Recently, we were again reminded of how dangerous high temperatures can be to livestock, pets, and people. But what about our trees and shrubs? Do they also suffer from the heat?
High temperatures are unfavorable for the growth of many plant species because the rate of photosynthesis (the basic process plants use to make sugar) begins to decline rapidly after a critical high temperature is reached. 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 food-making process for many plants. Unfortunately for plants, respiration (the plant process that releases the energy of stored sugars to fuel growth) is not quite as sensitive to high temperatures, and continues day and night, depleting the food reserves of the plant. If extreme heat continues for weeks at a time, plants can actually die from a depletion of their food reserves. Finally, high temperatures may simply 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 can be killed.
Unlike our last bout with extremely high temperatures in 1988, our recent period of scorching heat lasted only for a couple of days while soil moisture remained adequate to good. These conditions should help trees and shrubs recover. But warn your landscape plants not to get too comfortable. After all, it's only late July!
This article originally appeared in the July 28, 1995 issue, pp. 1995 issue, pp. 116-117.