Heat stress in trees

We have had a pretty hot and dry summer throughout much of South Carolina and, although we are seeing some precipitation now, many trees are showing signs of stress.

Heat, especially combined with drought conditions, can damage trees in several ways. Most trees grow best in the 75°F to 85°F (24-30°C) temperature range. Above about 95°F (35°C) photosynthesis becomes less efficient, using more energy than it creates. Most plants cannot survive temperatures above 112°F (44°C). At these temperatures enzymes denature, nucleic acids (DNA and RNA) unravel, and cellular function ceases. Trees can cool down, most efficiently by transpiration, but this requires water, lots of water! When temperatures stay high for multiple days, trees feel it!

Trees respond differently to excessive heat and drought. Some species, like many pines, tulip poplar and sycamore, will drop older foliage to reduce transpiration loads and water loss. Others, like maples and many oaks, keep their foliage on but are thrifty with available water so that many leaves get a scorched appearance, usually around the edges of the leaves. Most of these trees will look poorly, but should recover once water is available and temperatures have cooled down.

Water is key to helping trees survive and is most effective BEFORE the trees are stressed. When weather is hot trees need more water. Watering in the heat of the day can be wasteful… much of the water is lost to evaporation so it is better to water in the evening. Avoid watering at the base of the tree; many of the roots that absorb water and nutrients are at or beyond the drip line, the ground with foliage directly overhead. Applying fertilizers can be counterproductive; stimulating new growth will increase the water requirements of the tree.

Trees are resilient. With a little extra care and the right weather, stressed trees should come out healthy next spring!

Maple leaves showing the scorched symptoms typical of draught or heat stress. Some fungal diseases cause similar symptoms, but usually when it is cooler and wetter.

These oaks are showing signs of stress from the long (more than one week) exposure to temperatures at or close to 100 degrees.