Scorching days have arrived! All but the hardiest of us wilt when outdoor temperatures hit the 90s and 100s, and we seek refuge in air-conditioned places or in water-cooled outdoor areas. Plants don’t have that luxury; they’re literally rooted where they are, and they sometimes need our help to deal with the Central Valley sunlight and heat.
Summer weather can damage plants by stripping them of the moisture they need or by exposing them to more heat or light than they’re adapted to handle. Higher than usual air temperatures, intense light, and overheated or too-dry soil can harm a plant’s leaves, stems, and roots. Wind can further worsen the effects of hot air.
Like humans, plants rely on water partly to cool themselves: We sweat, plants “transpire.” Transpiration is the process by which plants absorb water through their roots, move this water upward through the part of their vascular system called xylem, then lose this water through tiny pores called stomata on the leaf surfaces. The transpiration rate rises in hot temperatures; a plant’s water loss generally doubles with every 18-degree increase.
Plant species vary in the amount of water they need to resist heat and maintain good health (hence their classification as low, medium, or high water use). New plant growth, tender seedlings, fruits and vegetables, and cool-season annuals are particularly susceptible to sun-related damage.
Plants exhibit different levels of heat damage, and it’s important to know the distinction. Wilting is the drooping or shriveling of plant tissues that occurs when they lack sufficient water; it’s reversible if plants are watered in time. (Large-leaved plants will usually wilt a little during peak daytime heat even with adequate water, but will recover when temperatures cool.) Heat stress is when plants begin to suffer irreversible heat-related damage; at this stage, some plants will try to conserve water by dropping leaves or buds. Sunburn (or “leaf scorch”) is when a plant’s leaves or non-woody parts are permanently and severely harmed by excessive heat or sunlight; leaves develop dried brown patches or margins and they eventually wither and fall off. Sunscald is the cracking, discoloration, and warping of bark that occurs when the trunk or branches of a woody plant get too much sun exposure; the damage is permanent and very harmful since it increases the plant’s disease susceptibility.
Follow these simple guidelines to minimize heat damage:
• Conserve soil moisture and protect plant roots from excessive heat by covering bare ground with a 2- to 4-inch-thick layer of organic mulch — wood chips, shredded bark, leaves.
• Don’t place inorganic mulches — sand, pebbles, rocks, shredded rubber — or black-tinted mulch near plants in sunny locations (with the exception of desert-adapted plants), because these materials collect and radiate heat.
• Follow the principle of “right plant, right place.” Select plants adapted to our Mediterranean climate and choose planting locations with proper exposure. (No shade-loving plants in full sun!)
• Don’t heavily prune trees and shrubs in summer, because this can suddenly expose tender bark to the sun’s intense rays. It also encourages a flush of heat-sensitive new growth and places additional energy and water demands upon heat-stressed plants.
• Avoid planting during peak summer heat; this stresses plants and compromises their chances of successful establishment. Delay planting until fall, or (if you must plant this season) wait until a cooler spell, plant in the evening, and water deeply after planting.
• Keep potted plants well watered and (if possible) move them to shadier locations. Use light-colored or plastic containers, which absorb and transmit less heat than dark-colored containers or those made of ceramic, cement, or metal. Hydrogels (water-retaining polymer granules) can be mixed into potting soil to help hold moisture.
• Whitewash trunks of young trees to help prevent sunscald. Mix equal parts water and white interior latex paint, then apply it from 1 inch below ground to at least 2 feet above ground.
• Use strategically placed shade cloth to shelter plants.
• Ensure that plants receive appropriate and consistent levels of water, and check irrigation systems for proper operation. Do this yourself, or enlist the services of a Qualified Water Efficient Landscaper (qwel.net).
For gardening-related questions, call the UC Master Gardener office at (209) 953-6112, or use our website: sjmastergardeners.ucanr.edu/CONTACT_US/.