In a recent study, researchers at Ohio State concluded that brown patch was the ultimate cause of decline in shaded turf plots that had received higher rates of nitrogen. Additionally, the thinner cuticle of shaded turf may make it more susceptible to attack by pathogens.

So this summer when you are taking refuge from the sun under the shade tree, remember the turf there is under stress. A few changes in your management practices may make the difference between having grass or bare ground.




(Steve Keeley)
Reprinted with permission from the Kansas Turfgrass Foundation.