Maryland Turfgrass Council – Trees and Turfs

December 9, 2021 | Maryland Turfgrass Council | PODCAST

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MTC Turf News – Larry Tankersley, Extension Forester, University of Tennessee Department of Forestry, Wildlife and Fisheries and Tom Samples, Ph.D., Turfgrass Science & Management, University of Tennessee Department of Plant Sciences

Trees and turfgrasses share the same basic requirements in order to live. Both capture energy from sunlight in order to produce carbon-containing substances that are used immediately to support growth, or can be stored in reserve. Photosynthesis, the combination of carbon, hydrogen and oxygen from carbon dioxide and water in the presence of light results in the formation of glucose and other sugars. Atmospheric carbon dioxide is also the source  of carbon in amino acids, proteins, starch and cell walls.

Trees and turfgrasses have vascular systems (xylem and phloem), and rely on water for oxygen and hydrogen, and to move essential nutrients absorbed from the soil to leaves, and sugars produced in the leaves to roots. Fourteen mineral nutrients are essential for their survival and reproduction. Nitrogen, phosphorus, potassium, calcium, magnesium and sulfur are classified as macronutrients due to the quantity of each that is required. Due to the very small amount (usually 100 ppm or less) of each found in plant tissue, the remaining eight essential minerals, chlorine, iron, boron, manganese, zinc, copper, molybdenum and nickel, are referred to as micronutrients.

Local air temperatures, annual rainfall amounts, and soil texture and fertility levels often determine if a particular species or variety of turfgrass or tree will thrive in the landscape. Although trees and turfgrasses growing in close proximity to each other must share resources (Table 1), this does not necessarily mean that they cannot co-exist. Successful management strategies assure that the fundamental requirements of both trees and turfgrasses are being met every year, even though growing conditions for each may be less than ideal.

Table 1. Fundamental Requirements of Trees and Turfgrasses.

Light Intensity, Quality and Duration. The rate at which photosynthesis takes place within a tree or turfgrass usually increases with increasing light intensity. Light intensity is described as the amount of energy, or ‘packets’ of light referred to as photons, hitting the surface of a leaf over some time period. Light stimulates stomates to open. As a result, high light intensities are usually associated with high water transpiration rates. Only an estimated one to two percent of the solar radiation a turfgrass is receiving is absorbed and converted to chemical energy. Most of this absorbed energy is reradiated at much longer wavelengths, resulting in the release of heat. Turfgrass leaves with a horizontal orientation are more efficient absorbers of solar radiation compared to those growing more upright. The surfaces of turfgrass leaves may also reflect solar radiation. Dull and dry leaves are usually less reflective than glossy or wet leaves. Turfgrass leaves may also transmit solar radiation that may be absorbed by other leaves.

In his book “Turfgrass Science and Culture” published in 1973, Michigan State University researcher and author, Dr. James Beard, reported that an estimated 20 to 25% of maintained turfs were receiving some level of shade from trees, shrubs or buildings. The canopy height of bermudagrass plants growing under low light intensities (less than 70 percent full sunlight) often increases by more than 100 percent compared to that of plants receiving full sunlight. Leaf elongation is often about 35 percent higher when cool-season turfgrasses are growing on sites with limited light intensity compared to the same species in full sun. This is believed to be the result of shaded turfgrasses producing more of the plant growth hormone, gibberellic acid. The root-shoot ratio of turfgrasses tends to decrease as the level of shade increases, and less energy reaches the roots. The cuticle, the protective, waxy surface layer of a leaf, often thins as the level of shade increases. This may cause shaded turfgrasses to be more susceptible to injury from drought, freezing temperatures, diseases and insects. Since air movement among plants is often restricted, turfgrasses managed in shade often take longer to dry after rain or irrigation.

In addition to the intensity of light, the overall health and performance of turfs under trees also depends on the quality of light they receive. Photosynthesis cannot occur without an appropriate amount of light of specific wavelengths. The quality of light reaching turfgrasses shaded by trees is often restricted. That is, a portion of the photosynthetically active radiation, or PAR (visible light wavelengths from 380 to 700 nanometers), has been intercepted and filtered by the tree canopy before reaching the leaves of turfgrasses. As sunlight reaches a tree’s canopy, it is 1) absorbed by the leaves and used for photosynthesis, 2) reflected back into the atmosphere or 3) transmitted to plants (turfgrass) below. Transmitted light is less intense and certain wavelengths, especially visible light important for photosynthesis, are filtered by tree leaves. When the tree canopy provides solid shading, turfgrasses receive only indirect light, totally depleted of many of the wavelengths that result in healthy growth. Absorption, reflection and transmittance of light as it passes through a cottonwood (Populus deltoids) tree leaf are presented in Figure 1. Notice that the dotted line or light transmitted below the tree crown, is very low. An alteration of the light spectrum directly affects turfgrass growth. Silicon photovoltaic sensors are used to estimate PAR, while other sensors predict the photosynthetic photon flux density (PPFD) of the PAR. The PPFD is the amount of photosynthetically active photons that are hitting the leaf surface per unit area per unit time. The PPFD is reported as micromoles of photons per square meter per second. These sensors are commonly used by growers to monitor and manage supplemental lighting in greenhouse plant production systems.

The intensity, photosynthetic activity and duration of light that a turf shaded by trees receives is influenced by its location in the landscape, and the size, form and species of trees. The level of shade in areas of a landscape receiving only four to six hours of direct daily sun is considered medium. Dense or ‘heavy’ shade is cast by trees with very dense canopies. Due to their ‘open’ form, pines (Pinus spp.) tend to allow more light to reach the turf surface than shade-tolerant, deciduous trees with ‘thick’ canopies, such as oaks (Quercus spp.), sycamores (Platanus spp.) and maples (Acer spp.).

The time of day that the turf receives direct sunlight also deserves consideration. The ideal time for a turfgrass to be in direct sunlight depends on the optimum temperatures for photosynthesis for the species being managed. More specifically, turf health and performance are dependent on the length of time that plants receive light within the PAR region at an air temperature at or near the optimum for photosynthesis. For example, during hot, dry periods in the summer, cool-season turfgrasses maintained with no irrigation and in areas of the landscape receiving moving shade, most often perform better when shaded in the afternoon, rather than in the morning.

The Turfgrasses. Shade tolerance varies among cool- and warm-season turfgrass species, and varieties within species. The relative shade tolerance of chewings, strong creeping red, hard and sheep fescues is high. Yet, the performance of these cool-season turfgrasses in shade is limited by a general lack of heat tolerance. Perennial ryegrass and Kentucky bluegrass rank low to medium in shade tolerance while tall fescue, the predominant cool-season turfgrass species in Tennessee, generally ranks medium in shade tolerance. Ranking of shade tolerance among warm-season turfgrasses is bermudagrass < centipedegrass and zoysiagrass < St. Augustinegrass. The adaptation of St. Augustinegrass in Tennessee is limited by a lack of cold tolerance. Although, as a species, bermudagrass ranks low in shade tolerance, the variety TifGrand® has demonstrated improved shade tolerance compared to several other sterile hybrid (Cynodon transvaalensis x Cynodon dactylon) varieties. Similarly, several newer, vegetative, clonal-type zoysiagrasses (Zoysia spp.) including ‘Diamond’ (Z. matrella), ‘El Toro (Z. japonica x Z. matrella), ‘Geo’ (Z. japonica x Z. pacifica), ‘Palisades’ (Z. japonica x Z. matrella), ‘Royal’ (Z. japonica x Z. matrella), ‘Zeon’ (Z. japonica x Z. matrella) and ‘Zorro’ (Z. japonica x Z. matrella) have improved shade tolerance compared to ‘Meyer’. Meyer, a variety of Zoysia japonica with good cold tolerance, was released in 1951, and is still being maintained in landscapes throughout the state.

The Shaded Turfgrass Microenvironment. Shading impacts the turfgrass microenvironment in several ways. Daily and seasonal fluctuations in the air temperature of the microenvironment are often reduced.

In the top several inches of soil, trees and turfgrasses compete for water. The first plant species established is generally the most successful absorbing water from the soil since it is the first to occupy limited growing space. A large tree with a well-established root system can often absorb water much better than a recently installed sod. Likewise, established turf will compete very effectively with a recently transplanted tree for available water.

Tree roots are opportunistic and spread well beyond the width of the crown, wherever conditions favor growth and adequate oxygen is present in soils (Figure 2). This is generally in the top one foot of soils. Some tree roots will penetrate the soil to greater depths, depending on the soil texture and bulk density. The majority of tree roots are located in the top few inches of soil. Many fine “absorbing” roots actually grow into mulch or thin turfs. As with turfgrasses, tree roots are often denser on the north (shaded) side of the tree where shade discourages rapid soil drying.

Relative humidity and carbon dioxide levels within a shaded microenvironment are generally higher, and air flow within the turfgrass canopy is often restricted, compared to those in ‘open’ areas of the landscape. When the relative humidity remains high for extended periods of time, and air flow among plants is severely restricted, turfs are generally more susceptible to disease. Due, in part, to a lack of sugars and starches, shaded turfgrasses are often less durable, and may be very slow to recover from injury.

Improving Turfgrass Performance in Shade. An annual review of the landscape management plan followed by adjustments in both tree and turf care can result in better turf quality.

Managing The Trees. The quality and intensity of light reaching turf below isolated trees may be improved with creative pruning (Figure 3). Generally, at least two-thirds of the tree’s total height should contain live branches. Regular pruning of the lower third, and removing drooping branches, is highly recommended to allow sunlight to the turf below. Crown thinning can also be helpful to reduce the crown density and leaf area of the tree allowing more sunlight to move further through the crown (Figure 4).

Another consideration is to plant trees that “naturally” allow more sunlight to pass through. For example, research indicates that maple and black walnut restrict far more radiation than honey locust (Table 2). Trees can also be selected based on shape to reduce the number of drooping branches that intercept light at lower angles. Without maintenance, most of our common trees restrict enough sunlight to stress all but the most shade-tolerant turfgrasses. Instances where tree cover is multi-layered, as in a forest, may preclude the use of any turfgrasses.