Gardening

Heavy shade curtails vigor of lawn grass

By Dr. William M. Johnson, Galveston County Extension Agent - Horticulture

August 22, 2007

Mature trees in the landscape provide us with many benefits. They provide pleasing beauty to the landscape, add to the economic value of property, and reduce summer air conditioning costs.

The cool temperatures of Spring were very pleasant. Until a couple of weeks ago, our summer had been relatively bearable—thanks to cloudy weather and frequent rainfall, temperatures ranging from the mid- to upper eighties ruled!

Then the clouds went in hiding, the rains stopped, and we melted over the next few days when temperatures blasted into the triple digit range. We are quickly reminded of another very welcomed blessing provided by mature landscape trees: they furnish us with a cherished spot of cool shade in the heat of summer.

Unfortunately, these same lovely trees can also provide not-so-welcome shade for our lawn grasses. "Why won't grass grow under my trees any more?" is a question that is frequently asked by Galveston County area residents. Oftentimes, the question is followed by the statement, "The grass had been growing great there for the last 25 years!"

Let's look at the situation a bit closer. A common scenario goes like this. One day you find that your lawn under the tree has "disappeared" entirely, leaving bare ground under the tree. You re-sprig, re-sod, or re-seed the area, but you ultimately find that you can't get the lawn grass to re-establish in the area.

The reason is not the lack of fertilizer or competition with the tree's root system nor insect or disease problems. The lawn grass thins out from lack of sufficient sunlight. Over time that young sapling grew larger and produced a wider and more dense area of shade—that's good because it's what we want. Then, the grass under the tree starts to thin out—that's bad because we've got to have a full carpet of green grass. However, we simply can't have it both ways. When there's not enough light there for the grass to grow, bald spots start to develop.

The problem will show up first with bermudagrass. Bermudagrass is unforgiving of shade. It will grow little and look bad even in a moderately shaded area. In fact, bermudagrass will not tolerate 20% or greater shade. There is no need to consider planting bermudagrass around or in the shade of large trees.

St. Augustine grass, on the other hand, is more shade tolerant. Understand that the term "tolerant" does not mean this grass thrives in the shade. It’s just that St. Augustine will do better than other types of lawn grass.

So let's look at your options. If you have grass growing in the shade of your trees now, then take steps to keep it going. The following suggestions will help to do this:

- Raise the height of your lawn mower bladee. The higher the lawn is mowed the less stressed the lawn is. Mow high—at least 3 inches.

- Practice deep, less-frequent watering durring periods of low rainfall. While causally watering the lawn by hand for a few minutes every evening or so is quite good for relieving human stress, this form of light watering can actually increase the stress on shaded lawn grass. This type of watering promotes development of a shallow root system.

- Avoid heavy foot traffic over the area ass this results in soil compaction problems.

- Thin out the crowns of the trees to alloww more light to penetrate. I must emphasize the word "thin" because heavy pruning can be harmful to mature trees. Moreover, thinning should be done in late winter. Keep in mind that thinning the canopy on older, mature trees is often done best by a professional arborist who can determine which branches should be removed without adversely affecting the tree.

- Remove fallen leaves promptly in the falll. If leaves are allowed to completely cover lawn grass, and especially if the leaves remain wet, then the grass will decline even more rapidly.

- Prune tree limbs up to a height of 8-10 ffeet from the ground to permit more sunlight to reach the grass. This can result in more morning and/or afternoon light provided a fence or other structure does not block the sunlight. And, it makes mowing less of an obstacle course.

If you have already lost the grass under your trees, consider planting a shade tolerant ground cover. Certain ground cover plants are much better adapted to shady locations than is lawn grass, and there are several from which to choose. Shade-loving ground covers include Asiatic jasmine, English ivy, ajuga, and mondo grass (monkeygrass). Unfortunately, they cannot take heavy foot traffic but they will cover the ground and perform admirably.

The most important thing to remember when creating a landscaped area under a tree is to respect the root system of the tree itself. Adding excessive soil or working the existing ground too deeply can eventually lead to the decline or death of a tree. Here are some tips: Avoid severing any roots larger than 1 inch in diameter. Where needed, use a garden fork (rather than a shovel or spade) to lightly work the very top layer of soil under the tree, since the fork will cut fewer roots. If you need to bring in extra soil to create the bed, use as little as possible—never add more than 1½ inches of soil to the bare area.

Dr. Johnson is a horticulturist with the Galveston County Extension Office of Texas Cooperative Extension, Texas A&M University. Visit his web site at http://aggie-horticulture.tamu.edu/galveston/index.htm

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