Thermal injury, or high temperatures
Plants are injured if temperatures become too high in the canopy or root zone, dehydrating tissues and killing plant cells. Damage is more likely the longer plants are exposed to excess heat and if they are moisture stressed, otherwise unhealthy, young, or a species more susceptible to high temperatures.
Identification
Similar damage is caused by aeration deficit, cold temperatures, exposure to herbicide or other phytotoxic chemicals, mechanical or gas injury to roots, root-disease pathogens, severe fire blight, and acute water deficit.
Fire is the most common cause of thermal injury. Fire injury can be diagnosed because it blackens bark in addition to scorching foliage. Usually the fire event itself was observed and reported. Often it is apparent that fire was the cause based on surrounding damage: one-sided injury along edges and distinct boundaries to the burn zone.
Steam or heat released by equipment, vents, and blacktop paving sometimes causes thermal injury to foliage. Usually there is an observable pattern to the damage and the injury develops over a relatively short-time period. Often the source of heat is apparent, and damage is most severe closest to the heat source.
Root injury can occur at soil temperatures above 105ºF. Root injury from heat is common in plant containers exposed to direct sun, but it is uncommon in landscapes because of the insulating effect of soil; exceptions include unshaded raised beds or when roots are near buried heat or steam pipes, usually when the pipes leak.
In new plantings where soil has been overly amended with undecomposed organic matter, roots can be killed because organic matter decomposition by microorganisms can generate substantial heat; this level of heat is common in compost piles.
Damage
High temperatures can cause brown, crisp foliage that appears scorched. Additionally, fire blackens bark and leaves. Trunk damage may not be seen immediately, but heat-damaged bark will separate from the underlying wood. As the trunk expands and callus develops, bark sloughs off, exposing the wood. Thermal injury can kill plants, most commonly after a fire.
Solutions
To avoid high-temperature injury to roots, plant properly, provide sufficient soil volume for root growth as plants mature, and irrigate adequately. Apply organic mulch on top of the soil. Shade the sides of containers and planting boxes.
Avoid planting near heat sources and choose species well adapted to local conditions. For example, giant sequoia, Monterey pine, and other species adapted to cool growing conditions become damaged or killed when planted where summers are hot, as in most inland valleys of California.
Before mixing organic matter into soil, compost it well. If topsoil is amended with undecomposed organic matter at more than about 20% of topsoil volume, do not plant in it until after soil has been moist and warm for 2 months or longer to provide time for decomposition.
For more information, see Abiotic Disorders of Landscape Plants.
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Heat injury from street paving
Leaf scorch above a heat vent
Fire-blackened bark and fruit
Fire burn-zone boundary
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