Throgmorton: Heat Stress of Plants

Our temperatures have rocketed from a cool, moist early June to July temperatures. Plants can’t go to the basement or turn on the air conditioning. Many may start suffering from the heat. And different plants react differently to heat stress.

The new growth on a spruce will look great but the older, inner needles may turn reddish-purple and begin to fall. They’re shedding needles to balance their moisture loss. Maple, ash and linden may have brown leaves. It may be only the edge of the leaf or the entire leaf may turn brown and crisp. Hackberry and young Honeylocust may completely defoliate. Check these plants for supple buds at the tip of the branches. Usually they go dormant early and are fine the next season.

Newly transplanted plants are most susceptible. This includes any plants transplanted in the past year. They have limited root systems. Their roots can dry out quickly. Sometimes their roots just can’t take in enough moisture to balance the moisture evaporating from their leaves on a hot, dry day.

Sprinkle the leaves of these plants in the cool of the morning or evening. This will help reduce moisture loss through evaporation. Keep the soil evenly moist. Put water close to the center of the plant to keep the original root ball moist. Water in a wider circle as roots grow into the garden soil.

Most of us have adjusted our watering schedule to keep up with the heat. Run drip systems more frequently and longer. The key is to re-adjust as the days get shorter and cooler.

Unfortunately, automatic sprinkler clocks aren’t always re-adjusted. Once the heat breaks, a lot of plants will get over-watered. Plants will stop growing vigorously. They’ll need less moisture. Plants will transpire or lose less moisture as the days cool. If clocks aren’t adjusted and watering curbed plants will get too much.

The tricky thing with plants is that their symptoms for under-watering are similar to symptoms for over-watering. Some shrubs wilt if they get too much water. Some plants get brown leaves. Others drop leaves when they’re over-watered. Check the soil. If it is wet and boggy, then the water needs to be cut back or off. If the soil is dry and hard, then it’s time for a thorough, deep watering.

Tom Throgmorton, of Throgmorton Plant Management. (970) 980-5111

Originally published on June 29th, 2011. Updated on June 11th, 2024.