Trees and Turf: Not an Easy Marriage

Published: September 1, 2010

In most Midwestern landscapes, trees and turf are planted in close proximity. Turf is an excellent ground cover and provides a sense of restfulness and openness. Trees create visual barriers, add interest and create shade for buildings and outdoor living and recreational spaces.

Unfortunately, trees and grasses are generally incompatible and often have adverse effects on one other. Most grasses are adapted to high light, prairie-like conditions that are often droughty. They use light energy to produce an aggressive root system in the top few inches of soil to harvest limited moisture and most have the ability to go dormant to survive drought. Most trees are adapted to a forest ecosystem in which shade from the tree canopy suppresses grasses. Soil on the forest floor is fertile and moist due to the mulch layer of decomposing leaves. Tree roots grow extensively in the top few inches of soil. When trees are planted in a turf area, they often struggle to become established because turf roots generally win the battle for moisture and minerals in the upper 6 inches of the soil. Trees and grasses grow successfully together in savanna ecosystems, but this type of association is relatively rare and requires regular fires to persist in nature. Grasses differ in there compatibility with trees. Tall fescue is particularly hard on new trees because, in addition to being highly aggressive, fescue roots exude phenolic chemicals that inhibit the growth of many other plants (even walnut). This chemical suppression is referred to as an allelopathy. However, Swanson found that a planting a mixture of perennial ryegrass and red fescue (Festuca rubra) as a groundcover in a nursery reduced the incidence of nectria canker on honeylocust trees by slowing growth in fall and thus reducing canker-inducing winter injury.

Trees, once established, often have negative effects on nearby turf. In addition to reducing light to sub-optimal intensity, trees can reduce air movement and keep a water film on grass leaves leading to attack of light starved grass plants by fungal pathogens. The combination of shade and surface growth of tree roots makes a particularly hostile environment for grass growth under an established tree. However, if a simulated savanna with producing less than 50% shade can be maintained by tree placement, pruning and thinning, most cool season grasses can persist under trees.

Recommendations for Facilitating
Tree/Turf Cohabitation

When Possible, Maintain Mulched, Turf-Free Zones Under Tree Canopies

• Plant shade tolerant shrubs, perennials and ground covers in mulched areas. See http://muextension.missouri.edu/explore/agguides/hort/g06911.htm

When Trees are Planted in Turf Areas,

• Use Good Tree Planting Techniques
• Wide, shallow planting hole, at least 4’ mulch ring, no mulch volcanoes
• Consider trees that provide filtered shade (honeylocust, KY coffeetree)
• Water new trees frequently and lightly- Do not keep the subsoil bathtub full of water by automatic, overhead turf irrigation.

Established Trees

• Prune up branches (lower 1/3) of established trees and/or remove trees to <50% shade.
• Irrigate infrequently and wet soil to 8”
• Do not over-irrigate mulched areas – (Promotes waterlogging and root growth in mulch).
• Do not allow herbicides to leach into mulched areas.
• Mulch leaves into turf under tree canopy with rotary mower to recycle minerals.

Managing Turf in Shade

• Plant shade tolerant grasses (such as fine fescues) See http://muextension.missouri.edu/explore/agguides/hort/g06725.htm
• Less than 2 pounds N/1000 ft2
• Mow at 3-4”
• Keep on dry side
• Mulch leaves before they mat
• Limit traffic and core aerify regularly
• Limit herbicide use, especially upslope from mulched areas around trees

References:

• Dunn, J. and B. Fresenburg. Grasses in Shade: Establishing and maintaining lawns in low light. http://muextension.missouri.edu/explore/agguides/hort/g06725.htm
• Green, T.L. and G.W. Watson. 1989. Effects of turfgrass and mulch on the establishment and growth of bare-root sugar maples. J. Arboric. 15(11): 268-272.
• Ham, D. and K. Townsend. 1997. Maintaining Tree/Turfgrass Associations: A Plant Health Care Approach. http://www.clemson.edu/extfor/publications/forlf22/
• Midwest Oak Savanna Conferences - http://www.epa.gov/glnpo/ecopage/upland/oak/
• Starbuck. Gardening in Shade. http://muextension.missouri.edu/explore/agguides/hort/g06911.htm
• Calkins, J.B and B.T. Swanson. 1996. Comparison of conventional and alternative nursery field management systems: tree growth and performance. J. Environ. Hort. 14:142-149.
• Calkins, J.B and B.T. Swanson. 1997. Susceptibility of ‘Skyline’ honeylocust to cankers caused by Nectria cinnabarina influenced by nursery field management system. J. Environ. Hort. 15:6-11.
• VanSambeek, J.W. and H.E. Garrett. 2004. Ground cover management in walnut and other hardwood plantings. In Proceedings of the 6th Walnut Council research symposium. Walnut Council Inc. Wright Forestry Center, 1011 N 725 W, West Lafayette, IN 47906-9431
• Watson, G.W. and D. Neely eds. 1994. The Landscape Below Ground. Proceedings of an international workshop on tree root development in urban soils. International Soc. of Arboriculture.
• Heckman, J.R. and D. Kluchinski. 1996. Chemical composition of municipal leaf waste and hand-collected urban leaf litter. J. Environ Qual. 25:355-362