The Science of Shade Grass Management

In efforts to find the perfect turf grass, there has been a lot written about the nutrient and light requirements of full sun grass, but there isn’t  much scientific data written about shade turf and its growth and light requirements.  

Although there are exceptions, it is a general rule that most warm season grass requires full sun. On the other hand, most cool season grass can handle shade better than warm season grasses, but there are also exceptions.  

Landscape designers or turf specialists really don’t have the luxury of trial by error or to use a “one size fits all” solution. It takes more than guesswork because wrong guesses can cost you or your customer money. 

Understanding the conditions that exist will help in choosing the correct grass. It’s a give and take process as some varieties of turf grass can handle shade, but can become disease prone in prolonged wet or dry environments or if all factors are not considered. Understanding the conditions that exist will help in choosing the correct grass. 

Let’s take a look at the overall scientific factors that will help take the guesswork out of determining the best shade grass  and turf quality for your conditions.

The Top 5 Considerations

 1. Amount and Quality of Sunlight

All grasses require light, which is a key driver of photosynthesis. Photosynthesis is necessary because this is how plants produce their own energy for growth. While some grasses can grow in heavy shade with little light penetration, they will not thrive long. Whether it is shade grass or grass in full sun, all produce energy through the electromagnetic radiation it provides. Where light is insufficient, the grass will thin and begin to die. This amazing process of converting light energy to chemical energy is called “Photosynthesis and Respiration”. 

Determining the amount and quality of sunlight needed for each individual set of circumstances can be overwhelming. Too much or too little can each present their own set of problems. 

Although shade is generally considered the enemy of turf, many turf grass varieties are better able to handle shade conditions if other conditions are monitored and maintained. 

In many areas around the country that have frequent heavy cloud cover, the need to add sunlight hours is increased for maintaining healthy grass. Kentucky Bluegrass, Merion Variety, for instance is one variety (as well as others) that can handle shade well. However Merion when exposed to too much moisture tends to develop powdery mildew, turning the blades white or grey. The mildew can easily become too thick and can thin the grass as it blocks the light preventing photosynthesis. Some newer varieties have been bred to be more Powdery MIldew resistant, but being less resistant doesn’t mean it won’t get the disease. Maintaining the right balance of sunlight exposure is vitally important.

As a turf manager, you must either rely on one’s own interpretation of light and shade along with the grass requirements and growth habits or you can rely on modern electronics that can give you exact measurements of light reading. Actually to do it right takes both. Common sense must also play a part. 

One such light measuring device used by professional turf managers is called a Quantum Light Sensor. It is a device that measures PAR (Photosynthetically Active Radiation) and some models can measure the accumulation of PAR over time called DLI.  The measurements for PAR are called Photosynthetic Photon Flux Density (PPFD). The units of PPFD are micromoles of photons per square meter per second (µmol m.2s1). 

PPFD only measures PAR and the range is from “0” in total darkness to as high as “2000” at midday in summer. While it sounds complicated, understanding the numbers are not. The machine does most of the work. See the common scenario discussed below to learn more on how it works.

 2. Amount of Moisture

Moisture is an important aspect for any grass. Some require more than others to maintain vigor and health. Buffalo Grass is a very drought resistant grass and can’t handle areas of frequent rain or prolonged moisture. Turf Type Tall Fescue is the opposite and requires much more irrigation, especially in hot climates, to maintain color and to prevent it from going dormant. 

In heavy shade moisture doesn’t evaporate as quickly as in the sunlight. The grass you choose will need to be able to withstand longer periods of moisture. If the moisture your grass receives is primarily from sprinklers or by hand watering, then water deeply but about half to one third as often as in full sun. If tree roots are competing for moisture, then irrigate more often. 

Rough Bluegrass and Supina Bluegrass are two species that do well on prolonged moist, shady sites. However, these grasses may not persist in dry, hot summers. You will have to pay close attention to make sure the ground is watered more frequently. Syringing the grass (lightly spraying water over the grass) during the hot part of the day may be needed to keep the grass cool, especially during unseasonably hot summers.

Since shaded soil holds moisture longer than sunlit soil, always irrigate in the early morning. This allows for plants to use what they need and gives the ground time to dry. If you water at night or late evening the moisture will be in the soil much longer. While you may think this is a good thing, it actually isn’t. Persistent prolonged moisture can be a death sentence for grass as it allows for diseases to take off. Many diseases require high moisture for longer periods of time and can be more difficult to control.

 3. Blade Height

Blade height is an important factor.  Turf that is maintained at lower heights require longer periods of sunlight to compensate for shorter blades. Turf Type Tall Fescue that is maintained at two inches high requires more sunlight than grass maintained at 4 inches. To compensate for the lower height, grass plants will put out more blades than the same plant maintained at 4 inches. Raising your mowers blade height may help compensate for shade. Be sure your grass type can be maintained at higher heights. Some, such as Centipedegrass, suffer if allowed to grow too tall.

 4. Temperature

All grasses grow best in a certain temperature range. Warm season grasses thrive in hot weather, where cool season grasses thrive in cooler weather. 

Many of the grasses that are used in shady sites were actually developed in cooler countries than the U.S. The Fine Fescues, for example, many originated in Scotland or similar locations where the temps are much cooler. 

As a result, while Fine Fescues can thrive in full sun, in the U.S. where temps are much warmer, they are relegated as shade grass and are sold as “shade grass seed”. With that in mind, even shade can get hot. Many of the fine fescues as well as some bluegrass varieties, such as Rough Bluegrass and Supina Bluegrass will struggle if the temperatures reach into the 90’s in shade, especially in the south. 

Where temps are high, even in the shade, Turf Type Tall Fescues, such as NightCrawler and Mustang 4 should be considered. Both were bred for  exceptional turf quality in shade or sun and have a high heat tolerance. The Nightcrawler is a rhizomatous grass and spreads by underground stems. Therefore, it is considered a self-healing grass, meaning damaged areas will fill in.

 5. Soil Nutrients

Life begins in the soil, therefore, before you plant is a good time to do a soil test to determine the amount of nutrients that are currently in your soil. Your local University Extension or County Extension Office often have kits for gathering the sample and can mail the sample in for you. 

WIth that said, all grasses have a minimum nutrient requirement. As a general rule, shade turf requires less nutrients than full sun grass of the same species. As much as one half the nitrogen as the grass growing in the sun. This is because there is less photosynthesis happening in the shade. 

High maintenance turf or high activity turf, such as on golf courses or sports fields for example, require more attention, maintenance and nutrients than most home lawns. (check out this article for pro tips on Sports Field Maintenance

It takes skill and knowledge to properly care for a lawn or sports field. Fertilization applications are based on the N (Nitrogen) needed to be put down for that application and for that time of year. Regardless of what is in the bag, you determine how much nitrogen is needed for that application and the other nutrients in the bag are along for the ride. If you are low in Potassium and you need a large amount applied, make sure the fertilizer you are using has a higher percentage of K. This way if you are needing just half a pound of nitrogen, then the bag has a large amount of K that is also going down.

Warm Season Grasses grow steadily starting late spring to fall. Depending on the grass’ nitrogen needs, the grass is fertilized while it is actively growing. The Nitrogen applied is divided up over the year during the growing season. When the grass is dormant there is no activity inside the plant.

Cool Season Grasses grow best in the cooler parts of the year. Based on how cool season grasses grow, three quarters of the entire year’s nitrogen is delivered in the fall. The last and also heaviest application is delivered after the mowers are put away for the year. This is because even though the grass isn’t growing, the roots are very active and are putting away nutrients for the spring growth spurt. 

Fertilization is a complex issue. To know how to fertilize properly be sure to watch for a future article on fertilization techniques.

 The following is a common scenario that many turf managers face when choosing the correct grass type based on light and shade quality:

A lawn care company receives an inquiry about the best turf grass selection for his front and back yard. A family just bought a home in north Texas. The front yard is mostly in full sun with a couple ornamental cherry trees about 15 feet tall but the canopy is narrow. The back has a large shade tree with features that complicate things. The new home owner really wants Bermudagrass in front and back. The turf manager must rightly choose whether the decision of using Bermudagrass is an acceptable choice. Here is one method of meeting this challenge:

A turf manager knows that Bermudagrass is a full sun grass. As a general rule, Bermudagrass is not considered a shade tolerant grass. Most varieties will not grow but a couple feet into dense shade and stop. Even though there are some shade tolerant varieties that have been developed, such as “Celebration” for example, experienced turf managers would advise not using it as a shade grass and only cautiously say that its shade tolerance is greater than standard Bermudagrass varieties. The shade variety has a lower cold tolerance and could suffer winter damage in north Texas with their occasional ice storms.

The big question is how much sun does the grass need per day and how much shade can it take? Bermudagrass needs about 8 hours of sun to thrive and flourish. The minimum may be 7 hours. Morning sun is usually not as good as evening sun. Clearly the front yard may receive enough sunlight to satisfy the demands for efficient photosynthesis. 

The turf manager takes a reading while standing in the front yard from his Quantum Light Sensor at 12 noon in full sun. The reading is 1505 µmol m.2s1

However, the backyard is another story. Under the canopy the ground is not covered in total shade, but very patchy with large circles of light shining though.  There is a considerable amount of reflective light bouncing off the house and fence so the shaded area appears much lighter than it would be if the tree was in a different location. The light coming through the trees moves as the sun moves across the sky.

The turf manager takes another reading in the shade and it reads 826 µmol m.2s1. The intensity of sun is 45% less in the backyard than the front yard at the same hour. (826/1505 = 54.8 or rounded up to 55%) There is no way he could recommend bermudagrass as the backyard grass. Although part of the grass at that moment was in full sun, due to the shade tree and the rising and setting of the sun, much of the backyard will never get the amount of sun required for good growth even if he trims the canopy to let more sun in. At the very best the grass will be very thin and will lose many of the qualities, such as a thicker cuticle that helps prevent diseases.

Light Sensor Logger - licor.com

The answer is either to have two different grass types in his yard, such as Turf Type Tall Fescue in the back and Bermudagrass in the front and sides. Turf Type Tall Fescue needs a minimum of 3 – 5 hours of direct sunlight. Or if the customer prefers only one type of grass, he could look at St. Augustine, which requires 2 less hours of sunlight for good growth than Bermudagrass and has a high shade tolerance. The problem with St Augustine is that its northern range steeply declines as you travel north of Dallas, Texas.

Regardless of whether your grass is growing in full sun or shade, Turf Formula® is the perfect choice for growing a beautiful lawn. It is formulated to increase the efficiency of photosynthesis and essential soil elements such as organic matter.  Check out this link to learn why Organic Matter Matters  

The tremendous increase in beneficial microbes that Turf Formula brings helps reduce disease problems while increasing root growth and delivering more nutrients to plants. 

Plants need more than fertilizers alone, and Turf Formula® picks up where fertilizers leave off. How It Works? Add Super-Cal® to the Turf Formula® and you have a bio-enhanced super calcium that greatly increases the plant’s health and multiplies your soil’s beneficial microorganisms and more.

Russ James
AgriGro Turf Specialist
Call Russ / Email Russ

Sources:
Dr. Nick Christians, Iowa State University 2017, Bell G. E. Turfgrass Physiology and Ecology: “Advanced Management Principles” 2011, Russ James, Lawn Care Academy