Published on 07/08/98

Hot, Dry Summer Especially Hard on Trees

Dust grit is between your teeth. Heat radiates from everything. The summer has turned oppressive. You may be slowed and burdened by the heat, but what about your trees?

Landscape plants, especially trees, are stressed by the heat. Trees stand tall and firmly anchored in the face of the blowtorch-like wind and heat radiated from hard surfaces.

Driveways, rooftops, patios and sidewalks absorb sunlight and radiate that energy as the heat we all feel. When the air moves this heat over to your landscape, it's called advective heat.

Advective heat means you must use your cooling power, water and shade to dissipate heat generated somewhere else. And this summer there is plenty of heat to move around.

Heat badly stresses trees in two main ways. One is through water loss. The other, much more insidious, is through accelerated food use.

Wilted foliage and drooping new twig tips are a sure sign of water shortage in the tree. The tree can take in water only from the soil.

In some places, little water remains accessible to tree roots. Either the soil particles are holding the water too tightly for roots to extract it, or the tree doesn't have enough roots to gather enough water. As water concentrations become too low, roots are shut down and don't grow.

Trees need water to make food and move essential materials. They also gain some heat dissipation from water evaporating from leaves. As water is allowed to evaporate from leaves, carbon dioxide for food-making moves into the leaves.

Water is the most valuable of raw materials trees collect. But as water becomes less available, trees must close up leaves to save the water that remains. Stopping leaf functions means the tree can't make food and will have its internal temperature climb.

As temperatures climb in and around the tree, many problems arise. For every 18-degree increase, water and food requirements double. As water becomes unavailable and leaves are closed, food continues to be needed in greater volume. All this food must be taken from storage.

Hot streaks mean healthy trees must close leaves and wait out the heat. It's like running on internal batteries. Old, young and badly stressed trees are much more at risk of damage, decline and death because they have little storage. The little stored food they have is being used at two to three times normal rates. Trees can starve to death during long hot, dry periods.

During the heat, watering can help if you don't overdo it. In heavy clay soils, it's easy to drown already starving tree roots. Your trees don't need a dip in the pool -- just a refreshing, deep drink.

Don't apply nitrogen fertilizers when soil temperatures are high and there is little water. Prevent mechanical damage and injury from landscape chemicals. Thin layers of mulch around a tree help conserve water and keep the root area cooler, which reduces food demands.

During hot periods, people and trees need each other. The shade that protects you, saves on cooling bills and blocks advective heat from terrorizing you is not free. You need to invest in the survival and health of your trees.

Kim Coder is a forester with the University of Georgia Warnell School of Forestry and Natural Resources.