Aug. 31, 2007

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OVERTON - - Homeowners across Texas are concerned their landscape trees may be diseased, but "don't panic," said a Texas Cooperative Extension expert.

"We are getting quite a few calls from people who are seeing the leaves on their trees turn brown, and they suspect that their trees are dying from disease, " said Dr. Eric Taylor, Extension forestry specialist. "But that's not necessarily the case."

More likely, Taylor said, trees are taking a double whammy from August's high temperatures and droughty weather. The trees are just moisture stressed, he said.

"It's especially puzzling to some because we've actually had an unusually wet summer until just a few weeks ago," he said. "But this actually has made the situation a little worse."

Because Texas trees had so much water for so long, they became "spoiled," Taylor said.

"Until recently, trees were able to pump so much water through their system that they were able to maintain much greater foliage than they would have during normal conditions."

Also, the nature of the leaves of many species of trees change in response to either high-moisture or low-moisture conditions. For example, under low-moisture conditions, leaves are smaller, thicker and may develop a thicker waxy coating.

"These adaptations help to reduce transpiration, the evaporation of moisture from the leaves," he said.

With more leaves and fewer of those leaves adapted for typical conditions, trees responded to August's conditions by wilting, browning up early or even dropping leaves.

"Many of our species, such as redbud and catalpa - - but especially some of those non-native species that we see planted in our area like yellow popular - - tend to shut their leaves off altogether and drop those leaves," he said. "That's what we see happening; an early fall, if you will."

There are several ways to reduce stress on trees: watering, pruning, avoiding the use of weed-and-feed lawn treatments, and reducing the number of trees in a given area, Taylor said.

- - Watering.

"A typical, young landscape tree , with a trunk less than 5 inches in diameter requires 10 gallons of water per week, which surprises most people," Taylor said.

Larger trees can use much more. For example, a mature oak tree can pump 100 gallons of water through its system in a day.

"More than 90 percent of the water that enters the plant passes through and is evaporated into the atmosphere," he said.

Water needs to applied slowly, he said. The amount and frequency of irrigation depends primarily on the soil type.

Taylor warned that automatic lawn sprinkler systems are not designed to water trees. The water applied with lawn sprinkler systems rarely reaches below the first few inches of the soil profile. To water deep enough to reach a tree's root system, he recommended the use of a soaker or drip-hose.

"Set the drip hose around the periphery of the tree's crown - - what we call the drip edges. Water enough so the water slowly saturates down to about 12 inches," he said.

- - Reduce the number of trees.

Many lawns and landscaped areas have too many trees, Taylor said. He recommended that homeowners and landscape managers choose those trees that they prefer and remove others.

When crowded together, trees compete for water, sunlight and other resources.

"If you have a clump of trees, then there's quite a bit of stress on the trees," Taylor said. "If you remove several trees from the clump and leave a select few - - or even just one - - oftentimes, you'll actually improve the crown cover."

A simple rule of thumb is to thin so that tree's crowns do not touch each other, he said.

- - Prune trees.

Trees don't "prune" themselves - - lose excess branches - - very well when grown in an urban setting, Taylor said.

"We see numerous trees that have lower branches – branches closer to the ground – that aren't doing much work," he said. "In fact, instead of becoming 'sources' of energy, they actually become 'sinks' of energy."

Removing lower branches can increase a tree's vigor and reduce the stress it experiences during drought and from insects and disease. However, Taylor cautioned to use good pruning practices, which include waiting until the first freeze before pruning.

"That way the insect population has been knocked back sufficiently so it's less likely to cause damage to those open wounds you have created on the tree," said.

Prune so that the live crown of the tree is about one-third of the total tree height.

- - Avoid weed and feed herbicides.

Homeowners often rely on these mixtures of fertilizer and herbicides to control weeds in their lawns. But all trees are susceptible to varying degrees to the herbicide used in these mixtures, Taylor said.

"It may not kill your trees," he said. "But it certainly can stress them further and cause major damage when it's very dry."

Labels on the products warn against applying within the drip ring of the tree.

"But that's not really good enough, because the tree's roots go way beyond that," he said. "A large tree's root system may span a city block, so it's best not to use weed and feed products at all (if you have trees). Fertilizer is OK, but reserve herbicides for spot treatments.

"The name of the game is keep your trees healthy and happy by reducing the stress on them."