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Plants Feel the Heat!

With over 50 days of 110-degree weather in the valley, the extreme heat is taking a toll on many species of trees and shrubs.

What happens to our plants in these conditions? 

According to Jeff Iles with the Department of Horticulture, high temperatures are unfavorable for the growth of many plant species because the rate of photosynthesis (the process by which plants convert the energy of the sun into chemical energy – complex sugars) begins to decline rapidly after a critical high temperature is reached, resulting in the breakdown of tissue.  Photosynthesis occurs during the day and at considerably higher rates than respiration (the process by which plants convert the stored chemical energy into energy to fuel growth) which continues day and night.  If photosynthesis is unable to replenish the stored chemical reserves used overnight by respiration, the plant will start to decline, exhibiting death of leaf tips and margins, branches and roots, eventually resulting in death of the plant.  If extreme heat continues for weeks at a time, more plants may die.

It is difficult to define one critical high temperature for landscape plants because it varies with species, however, temperatures in the 90's and 100's undoubtedly slow this important light-energy to chemical-energy conversion for many plants.

Finally, high temperatures may cause severe water loss (desiccation) when transpiration (the process by which leaves release water vapor to the atmosphere) exceeds moisture absorption by the roots. As the water content in leaves decreases, leaves wilt slowing the rate of water loss, but this causes leaf temperatures to increase because of reduced evaporative cooling. Again, if unfavorably high temperatures persist, this cycle can worsen so that a portion, or all of the leaf will die.

How should we care for plants in hot weather?

• It is recommended to water plants deeply, ideally first thing in the morning while temperatures and transpiration rates are lower.
• Mulch or apply granite to help keep soil temperatures lower, retain moisture, suppress weeds and improves soil health.
• Move potted plants to a shaded area. Beware of potted plants close to a wall that might receive radiant and or reflective heat during the day.
• Set up shade cloth for young tender plants.
• Watch the leaves on your plants.  Leaves wilt during extremely hot and sunny days as a defense mechanism.  If water regimens are sufficient but the plant does not perk up in the evening or after watering in the morning then it might be a sign that soil water is not the problem.
• Practice sustainable landscape management. The best defense for extreme weather is having healthy plants.  Caring for them year-round with the right fertilizers and amendments, seasonal pruning, and good watering practices are the best way to help your plants make it through extreme weather.