Dirt on Gardening

There’s a summer term I dread: Heat index. If you think that you’re uncomfortable outdoors when the heat index is over 100 degrees, some of your garden plants aren’t too happy either.

The heat index is what the combination of heat and humidity “feels like” to us humans. Plants cannot feel a heat index, but they can react to extreme amounts of heat and humidity.

One of the common responses from plants to extreme heat is for the plant to wilt. Wilting is not always due to a lack of water. Plants, especially young plants, can have perfectly hydrated soil and still wilt in the heat.

Wilt with adequate water is caused because of transpiration. The leaves of the plant transpire, or lose moisture, faster than the roots of the plant can absorb moisture in the soil. Usually, a mature plant will develop a root system that can equal transpiration and absorption in the plant. If you are having this problem with a container plant, the plant should be moved to a larger pot or placed in a shadier location.

Something to watch for in landscaped areas is “urban heat islands.” An urban heat island is caused in areas where there’s lots of pavement and/or concrete. Combined with hot weather the concrete and pavement can raise the temperature around the plants by as much as 10 degrees. A similar effect is found with container plants on patios. Plants in these areas may need water every day to survive.

It is possible for plants to make the environment cooler through evapotranspiration. Evapotranspiration occurs when a plant transpires moisture through its leaves, drawing heat as the water evaporates and cooling the air in the process. A large tree such as an oak can release up to 40 gallons of water in a day.

Evapotranspiration combined with the shade cast by a large tree can make a huge difference in the heat in some areas. This is another reason to consider adding some large trees to your home landscape; they can provide cooling effects and save energy costs, especially on the south and west sides of your home.

In times of high heat and reduced or no natural moisture, it’s important to remember to water well rather than frequently. Instead of frequent small watering that barely permeate the soil, give your plants a once a week deep soaking. When you give a deep soaking, you encourage roots to grow deeper into the soil in search of water. This will help your plants be better equipped to handle drought and high heat conditions.

Questions or comments related to gardening? Contact Joleen at missourigardener@hotmail.com

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