While it’s no surprise that a heat wave can cause struggling plants to wilt, even well-established trees and shrubs can suffer setbacks. Although we can’t control Mother Nature’s heat waves, we can help our green companions by identifying heat stress symptoms and learning how to care for heat-stressed trees.
Understanding Heat Stress in Trees
Plants generally thrive in temperatures between 15°C and 30°C. When the temperatures go above 32°C and stay there, you might start noticing signs of heat stress. These symptoms become more pronounced when temperatures exceed 40°C. The National Weather Service defines a heat wave as a period of abnormally hot weather lasting several days, often accompanied by drought. This combination of heat and lack of water leads to the symptoms we observe in our landscapes.
Think of it like heat stroke in people. Here are some symptoms to watch for:
- Leaf rolling and cupping – Wilting
- Scorched leaf edges
- Dead leaves or leaves falling off – Some trees, like hackberry, are more prone to this and may enter summer dormancy if the heat is severe.
- Little or no new growth
- Sap oozing from the trunk – This can happen on the south or southwest side of the trunks where the sun’s heat burns the tree bark.
- Blossom and fruit drop – Many ornamental plants will drop buds or flowers after prolonged exposure to high temperatures to conserve resources.
If any of these symptoms were present before the heat wave, something else might be causing them.
Effects of Heat Stress on Trees
Extreme heat can impact many functions of a tree. Photosynthesis is reduced, leaves may fall off, and the growth rate of the remaining leaves decreases. Overall, heat stress can slow the tree’s growth.
Long-Term Impacts
While some tree species are more tolerant than others, prolonged heat exposure can exacerbate existing issues. Without sufficient water, trees may not be able to take up enough water from the soil to support their canopy. As temperatures rise and water becomes limited, trees close their stomates, reducing photosynthesis and slowing growth. This can weaken the tree, making it more susceptible to pests, diseases, and even mortality.
Immediate Actions During a Heat Wave
Wondering how to reduce heat stress in your plants? Follow these tips:
- Water deeply but infrequently. A weekly, deep soak is better than a little water every day. Follow any water restrictions in your area.
- Different watering needs. Young trees need more water to establish – about 5 to 15 gallons per week, especially between April and October.
- Water the roots. Apply water directly to the soil to avoid evaporation.
- Refresh mulch. A 2- to 3-inch layer of mulch can insulate roots from heat and minimize moisture loss. Keep mulch away from the trunk to prevent fungal problems.
- Check irrigation systems. Ensure your system is not over- or under-watering any area. Sprinkler heads should not hit trees directly with water.
By taking these steps, you can help your plants and trees survive the current heat wave and thrive despite the heat.