Sometimes when plants, turf, and trees look sick or appear to be under attack by insects, the symptoms are actually a sign that the plant is being stressed by environmental factors, such as heat stress. Here in Southern California, turf, various trees, and plants can suffer from forms of heat stress due to our somewhat dry, hot climate and periods of extreme heat caused by high temperatures. Here are some common symptoms of stress and the conditions that cause them.
Wilting Isn’t Always from a Lack of Water
Wilting can indicate insect or disease problems, but is most commonly due to a lack of soil moisture. Don’t assume your trees and plants have enough water if the soil surface is moist. Dig down and make sure it is moist to a depth of at least 6″ for most vegetable plants and other annuals. If soil is too dry, water plants and trees thoroughly; they should recover within 24 hours. Wilting is also a normal response to extreme heat. This physiological reaction indicates that the plant has temporarily shut down to minimize moisture loss. Wait to see if the plants recover in the evening when temperatures cool.
Turf is Vulnerable to Heat Stress Too
Like any plant, grass reacts to summer’s high temperatures and lack of water with wilting, browning, or even death. The Inland Empire, Coachella Valley, and other inland areas of Southern California are especially key areas where heat stress can occur in plants and turf grass. Here’s how to detect drought or heat stress:
Locate a brown patch, and pull on the grass. If it won’t pull easily from soil and is firmly rooted, it’s likely brown due to drought.
Push a screwdriver into soil in brown and green lawn areas. If the blade slips easily into green lawn and won’t penetrate in brown lawns, soil is dry.
Look at the lawn as a whole. When drought is the culprit, brown patches appear randomly and in rough patterns. Lawns near a sprinkler head may be green, while a lawn further away is brown.
Learn the early signs of heat stress. In general, footprints remain on grass after it’s walked on. Kentucky bluegrass develops a grayish cast, while other grasses become a darker hue. Grass blades may also wilt.
“Bleached” Areas are Another Warning Sign to Heed
Bleached areas on the foliage of new transplants, trees or other plants that have been moved from indoors to outdoors can indicate sunburn. Discoloration will be most pronounced on the leaves most exposed to the sun. To prevent sunburn, seedlings and other tender plants should be exposed to direct sunlight gradually, over a period of several days. Plants will usually outgrow minor sunburn.
Dormancy Can Be Your Plant’s Best Heat Stress Defense
During periods of hot, dry conditions, both cool and warm-season grasses can go dormant as a protective measure. If grass receives sufficient moisture, growth slows and blades remain green. During times of prolonged drought without irrigation, turf will turn brown. If grass turns brown, don’t irrigate it unless you plan to continue watering the rest of the summer. In addition, don’t let newly established lawns go dormant because with a limited root system, a new lawn might not survive dormancy without extensive injury.