Hot enough for you? Well, it’s August, and most of the country seems to be dealing with unusually high temperatures, south to north and east to west. Plants of course are “cold blooded”, meaning they can do little to regulate their temperatures. It is known that plant respiration increases at higher temperatures, but the mechanism for this is not well understood. The enzyme- controlled reactions within plants are temperature sensitive. These reactions are very well organized, and they generally sequence best in the temperature range to which plants are adapted. In other words, tropicals will generally have their peak respiration and metabolism at warmer temperatures than say Alpine or Arctic species.

There is such a thing in plants as “heat delay”. Think of the chemical reactions in a crop functioning sequentially at optimal am temperatures as if they were the cylinders in the engine of your truck. When everything is firing on time, the engine runs well and the truck moves forward quickly. In a plant where temperatures are too high, the reactions don’t all fire at the optimum speed. The plant becomes somewhat like an engine that badly needs a tuneup. It runs roughly and doesn’t accelerate well. Some poinsettia varieties will finish 2 to 3 weeks later at high temperatures.

Heat delay is better understood in time sensitive crops such as holiday flowers, as opposed to plants like trees and shrubs which are less time sensitive in the marketplace. Their heat delay factors are less well-known, but they are likely just as real. When we are hot, we can perspire, drink cold water and either turn on the AC or move into the shade. Plants can do none of these things. They have to sit out there and take it. Many plants however can close their stomates, or curl or twist their leaves away from the sun. Some will simply softly wilt in the afternoon heat, changing the angle at which their leaves are exposed to the sun. They resume normal turgidity  when night comes or the temperature falls.

It’s not just the daytime temperatures that are important. Night temperatures are important too. It is for example known that some tomato varieties will produce sterile pollen when night temperatures are above 78°. At night, plants assemble the raw materials they make in the daytime into larger molecules like proteins and starches. That production can be slowed during high night temperatures. Also, the optimum temperature for roots is usually lower than that of shoots. In the spring, plant roots begin to grow before the leaves begin to grow. At higher temperatures, moisture requirements, nutrient requirements and plant energy requirements all increase.

I have been seeing more heat damage in nursery crops and landscapes  recently than I usually do. Leaf burn, marginal scorch and bleaching have been common in heat sensitive varieties. Some plants don’t manufacture chlorophyll well at high leaf temperatures. Heat and light are interrelated. Sometimes we shade crops more for temperature reduction than light reduction . I saw some crops of New Guinea impatiens the other day that normally grow fine in 30 to 40% shade, but in the Florida heat they were doing much better in 63% shade. The difference is clearly temperature and not light level, as I have observed many New Guinea crops in those houses over the years. The white varieties with lighter green foliage seemed to take it harder than the varieties with more red pigment. Variegated plants will often show leaf burn in variegated portions of the leaf as opposed to normally pigmented tissue.

What can growers do to help protect their plants from heat stress? Obviously, avoid drought stress in your plants as best you can in hot weather. Grow under heavier shade where feasible. Try to keep your roots as healthy as possible. Mist  plants briefly 2 or 3 times a day during the hottest weather, at least for those crops showing heat stress symptoms. Irrigating during the middle of the day instead of early in the morning or at night may help provide evaporative cooling during the hottest part of the day. Keep potassium up in your feed program to help ensure proper stomate function. Finally, make sure soluble salts are not excessive. CRF releases more quickly in higher soil temperatures. Temperature in container media generally fairly closely follows the ambient air temperature. Stay cool. Footballs will be flying through the air soon.