With the exception of one record-breaking weekend, temperatures in June averaged below normal this year. We have yet to see signs of heat stress in our gardens — just a few scorched leaves, maybe a dead late transplant or two — but not the full effect of extreme heat that can blight even the most carefully tended gardens in July.
Heat stress is often accompanied by drought stress (insufficient water) but not always. Plants react to high temperatures with several survival mechanisms and when those fail, they can die from heat stress even when water is available in the soil.
The first sign of heat stress is wilting or folded leaves. When temperatures are well above 100, the time window between the plant wilting, the leaves scorching and turning brown and the plant dying can be extremely short — an hour or two. New transplants are most susceptible to terminal wilt since their immature root systems cannot draw water up quickly enough to prevent water loss from all plant tissue. New plantings may need watering twice a day during really hot spells. Container plants also lose moisture more quickly than established landscape plantings; the pots retain heat that bakes the roots and moisture evaporates through porous pot walls. Plastic pots and glazed ceramic pots hold more heat than unglazed terra cotta pots, which are a better choice for sunny areas. Monitor your container plants and increase watering at the first signs of a floppy leaf or stem, move them into shady spots or create shade with umbrellas or shade structures in July.
Do lawn grasses wilt? yes, they do. And when they wilt, you can clearly see your footprints on the grass; the heat-stressed grasses don't rebound after being stepped on. Bermuda and other warm-season grasses need an average of 85 minutes of weekly irrigation in July and fescue and other cool-season grasses require about 113 minutes. Adjust watering times but not frequency to irrigate more deeply while still following the mandated schedule and adjust mower blades to cut off only 1/3 of the length of the grass blades; longer grass will shade the grass roots and keep them cooler.
A secondary sign of heat stress is loss of color in leaves, flower petals and grass blades. It is most visible in Bermuda grass, which turns a grayish blue color when stressed and in thinner rose petals, which can fade to beige in July. (Roses also lose fragrance when it's hot). There isn't much that can be done to correct for heat-induced color loss except to choose lawn grass hybrids or varieties that can withstand high heat and choose roses with thicker petals and a higher petal count.
Plants will drop leaves as one of the heat stress survival mechanisms. Flower production slows or stops and new growth is smaller and stunted. Fertilizing won't help heat-stressed plants — they should begin to recover as soon as day and nighttime temps cool down, usually by mid-August when normal fertilization schedules should resume.