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Hot temperatures affect plant health
The Coast Gardener Newspaper and Web Column - August 11, 2001

August and September can certainly bring two months of hot weather to Southern Mississippi. We have received much needed rainfall the past several weeks that have helped alleviate the drought situation. We can expect hotter temperatures when the rain stops and the clouds break.

High temperatures do affect the overall health of plants. Trees and shrubs grow best at temperatures between 70º and 85º. Plant tissue can even be injured when temperatures go above 90º. The degree of injury is determined by how hot it gets and how long the heat hangs around. Some plants adjust better than others to temperatures in the nineties. The longer the heat lasts, the more plants will suffer.

Flowers have trouble setting fruit and forming seed when day temperatures exceed 90 degrees and night temperatures exceed 75 degrees. Heat either destroys the pollen or interferes with the fertilization process. This is especially true with tomatoes.

Plants lose heat by several methods. The primary way that plants stay cool is through a process called transpiration. Transpiration is the process of water loss through the leaves. Plants can transpire lots of heat if the temperature is not too high. Unfortunately, high temperatures increase the dryness of the air, which increases the water loss through the leaves.

As this happens, the pores in the leaves, called stomata, close. This stops the cooling process causing the leaf tissue to heat up. Leaf injury often results.

Sometimes the leaves loose water faster than the roots can replace it. This form of heat injury causes a condition known as leaf scorch. During times of extended hot and dry weather, many people wonder "What's causing the leaves on my tree to turn brown along the edges before they fall off?"

This problem affects many trees such as dogwoods, maples, sycamores, fruit trees and Bradford pears. Although we are now receiving rain, some trees may still show symptoms of leaf scorch from an earlier drought period.

Water is the most important nutrient for plants. Plant processes cannot occur without water. Day water shortages in the plant are ideally corrected at night. Night water uptake can be 40 percent of all the water a plant needs.

Too much water, however, can be as bad as too little. We all ask how much and how often to water. It is important to only water when plants need it. How do you know when to water? Your plants will show you.

For example, wilting is the most obvious symptom that a plant needs water. Dig down and feel the soil. Plants will also wilt when the soil is too wet. This is because injured roots in saturated soil cannot absorb water either. Confusing, right?

Folding and cupping of leaves can happen when plants are dry. Lawns are excellent examples. Grass blades fold like a book or roll like a newspaper when they need water.

Always water deeply and thoroughly. Shallow waterings are not as good a deep waterings. Watering early in the day is best.

Take time to check your plants and water as needed. Your thirsty plants will thank you for it. Remember, however, there can be too much of a good thing--water.

These archived gardening columns were written by Chance McDavid, former Harrison County Extension Agent.