LIVING in the SHADOWS
Turf in shaded sites requires different management practices than turf in full sun. Realizing the differences between sun and shade environments will help you manage shaded turf successfully.
Lack of sunlight is the most important factor affecting shaded turf (but it's often not the only one). Most turfgrasses evolved under sunny conditions and cannot easily adjust to low light levels. Tree shade also can reduce the quality of light by filtering out blue light and leaving an abundance of red light. Blue light is more energetic and photosynthetically active than red light.
Wind speed and temperature are affected by shade, too. Shaded sites almost always have lower wind speeds than sunny areas. This has three effects:
Turf uses less water;
The low wind speed decreases evaporation, so turf leaves stay wetter longer after rain or irrigation, which can increase diseases; and
During hot spells, there is little wind to help cool the turf. On the other hand, temperatures in the shade are generally cooler to start with, so turf in light to moderate shade nevertheless may stay green during summer while turf in full sun goes dormant.
How shade affects turf
Turf in the shade dies for three reasons: Lack of light, diseases and tree root competition for water and nutrients.
Lack of light. Most cool-season turfgrasses potentially can survive in as little as 5 to 10 percent sunlight. However, the amount of energy they receive through photosynthesis isn't enough to allow them to withstand environmental stresses such as heat and cold, traffic and disease. The turf may survive the first year it is established, but it eventually will die or thin out severely.
Turf plants in shade usually have fewer tillers (shoots), leaves and roots than turf in full sun because there is less solar energy to use for growth of these parts. The leaves and shoots become elongated as the plant tries to stretch for light. Because energy is limited, the elongated leaves and shoots are thinner than if they were in full sun. The turf becomes flaccid (weak) and tends to bend when mowed, resulting in a poorer quality of cut. Leaf tissue takes priority over roots for energy, and root growth is reduced. Poor rooting may not supply enough water and nutrients needed for growth and allows the turf to be easily torn from soil.
The combination of fewer plants (thinned turf) with reduced growth, less leaf tissue and weak plants results in turf with low tolerance to traffic and environmental stresses. In addition, wood violet, ground ivy and other broadleaf weeds that tolerate shade well often encroach into the turf. Moss often grows where the turf has died and the soil is moist.
Disease. The moderated temperatures and extended periods of leaf wetness, combined with weak, succulent turf provide an ideal situation for fungal diseases. Microdochium patch (also known as pink snow mold) can be a problem on cool-season grasses throughout the year. Powdery mildew disease can devastate Kentucky bluegrass. Other diseases may also occur.
Competition. Large tree roots compete with turfgrass for space when they grow on or just below the surface. Smaller tree roots can absorb water and nutrients needed by turf.
Made in the shade
Removing all shade from over turfgrass would be virtually impossible, not to mention impractical. So you need to learn how to get the most out of grass that must exist in shade. Here are a few things you can do to maximize turfgrass health despite shade cover.
Grass selection. Some grasses are more shade tolerant than others because they capture or use energy from sunlight more efficiently. Cultivars of Kentucky bluegrass sold as “shade-tolerant” often simply have better resistance to powdery mildew. In temperate areas, use fine fescue for dry, shaded sites (see Table 1). Plant seed mixtures or sod containing rough bluegrass (Poa trivialis) or supina bluegrass (Poa supina) in moist, shaded sites (but remember that both rough and supina bluegrasses can spread by stolons to other parts of the lawn). The improved turf-type tall fescues can be used in both cool- and warm-season climates and exhibit a fair degree of shade tolerance.
Most of the warm-season grasses have poor traffic tolerance when growing in shade. Breeders have been working to improve this. For example, ‘DeAnza’ and ‘Diamond’ zoysiagrass offer better shade tolerance than other zoysias, but supplies still are limited. Some cultivars of St. Augustinegrass and centipedegrass are suitable for shaded environments.
In some cases, annual seeding of annual or perennial ryegrass may be the best option. These turfgrasses may die out after several months to a year but are easily re-established, so they may be an option for maintaining turf cover in difficult shaded areas.
Pruning and replacing trees and shrubs. Prune trees and shrubs to increase air movement and sunlight. What is practical or desirable will depend on the specific site. Pruning should be performed by a qualified arborist or horticulturist to minimize tree damage and disease potential.
Where practical, the following steps will improve air movement and sunlight: remove tree limbs to a height of 10 feet aboveground; thin tree canopies to allow flecks of sunlight to reach the turf; remove borders of shrubs or brush to increase air movement and early morning or late afternoon sunshine penetration; eliminate dense tree stands by selective removal; replace dense shade trees such as oaks, maples and pecans with trees having more open canopies such as lindens, birches and some pines.
Increase mowing height as much as possible. A good target is 25 to 50 percent higher than your standard mowing height, though increasing the height even 10 percent will help. The additional leaf material can increase the amount of energy produced by photosynthesis, and it will cushion the turf better, improving traffic tolerance. Never mow off more than one-third of the leaf tissue. You do not need to collect clippings if you follow the one-third rule. Just be sure to keep the blades sharp.
Fertilize at the same frequency as turf in the sun but at half the rate. Shaded turf needs less nitrogen (N) because its growth is diminished in shade. Use a combination of fast- and slow-release N sources. Research being conducted at the University of Wisconsin shows that bluegrasses may perform better in shade when granular forms of nitrogen are used as opposed to liquid forms.
Irrigate thoroughly but infrequently. Grass in the shade uses about half as much water as turf in full sun. Irrigate early in the day to allow the turf surface to dry and reduce disease potential.
Keep traffic off the turf. Dogs are especially hard on turf when confined in a fenced area or chained. Walking paths should be paved or mulched. Use wide turf tires on large equipment to reduce compaction problems.
Plant growth regulators may help. Plant growth regulators (PGRs) that prevent turf from producing the hormone gibberellic acid (sometimes referred to as Type II PGRs) have been shown to improve shade performance of several grass types, notably zoysiagrass, Kentucky bluegrass, supina bluegrass and tall fescue. The effects are less noticeable on perennial ryegrass. PGRs that have been tested for this use at University of Wisconsin, Michigan State University, Texas A & M, and the University of Missouri include paclobutrazol (Syngenta's Trimmit and Anderson's TGR and Turf Enhancer), flurprimidol (LESCO's Cutless) and trinexapac-ethyl (Syngenta's Primo).
Use PGRs at about half the label rate and apply at 4- to 8-week intervals throughout the growing season. Begin applications when turf has begun active growth in the spring and stop applications approximately 4 to 6 weeks prior to winter dormancy.
How to determine the amount of shade
Turf that receives at least 4 to 6 hours of direct sunlight a day should be fine. If not, there is another problem affecting the turf. For turf that receives less direct sunlight, or none, use a light meter to determine if there is enough sunlight to grow any type of turf. Avoid an exercise in frustration: Know that less than 10 percent sunlight will make it exceedingly difficult to maintain a successful stand.
Between 10 and 20 percent sunlight will make it practical to grow species with good shade tolerance (see Table 1, above), or use PGRs to improve turf quality. Otherwise, you'll need at least 20 percent sunlight to maintain good turf quality with most turfgrasses.
Light meters can be expensive, though some may cost less than $100. Different types of meters use different units to measure light (see Table 2, above). Light comes in a variety of wavelengths, much like a rainbow. Not all sensors measure the same bands, so measurements can differ among light meters. Light meters intended only for indoor use typically do not have sufficient range to measure full sunlight. Table 2 indicates some of the more common units and the corresponding level of full sunlight during mid-summer.
Maximum sunlight occurs at midday. However, some sites will receive the most intense light in the morning or afternoon if those are the times when the angle of the sun allows light to reach the shaded area. Therefore, collect measurements every 1 or 2 hours during the day under clear skies to determine if enough sunlight exists for turf. For example, beginning at 7 a.m., place the meter on the ground in a sunny area and record the amount of sunlight (do not shade the sensor by standing too close). Then measure the amount of light in the shade. Repeat this sequence every hour or two until the end of the day. Collect light measurements from throughout the shaded area as some locations will be less shaded than others. At the end of the day, average the percent of sunlight in each location and record it on a map of the site. Graph paper works well. This will give you the information necessary to know where it is feasible to grow turf and where you should consider alternatives (see illustration, bottom of page 24).
When all else fails
Occasionally, turf will fail in the shade because there is too little light, too much traffic, disease or tree root competition. In these situations the best solution is to plant an alternative groundcover or use some type of mulch.
Dr. John Stier is professor of environmental turfgrass science at the University of Wisconsin (Madison, Wis.).
GETTING TURF TO GROW IN THE SHADE
Use the following strategies to help turf grow in the shade.
Plant shade-tolerant grasses (see above list).
Prune tree branches and remove surrounding shrubs or brush.
Replace dense shade trees with open-canopied species.
Increase the mowing height 25 to 50 percent.
Irrigate infrequently, and then early in the day.
Use fertilizer containing at least 30 to 50 percent slow-release nitrogen sources.
Keep traffic off turf.
Use plant growth regulators that inhibit gibberellic acid (GA) production in turfgrass.
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