How to help your trees during Dallas' looming extreme heat wave

Texas Trees Foundation, a nonprofit that advocates for trees, has tips for how to treat trees during the extreme triple-digit heat forecast for Dallas in the next few weeks.

Trees may appear impervious, but they suffer during the heat just like humans; they just can't complain about it incessantly like humans do. If trees go through heat stress for an extended period, they can reach a point of weakness leading to pests and disease.

Watering is key but Texas Trees Foundation has some intel, including understanding what's going on beneath the bark.

How do trees sweat
Humans regulate body temperature via perspiration – releasing liquid through the body’s sweat glands. Plants "transpire": moving water up, against gravity, to evaporate from leaves, stems, and flowers. The self-sustaining process of transpiration is what helps cool the temperature immediately surrounding the tree. A large oak tree can transpire up to 40,000 gallons of water per year.

The higher the air temperature, the more a tree transpires. When temperatures get too hot, a tree will become unable to absorb water from the soil fast enough to support its canopy – the layers of leaves and branches that create shade.

This is worsened during drought. In times of little to no rain, trees close their stomata (small pores generally on the underside of leaves), to reduce the amount of water lost through transpiration. This is an effective way to conserve energy in times of drought.

But when drought is combined with heat, the tree can't transpire, and that's when it enters a state of heat stress. Symptoms include leaf wilting, scorched leaf edges, dead or dropped leaves, little to no new growth, or premature blossoms and/or fruit drop.

Some trees drop their leaves as a reaction to extreme heat and enter a state of summer dormancy. Many ornamental or fruiting trees will drop fruit or flower buds prematurely to conserve energy – or they will not bloom or fruit at all.

Heat stress tips
It's all about how much to water and when.

• Water deeply, but infrequently. A weekly, deep soak is much better than just a little water every day. (Note: Follow any water restrictions in your area.)
• Allow the soil to drain between waterings. Don’t let it stay soggy or muddy.
• Be aware that there is a difference in how much to water young trees versus old trees. Newly transplanted or younger trees need more water – approximately 5 to 15 gallons of water each week, particularly between April and October.
• Water the roots of the tree by applying water directly to the soil (as opposed to overhead irrigation); this way, you don’t lose any water to evaporation.
• Refresh the mulch over your tree's root zone. If there is no mulch, a 2- to 3-inch layer can help insulate the roots from heat and minimize moisture loss in the soil. But keep mulch away from the tree trunk. Mulch against the bark can lead to fungal problems on your tree.
• Sprinkler heads should not hit trees directly with water.

Best heat-tolerant trees for North Texas
Fall tree planting season begins in late October. As you plan for tree planting on your property, choose these heat-tolerant species for North Texas:

• Crepe Myrtle
• Cedar Elm
• Chinquapin Oak
• Bur Oak
• Lacey Oak
• Live Oak
• Southern Catalpa
• Shumard Oak
• Shantung Maple
• Texas Buckeye
• Desert Willow
• Bald Cypress

The Texas Trees Foundation (TTF) is a non-profit tree planting organization dedicated to greening North Central Texas. Established in 1982, TTF manages the nation’s largest non-profit urban tree farm and plants trees on public property. Contact TTF for questions regarding summer watering schedules, tree care and planting guidelines for North Central Texas. If you are interested in a planting project in your community call 214-953-1184 or visit their website.