Bright sun and excessive heat, especially if accompanied by wind, can dry plants quickly. Even if the ground is moist, plants may be unable to draw up enough moisture through their roots to replace the moisture transpired from their leaves. The plant wilts as a defensive mechanism. When water is inadequate, the water needed to hold cell walls rigid is not present and the plant collapses. Usually, if the soil is moist, the plant will recover without damage after the period of stress.
However, plants don’t always escape unscathed, especially if the soil is dry. Leaves burn at their edges or shrivel up entirely. The bark of dark-colored trees may develop sunscald roughness, a vulnerable opening for future fungal or bacterial invasion.
Shade-loving plants, which often also like a moist soil environment, frequently suffer stress in times of heat and bright sunlight. Water your camellias, rhododendrons, and azaleas attentively in these circumstances because their shallow roots dry out quickly.
Be especially alert to watering during heat waves. With enough water, plants keep themselves cool pretty well, but they are much more susceptible to heat when they can’t get the water they need. See for more information.
Protecting Plants from Sun
Young plants and plants recently moved to a new location are particularly susceptible to damage from strong sunlight, wind, and rain. Temporary shading devices can be used to protect germinating seedlings and young transplants; they can be removed when the plants become accustomed to their environment. One to three weeks of shading is usually enough.
A shingle shoved into the ground can shade a single plant. A newspaper tent can shade several plants. A board on two bricks can shade a newly-seeded row until the seeds germinate. Row covers can shade a whole row of plants. A cardboard box or wooden vegetable crate provide emergency shade for a stressed plant. A wire cylinder draped with burlap makes a handy movable screen.
Put landscape plants where they will not need temporary shading structures every summer. Plants that cannot tolerate hot sunlight should be planted under trees, under patio shade structures, or on the east or north side of the house. Larger shrubs or vines trained on a trellis may also provide shading for more sensitive plants planted under them.
The trunks of young trees may scald when exposed to intense sunlight, especially trees with dark bark. Bark that is newly exposed to sun by heavy pruning is susceptible to sun scald. You can reduce sun scald by painting the trunk with whitewash or white interior latex paint, or by wrapping the trunk with tree wrap, burlap or a similar material.
You may need to provide shading for some individual fruits and vegetables as they mature. Sun scald on melons can be avoided by shading the fruits with newspaper for the last few weeks before harvest. Tomato plants trained on a cylindrical wire cage produce fruit in the shaded interior and are less likely to have scalded fruit than plants grown on stakes.