Just like people, plants can get dehydrated, sunburned and even severely injured by intense sunlight. Your options for protecting plants depend on whether they will be vulnerable for only limited periods, as with seedlings or cold-weather crops, if they are shade-lovers that require permanent shielding from harsh sunlight and the season in which the burn occurs.
Portable Protection - Young seedlings benefit from protection provided by a piece of plywood or other flat scrap material. Use a bucket, rock or sturdy wooden stake to hold the flat surface at a roughly 45-degree angle over the plant during the most intense periods of sun. If you need protection for more than a few hours or a few days, set a portable frame around the bed. Over this frame goes shade cloth, which comes in various densities from 90- to 30 percent. Depending on how much sun-shielding your plants need, determines the density of cloth required. A density of 30 percent to 50 percent protects vegetables, while shade-loving plants need 50 percent or more. Be sure the shade cloth is firmly attached to the frame so it's not dislodged in windy conditions, as fallen shade cloth can injure or break limbs and foliage.
Portable Plants - When you can't bring shade to your plants, bring your plants to the shade. Container gardening enables you to move your plants in and out of the sun as needed, whether to different parts of the patio over the course of each day, or to a different part of your garden as summer progresses and intense heat becomes the norm. Use lightweight pots and potting soil to make this an easier task. Larger containers on wheels offer additional portability.
Painted-On Preservation - As tough-looking as tree bark is, some trees are vulnerable to sunburn in the summer and sunscald in the winter. Young trees are especially vulnerable to tissue damage from intense sunlight. Once bark is damaged, the resulting cracks and wounds provide entry points to fungal spores, bacteria and pests. Prevent this damage by painting the trunks with white, water-based latex paint, which is diluted with plain water at a rate of 1 to 1. The white color helps reflect sunlight, rather than allow damaging rays to penetrate the bark. For the same reason, wrapping the tree trunk with either a loosely fitting white plastic trunk guard, or a softer paper-based white wrap, will also prevent sunscald. Make sure the wrappings are not tight against the trunk or water and air cannot flow freely around the bark. Do not use paper-based wraps in summer to prevent sunburn or the bark can be damaged, according to Washington State University Extension. Both solutions are sold at garden centers.
Prudent Positioning - Plants that wilt from too much heat benefit from positioning in which they will receive afternoon shade. In most cases, this will be on the east side of your house, but careful observation of light patterns over the course of a few sunny days will confirm the spot that will best protect plants, especially during the dog days of summer. The north side of your house is appropriate only for those plants that prefer little or no sun, such as hostas (Hosta spp.), which thrive in U.S. Department of Agriculture plant hardiness zones 3 to 8. Alternatively, set shade-lovers under trees or shrubs to block out the worst of the sun's rays.
Precautionary Practices - Once you get your seedlings settled, a few measures will help them weather occasional droughts and temperature spikes. Unless your plants are extremely sensitive to the sun, as with lettuce in summer, covering the ground in which the plants are growing, rather than the entire plant, may be all the protection they need from the sun. Baked soil can harm plant roots, as well as keep water from reaching those roots. To prevent this, mulch the area around your vulnerable plants with a 2-inch layer of shredded bark, straw, compost or other mulching material. Avoid gravel or pebbles, which intensify heat. Water plants sufficiently to help them survive prolonged sun exposure.