Extension Forestry & Natural Resources

Wildlife & Fisheries Biology  -  Environmental & Natural Resources  -  Forest Resources

Maintaining Tree/Turfgrass Associations:
A Plant Health Care Approach

^{Forestry Leaflet 22: November 1997}

Natural Adaptations

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.

Competition Problems

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.

Maintenance Conflicts

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.

• Frequent irrigation that is often required to maintain green turf in the hot summer is detrimental to some trees. Water that strikes the tree trunk and collects at the base can cause root and stem rot.
• Many herbicides used to control weeds in turf are absorbed by tree roots and can severely injure or kill trees.
• Recommended fertilization rates differ between the two plant types. Typically, fertilizer applied at rates recommended for turf (1 lb. nitrogen per 1000 sq. feet of surface area) is rapidly utilized by the grass, and trees in the same area receive little benefit. The higher fertilization rates (2-4 lbs. nitrogen per 1000 sq. feet of surface area) recommended for trees may "burn" turf.

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.

• Designate turf-free areas under the canopies of trees. Turf outside of the tree’s canopy area will have ample sunlight while competition for rooting space within a major portion of the tree’s root zone will be eliminated. Remember to establish boundaries according to the ultimate size of the tree or plan to enlarge the turf-free zone as the young tree gets larger.
• Mulch with wood chips, bark, or pine straw within the turf-free zone surrounding trees. The larger the mulched area, the better for the tree. Apply mulch up to six inches deep, but avoid piling it directly against tree trunks. This creates moist conditions that can lead to rot or insect invasion of the tree trunk. Use only herbicides registered for use around trees to kill any dense patches of grass before mulching. If grass growth is already suppressed by shade conditions, then mulching alone will most likely eliminate it. Mulch creates favorable soil conditions much like a tree would have in its native environment. It moderates soil temperatures, improves water infiltration and retention, and enriches the soil with nutrients and organic materials as it decomposes. Studies have shown that the growth rate of a mulched tree can be double or even triple that of an unmulched tree, particularly if that tree is surrounded by turfgrass.
• Plant shade tolerant shrubs and ground covers in mulched areas if more "green" is desired under tree canopies. Although these plants also compete with the tree, their root systems are much less aggressive than that of turfgrass. They also are better adapted to living within the shade of trees. Ideally, these plantings should be done while a tree is young. However, if you must plant around mature trees, be careful not to cut large roots or disturb many of the finer, feeder roots in the top 4 to 6 inches of the soil. Tilling the planting area is not recommended as it will sever a large portion of those finer roots. Instead, dig individual planting holes using hand tools.
• Supplement water and nutrients to reduce the effects of competition in areas where trees and turfgrass must grow together. You may irrigate turf with sprinklers during dry periods, but avoid wetting the base of tree trunks. Young trees will respond better to occasional deep soakings than to the more frequent light waterings for turf. When fertilizing trees in turf areas, use slow release fertilizer or divide the total amount recommended into several staggered applications to avoid burning the turf.
• If you choose to exceed the recommended turf boundaries and maximize its area around mature trees, you may consider pruning low-hanging limbs to alleviate shade problems. This will allow more side light to reach the turf below the tree canopy. Take care not to strip too many lower limbs from a tree, as those limbs lend support to the tree trunk. A basic rule to keep in mind is to remove branches on only the bottom third of the trunk. Consult with an experienced arborist about how to properly prune your trees.

Donald L. Ham, Extension Forester and Professor
Karen Townsend, Research Assistant

Department of Forest Resources