Does Evaporative Cooling Cause Overheating in Plants?

More Than Sun Relief

• Getting out of the sun is only part of the reason it feels cooler under the shade of a big tree. During the day, plants are constantly releasing water vapor through tiny openings, or pores, on the underside of their leaves known as stomata. Water vapor is released through the stomata into the air around the plant, cooling it and anyone taking their ease in its shade. This evaporative cooling, works much like a water cooler in the home does to cool you. It cools the plant too and is one of the reasons for transpiration, the opening of the stomata and releasing of moisture. Another reason for this is that as the plant expels water from its leaves, it draws more water up from its roots. Additionally, through the open stomata, that plant takes in the carbon dioxide it needs for photosynthesis.

Overheating and Transpiring

• As the air temperature around a plant increases, transpiration increases, that is it begins to lose water from their leaves more rapidly, as people do when they perspire on a hot day. Once the air around the leaves of a plant reaches a certain temperature, the stomata close to prevent the plant from losing too much moisture. When plants are exposed to warm temperatures that are just below this threshold, they lose the most water, which is when the evaporative cooling effect of plants releasing water dehydrates them quickly. The curled leaves, wilting stems and brown foliage we associate with summer overheating are mainly the result of water loss as the plant tries to cool itself.

Providing Evaporative Cooling

• If your plants are suffering water loss when temperatures are high, you can use evaporative cooling to keep them upright, green and looking good. Spraying the leaves of your plants with a fine mist of room temperature water at the height of midday heat reduces the temperature of the leaves and some of the stress on your plant. You will notice the drop in temperature too. Enjoy it. But don't mist them so late in the day they don't have a chance to dry before dark because soggy leaves are vulnerable harmful fungi growth.

Environmental Effects

• The amount of water that your plants release to cool themselves off depends on the environment around them. When the air is saturated with moisture on a humid day, plants lose less water through their leaves because the air does not absorb it as easily. Plants that are growing in dry air lose water much more rapidly since the dry air tends to pull moisture out of the leaves. Strong winds pull the moist air created by evaporative cooling away from plants increasing the amount of water vapor they expel. Plants that grow in direct sunlight also lose more water vapor and are more susceptible to overheating.