As the summer temperatures start to rise above 85 F., many of our plants will inevitably start to feel the adverse effects of heat stress. Heat stress occurs when temperatures are hot enough for a sufficient period of time to cause irreversible damage to plant function or development. Signs of heat stress include wilting, yellowing leaves and drying up. Plants will also drop leaves,, flowers, blossoms and fruits in an effort to conserve water. Unfortunately there isn’t much that can be done for trees and plants that have sustained heat related injury but there are several things we can do to help minimize or prevent heat injury in the future.
It is important to deal with heat stress as soon as you notice it. First, check the soil several inches below the surface to see if it is already damp. If the soil is damp but your plants still look wilted, do not add more water – many plants simply wilt in intense heat (tomatoes are notorious for this), and will perk up once temperatures drop in the evening. However, if the soil is dry, it is important to water your plants immediately (don’t wait until your next scheduled watering cycle as irreversible damage could set in rapidly). Plants in containers should be watered daily and even twice a day in extreme heat. Make sure to give them a good soaking. Trees and shrubs should be watered regularly and deeply with a long slow trickle to ensure all the moisture is absorbed into the root systems. If water is running off dry compacted dirt, give it a short watering to moisten the surface and hit it again later with a more thorough soaking once the ground is able to absorb. Applying organic mulch is a great way to lock moisture into the soil to prevent evaporation and regulate soil temperature. Shade cloth and ground covers will also provide your plants with a little instant relief.
Originally published on July 1st, 2015. Updated on April 30th, 2019.