Issue: June 7, 2004

Grass under tree died

My wife had a rather good crop of grass (type unknown) under a large white fruiting mulberry tree. Sometime after the fruit fell from the tree, the grass died. She blames the mulberry fruit. I maintain the culprit was the shade. Who is right? I believe the watering was properly maintained.

Melvin B.
via Internet

Without seeing the tree and the amount of fruit produced and dropped to the ground, I cannot give a definitive answer. However, this is a good question for explaining several factors important in growing trees in New Mexico. I will take advantage of this.

The first point is that trees and grass are usually not found growing together. In forests, the trees predominate because they shade the grass. In landscapes, as trees grow the shade increases and any grass in the shaded area declines or dies. For that reason, we recommend planting trees in a tree/shrub/groundcover area away from the lawn.

Recently I spoke to a county agent in New Mexico about a similar problem. A gardener in his county has a tree and lawn in the same area. The grass is a native grass adapted to low water conditions, and the tree is a hybrid willow that supposedly is more water efficient than other willows. The result is a lawn that once looked okay but now has dead spots. A final piece of information from the county agent was that the landscape was on a rocky site with a thin layer of "top soil" added. He was convinced that in this situation the problem was competition for water in the shallow soil. I agree. A shallow rooted hybrid willow can compete with a native grass for both water and light. In this case, the tree was winning. However, this fast-growing tree will not survive long in this environment. Over the long run, the grass will probably prevail.

Although you think water is not the problem in this case, it should be considered. Under-watering or over-watering are common problems in New Mexico, and other readers should consider water issues. In general, grass is the better competitor for water compared to trees. However, the competition for water is often a problem, especially if the soil is shallow in the landscape. How deep is the soil in your landscape, and how deeply is it irrigated when you water? The mulberry tree is an efficient competitor for water, so it may be competition for both light and water that caused your problem. The mulberry tree, when in leaf, sheds water to the treeâs drip line. Under the tree canopy, the soil is dry and there is relatively little light. Could this combination of factors be the source of the problem?

Finally, it is possible that the fruit caused the problem. If the fruit was produced in sufficient quantities that it covered the grass when it fell, it could have blocked the little light reaching the grass. The sugars from the fruit could have fed the microorganisms in the soil allowing a "bloom" of fungi and bacteria. If there were sufficient pathogenic fungi in the soil, this could have resulted in disease in the grass. However, the sugars would have benefited the beneficial fungi as well as the pathogenic fungi, so this would require the right conditions to cause problems. You can collect a sample from the border of the grass (where the dying grass meets the living grass), and take the sample to your local NMSU County Extension Service agent to determine if a disease was the problem. If necessary, the county agent can send the sample to the NMSU Extension Plant Pathologist to identify the organisms causing problems and recommend treatment.

I think that light and water competition are the most likely candidates for causing the problem, but without seeing the situation, the effect of the fruit must also be considered.

Please join us on Southwest Yard & Garden, a weekly program made for gardeners in the Southwest. It airs on KRWG in Las Cruces Saturdays at 4:30 p.m., on KENW in Portales on Saturdays at 10 a.m., and on KNME in Albuquerque on Saturdays at 9:30 a.m.

Send your gardening questions to:
Yard and Garden, ATTN: Dr. Curtis Smith
NMSU Cooperative Extension Service
9301 Indian School Road, NE, Suite 112
Albuquerque, NM 87112

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.