By John Sorochan, PhD
When it comes to managing turfgrass in shade, an understanding of the actual causes of shade is important. Shade is simply the lack of necessary light for optimal turfgrass growth. Reduced light, or shade, results in reduced photosynthesis, in turn causing the induced turfgrass stress. A simple definition for photosynthesis is light energy plus carbon dioxide plus water yielding chemical energy (carbohydrates) plus oxygen plus water. Light (sunlight) is the ultimate source of most terrestrial life on earth. The sun provides light in abundance and does not appear to be a limiting factor for most turfgrass growth. However, changes in light, either quantity or quality, has dramatic effects on plants, thereby making it a limiting resource.
The components of light include light quality and quantity. Light quality is the wavelengths of light (measured in nanometers, nm), and range from very short (cosmic or x-rays) to long (radio) waves lengths. Turfgrasses, like all plants, require the visible light form 380 to 700 nm in order for photosynthesis to occur. The visible light spectrum (380 – 700 nm) is known as photosynthetically active radiation (PAR). Within PAR are the blue and red light wavelengths, which are important components for turfgrass growth and development. Blue light (~ 380 – 500 nm) is important for photosynthesis, and is the stimulus for short sturdy growth. Conversely red light (~ 600 – 700 nm) is also important for photosynthesis, and is the stimulus for turfgrass cell elongation. Green light typically is not important for photosynthesis and is reflected giving the turf its green color.
Light quantity is the actual particles of light providing the energy necessary for photosynthesis to occur. Light quantity (energy) is the most important light component for photosynthesis to occur.
Variations in light quantity occur with the time of year, latitude, time of day, atmospheric screening, structures (stadiums, fences, etc.), and topography. During the summer light quantity is greatest; while, winter provides the least light energy. Many sports are played in spring and fall, and even into the winter months where light quantity for turfgrass growth may be limited. Depending on the time of year and latitude, the light quantity can vary greatly. In the northern hemisphere the farther north you go during he summer the longer the days are and greater the light quantity. Between 12:00 and 14:00 the light energy is most abundant and is significantly less during the time of day before and after the time when the sun is at it solar zenith. Atmospheric screening reduces light quantity, and is caused by anything that has a potential to interfere with the light wavelengths. Clouds, air pollution, humidity, and even stadiums and fences are some examples of atmospheric screening that can limit growth on athletic fields.
Dr. James Beard (1995) estimated that over 25% of all managed turf is under some sort of shade stress. Thus, shade stress likely occurs anywhere turfgrass is managed which can include golf courses, athletic fields, and home lawns to name a few. Shade (reduced light) is a reduction in both light quantity and quality. Simply having a shade situation means not enough light energy is being supplied to the turf for efficient photosynthesis to occur. Reduced photosynthesis results in reduced carbohydrate synthesis, and in turn causes turfgrass stress conditions resulting in insufficient growth and development. Tree shade greatly reduces both blue and red light quality, with the blue light being affected the greatest. As a result the short sturdy stimulus for turfgrass growth is reduced and turfgrasses elongate from the more abundant red light stimulus, and continue to lack the necessary light energy important for optimal photosynthesis to occur.
Morphological changes that occur as a result of shade stress include decreased leaf thickness, decreased density, decreased tillering (rhizome and/or stolon growth), decreased root to shoot ratio, and increased leaf height and elongation. Turfgrass physiological responses to shade include reduced carbohydrate reserves, reduced transpiration, reduced respiration, reduced cuticle thickness, and increased succulence. In turn, these morphological and physiological changes, as a result of shade, increase the susceptibility of athletic fields to increased wear while reducing it capacity to recover from traffic damage.
Environmental conditions that typically accompany shade stress situations include increased relative humidity, more moderated temperatures, and restricted air movement. In turn, athletic fields become more susceptible to common low light turfgrass diseases like pink snow mold and powdery mildew.
Proper implementation of cultural practices can help when managing sports fields under shade stress conditions. Because root depth is limited as a result of shade a light and more frequent irrigation schedule should be used. However, avoid over watering! Irrigate only as needed to maintain adequate soil moisture for the turfgrass. Also, avoid excess nitrogen. Too much nitrogen will stimulate increased shoot growth; thus, making the turfgrass plant more stressed. A good rule of thumb to go by is to fertilize at half the recommended rate for nitrogen requirements for the turfgrass species being grown. If possible increase mowing height to enable more surface area for light absorption. Unfortunately, for a shaded putting green increasing the mowing height is often not possible, because of the increased demands for faster putting surfaces. Finally, turfgrasses under shade stress conditions have an increased susceptibility to fungal turfgrass diseases. Therefore, if possible fungicide applications are often necessary for turfgrass survival.
Other management practices aid in turfgrass shade stress situations include, limiting or redirecting traffic, tree canopy and root pruning, plant growth regulators, and increasing morning light. Limiting traffic is difficult, particularly on putting greens. However, any reduction in wear will help alleviate added stresses to the turf already under shade stress conditions. Trees are usually the major cause for shade problems, especially in golf course situations. Unfortunately, trees are also an important component to the golf course landscape. Trees add depth and aesthetic value to complement any golf course design. However, trees can also grow to exceed their original benefit and cause problems such as turfgrass shade stress. If the tree causing the shade cannot be removed, pruning both the limbs and roots will help reduce some of the problems being caused to the turf. Pruning the limbs will allow for more light to penetrate to the turf surface, and root pruning will lesson the competition for nutrients and water. Several research studies have been conducted to show the benefits of using plant growth regulators (PGRs) on turf under shade stress conditions. Stier and Rogers (2001) found that the use of PGRs like Trinexapac Ethyl (Primo), have shown to limit shoot elongation and improve photosynthetic efficiency. Finally, when at all possible, any attempts to provide morning light will greatly help with dealing with shade stress conditions. Morning light is when cool-season photosynthesis is at is greatest; thus, enabling for maximum photosynthetic efficiency.
Selecting the proper turfgrass species is one of the most important decisions to make when trying to manage turfgrass in shade. Unfortunately for golf course putting greens turfgrass species selection is limited; however, for tees and roughs using a turf well adapted to shade will significantly improve your turf quality and golf course conditions. Poa supine is one of the best, if not the best turfgrass for shade situations. A native turf of Austria, Poa supine is well adapted to the climate and performs exceptionally well in the shade.
More then 140 stadiums worldwide use grow lights to compensate for shade stress conditions. These lighting systems use high-pressure sodium (HPS) lights similar to grow lights used in greenhouses. Another benefit from the HPS lights are the increased temperatures under the lights which also improves growing conditions for stadiums in cool climates, like Lambeau Field in Green Bay, WI.
In conclusion, turfgrass management under low light/shade is common in many sports turf situations and an understanding of the cause and effects of shade stress can help a turfgrass manager improve turfgrass conditions in the shade.
John Sorochan, PhD, is Distinguished Professor-Turfgrass Science and Co-Director for the Center for Athletic Field Safety at the University of Tennessee in Knoxville.