Originally created 08/04/06

Trees will go to war for water



People try and try to grow grass under trees, but it can be difficult, even when you choose a shade-tolerant grass such as St. Augustine or zoysia. A summer that is hot and dry, only makes it harder.

Why is it so hard when there is enough light getting to the grass? The answer: root competition. The weather conditions this summer help produce rings of thinned-out, yellowed or low-growing grass around non-irrigated trees or those that do not receive enough irrigation. The tree roots are creating the grass rings as they compete for water.

All plants use a lot of water to transport nutrients from the soil to the leaves. Once the water reaches the leaves, it evaporates into the air. Because water molecules stick together, as one molecule evaporates another one is pulled into its place. Water in a plant is in a continuous ribbon from the soil to the leaf surface.

The problem is that all plants in the same soil area are after the same water. This competition can be intense during hot, dry times.

Both grass and trees use a type of chemical warfare to ensure their success by injuring other plant roots. Whichever collects and controls the most water will be successful.

To take up as much water as possible, trees use many small roots in dense mats or clumps. The greater the density of small roots, the more surface area of the tree will contact the soil. The more soil contact a tree has, the more water it will take up. A good soil will have millions of fine root tips from plants trying to compete for the same water.

A problem with root competition is that some plants win and some lose, depending on their water-use strategy. Some use lots of water quickly, while others are more miserly. Some closely monitor and control their water use and slow down as water becomes hard to get. Others use water as fast as possible until there is nothing left.

A plant with more fine roots in an area will have a greater chance at collecting any water that's in the soil or that falls during a brief rain shower.

Trees have three main types of roots: large pedestal roots at the base, which hold the tree up; long, thin woody roots that move water and essential elements form the collection site to the stem; and absorbing roots that are extremely small and fragile and collect water and elements. These grow in large mats in the top few inches of the soil and can quickly sweep up small amounts of water from brief showers or a sprinkler.

About half the absorbing roots can be found directly under the branches of the tree. The rest colonize soil many yards away, growing well beyond the overhanging branches of the tree, most as much as twice the length of the branches. They constantly battle grass roots for survival.

Some yard rings around trees are caused by shade of the leaves and branches. Others, sometimes called fairy rings, might involve fungi consuming organic materials.

Drought rings happen around trees when large mats of absorbing tree roots collect all the water from the soil and leave the rest of the plants with little. Treat drought rings by irrigating well. Sometimes, fertilizing or creating ground cover may be required.

Sid Mullis is the Director of the University of Georgia Extension Service office in Richmond County. Call 821-2349, or send e-mail to smullis@uga.edu.