Harmony in the landscape

Arborists and agronomists can meet in the middle when it comes to trees and turfgrass.

Photo courtesy of George Weigel

Tree care professionals and turf care professional have been at odds for a long time. Arborists claim that turfgrass is evil and must be eliminated from tree areas. Agronomists believe that their best tool is a chainsaw!

It is a given fact that in nature, trees and turf do not co-exist. Take a look at a forested area. You will notice the absence of turfgrass primarily due to shade. This did not occur overnight. In ecological succession, turfgrass establishes first, then secondary invaders arrive, such as low growing trees and shrubs. As the community matures, larger trees start to shade out the understory and the turfgrass dies out.

On the other hand, in a prairie setting, there is a dearth of trees likely due to recurrent brush fires that eliminate tree saplings. Where trees and grasses do co-exist in natural savanna landscapes, bunchgrasses, not turfgrass, are typically present and the trees are adapted to be tolerant of frequent grass fires.

When we care for landscapes, we try to optimize the growth of both turfgrass and trees to maintain aesthetics.

Height matters.

Trees occupy a higher stratum than turfgrass; as a consequence, tree canopies will intercept most of the sunlight and only filtered light will reach ground level. Even though turfgrasses, in general, prefer full sun, there are certain types that show good shade tolerance. Tall fescue and Red fescue tolerate shade well, whereas Kentucky Bluegrass does not. Warm season grasses such as Bermudagrass and St. Augustinegrass, prefer sunny conditions.

However, there are some cultivars with good shade tolerance such as St. Augustinegrass ‘Bitter Blue,’ Bermudagrass ‘Celebration,’ and Zoysiagrass ‘Emerald.’ In addition to intercepting sunlight, trees indirectly shade the turf through fallen leaves and spent flowers.

What about artificial turf? This product maintains its green color regardless of light levels. It does not require fertilization, irrigation or mowing.

However, depending on the product used, the underlayment may be made of non-porous material that does not allow water, air or nutrient movements into the soil. This may result in poor growing conditions for tree roots.

Underground conflicts.

Turfgrass and tree roots continually compete for water and nutrients. Trees consume large amounts of water that is lost to the atmosphere through transpiration. As the water supply is depleted in the soil, turfgrass begins to show symptoms of wilt. Roots also compete for nutrients as shown in many studies in the literature.

This is especially true with aggressively growing turfgrasses such as Bermudagrass. Surface applications of granular fertilizers applied to turf are not likely to benefit trees growing in the same environment. Deep root injection of tree fertilizers would be a better option to care for trees. Removal of fallen leaves to free turfgrass from their effects also deprives the trees of their natural mineral recycling method, further reducing their supply of nutrients.

Allelopathy (or chemical warfare) is another phenomenon to consider when placing plants together in the landscape. Some plants exude chemicals through the roots which inhibit the growth of other plants. A famous example is Juglone produced by Walnut trees.

Certain turfgrasses, such as Kentucky Bluegrass, Perennial Ryegrass and Red fescue may produce harmful leachates from the roots that adversely affect the growth of small trees such as Flowering Dogwood.

Trees and turfgrass have different optimal pH ranges; so it is impossible to optimize soil pH for both at the same time. Living under the wrong pH conditions will cause the plant to become deficient in some nutrients, while other nutrients may reach toxic levels.

The Solution.

Create a mulched zone around trees helps to mediate these conflicts. The area inside the mulch ring is managed for the tree needs, while the area outside the mulch is managed for the turf needs. Fertilization, pH adjustment and irrigation can then be tailored for their very different needs.

As can be deduced from the above, trees and turfgrass may be able to get along in a precarious peace treaty. However, that treaty will need the support of turfgrass professionals, arborists and landscape architects in order to maintain healthy and aesthetic urban landscapes.

Dr. Christopher Fields-Johnson is technical advisor, Mid-Atlantic region; and Dr. A.D. Ali is technical advisor, Southeast and South regions. Both are with the Davey Institute, The Davey Tree Expert Company.

March 2017
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