Forestry Leaflet 22: November 1997
Trees and grasses are, as a rule, adapted to two separate and distinctive natural habitats. Consequently, their strategies for survival differ dramatically. Most of our landscape trees are native to forest ecosystems. The dense shade produced by the forest canopy prevents shade-intolerant plants (including many grasses) from becoming established. Forest top soils are also moist and fertile from the constant decomposition of leaf litter and other organic matter. These conditions are highly favorable for tree root growth. Many grasses, on the other hand, are native to dry, prairie settings. They have adapted to low water availability by developing dense, aggressive root systems and the ability to go dormant in periods of drought. The absence of an overstory allows grasses to capture plenty of light to fuel their aggressive growth.
Trees and turfgrass are commonly grown together in man-made landscapes because of our desire to have both lush, green lawns and the shelter of large shade trees. This unnatural association is plagued with competition problems made worse by the limitations of urban soil conditions.
Roots compete for limited nutrients and water. Turfgrass roots create problems for trees because of their ability to aggressively colonize the top 2-3 inches of soil. Tree roots also favor the top portion of the soil profile for colonization, especially in manmade landscapes where soil compaction is common (see tree root distribution drawing at right). Soil air composition at greater depths is often too poor to support root function. Trees generally lose out in competition for limited nutrients and water in this region when turfgrass is present. Turf competition for water and nutrients can affect any age and size of tree, but is especially detrimental to young trees lacking well-established root systems. Trees with poorly developed root systems suffer from stunted growth. They often succumb to pest attacks and environmental stresses such as drought and nutrient deficiency.
Grasses can chemically retard tree growth. Some grasses, such as Kentucky bluegrass, red fescue, perennial ryegrass, bermuda, and bahiagrass, release chemicals into the soil which actually suppress the growth of tree roots. Allelopathy is the term which describes this competition strategy. Depending on the type of grass and the tree species, tree growth can be stunted by as much as 65 to 75 percent, even with supplemental watering and fertilization.
Tree root distribution. A tree’s root system is not like a carrot’s! Contrary to the popular belief in deep taprooting, for most tree species, the majority of the root system is actually concentrated in the top 6 to 8 inches of the soil profile where soil aeration is best. In fact, roots from a single tree may spread horizontally to a distance of up to three times the canopy width.
Trees block sun exposure from turf. In areas where trees are already well established, grass is often sparse and unhealthy due to lack of sunlight. Some shade-tolerant types are able to get by with less light than others; however, all perform poorly in the heavy shade of a densely canopied tree. Heavy shade conditions can also create temperature and moisture conditions that are favorable to turf disease organisms.
Tree species vary in their susceptibility to turf grass competition. The following tree root density table taken from a study at the Morton Arboretum (Watson, G., Grounds Maintenance, October, 1989, page 30) illustrates this point. For all tree species, mulch and mulch-covered soils contained greater total numbers of tree roots than bare soils or soils with turf cover. However, the Norway maple showed greater root densities than other tree species across all root zone treatments, thus indicating its ability to compete aggressively with turf roots.
Tree root densities were found by measuring the surface area (in centimeters) of roots in 300 cc (or about 1¼ cups) of soil or mulch, so the units are cm2/300 cc.
Tree Root Density
|Tree root densities were found by measuring the surface area (in centimeters) of roots in 300 cc (or about 1¼ cups) of soil or mulch, so the units are cm2/300 cc.|
Closely examine any tree that has grass growing up to its trunk, and you are likely to find old wounds at the trunk base caused by lawnmowers and stringline trimmers. Exposed tree roots which run along the surface of the ground also fall victim to these machines. Wounds make a tree more susceptible to decay and insects which can eventually compromise its health and structural stability. These wounds to the trunk remove not only the bark, but also tissue just inside of the bark which is responsible for diameter growth and food transport. When enough of the circumference of the trunk is surrounded by wounds, the roots are cut off from the food supply and the tree dies.
In several other ways, the practices that we use to maintain green turf are not compatible with trees’ cultural requirements.
Plant Health Care Solutions
Instead of struggling to keep your competing trees and turfgrass healthy with heavy inputs of water and fertilizer, the best approach is to eliminate or reduce the competition and conflicts by mimicking the conditions under which these plants grow best in nature. The result will be fewer insect and disease problems, less time and expense in maintenance, and a more attractive, longer-lasting landscape. Here are a few of the ways to accomplish those objectives.
Donald L. Ham, Extension Forester and Professor
Karen Townsend, Research Assistant
Department of Forest Resources