Heat Stress
Temperature Abiotic disorder
Last updated
This profile contains basic abiotic disorder data. Regional field notes and expert review are in progress.
What Causes It
Sustained high air and soil temperatures push plant metabolism beyond the temperature range where enzymes and membranes function normally. Photosynthesis rates fall as Rubisco activity becomes unstable above roughly 95F on most temperate landscape species; respiration rates continue to climb, producing a net carbon deficit. Membrane lipids become fluid and leaky, disrupting photosynthetic electron transport. Stomata may close to conserve water, further reducing cooling and carbon uptake. Root zones in shallow soil or containers can reach lethal temperatures during heat events, damaging feeder roots and compounding water stress. Heat stress usually combines with water stress, light stress, and wind stress during extreme weather; the multi-stress combination is what drives severe damage. [VERIFY]
Quick Reference
Symptoms
Wilting during mid-day heat that recovers overnight in mild cases. Sustained wilting that does not recover indicates damage past the turgor loss point. Bleached or bronzed foliage on sun-exposed surfaces, especially on broadleaf evergreens. Premature leaf drop on deciduous species during extreme heat. Scorched leaf margins similar to drought scorch. Flower and fruit abortion during heat events. On container plants, rapid decline as root balls overheat. Conifers may show bleached needle tips or whole-branch browning on sun-exposed surfaces. Symptoms often overlap with sunburn and drought injury; heat stress can be thought of as the shared cause that drives both. [VERIFY]
Heat stress symptoms are tightly correlated with specific heat events and regional weather. Region-wide damage across many unrelated species during a documented heat wave strongly indicates heat stress as the driver. The 2021 Pacific Northwest heat dome is the clearest local reference event: canopy bleaching, leaf drop, and fruit damage appeared across species that had never shown heat damage before.
Timeline: Acute symptoms during and immediately after a heat event (within hours to days). Cumulative effects (leaf drop, flower and fruit abortion, reduced growth, predisposition to secondary pests) continue for weeks to a season.
Triggers & Conditions
Air temperatures above the plant's thermal tolerance, especially when combined with high solar radiation, low humidity, and water stress. Container plants and plants on south and west exposures near reflective hardscape experience amplified heat loads. Cloudy weather followed by sudden clear hot weather is a common trigger for the worst damage because plants have not acclimated to the light load. The core physiological mechanism is: photosynthesis becomes unstable above ~95F (Rubisco denatures, electron transport breaks down); respiration continues to rise, creating net carbon deficit; membrane fluidity increases, compromising stomatal function and cellular integrity.
Summer heat events, especially multi-day heat waves. Newly planted stock and container plants are vulnerable throughout the growing season on any hot clear day. June through September heat events are typical.
Heat stress was historically a minor concern in the Puget Sound lowlands where summer temperatures rarely exceeded 85F for more than a few days at a time. The 2021 Pacific Northwest heat dome changed that baseline. Temperatures reached 108F in Seattle and 116F in Portland, and landscape damage was region-wide: Japanese maple leaves bleached and crisped, western red cedar foliage bronzed on sun-exposed sides, hydrangea flowers and leaves collapsed, containerized plants died in hours, and raspberry and blueberry canopies scorched. Subsequent 2022 and 2024 heat events added cumulative damage. The regional species mix was selected during a cooler climate regime, and many established plantings are now at or past their heat tolerance during the peak of summer heat events. Design for future plantings on exposed sites should account for this shift. [VERIFY]
Management
Prevention
- Select heat-tolerant species on exposed sites
- Mulch root zones to moderate soil temperature
- Water deeply before forecast heat events
- Shade or shelter vulnerable specimens
Mitigation
- Evening misting to cool foliage
Design for future heat: use reflective mulches and light-colored hardscape on exposed sites, provide shade canopy from heat-tolerant trees for sensitive understory plantings, and avoid dense planting schemes that increase afternoon air temperatures within the canopy.
Plant Tolerance
Temperate-zone landscape plants generally. Species from hot climate origins tolerate heat better; species from cool maritime and mountain origins are most sensitive.
More Tolerant
- Mediterranean-origin shrubs (Cistus, Rosmarinus, Lavandula, Teucrium)
- Quercus species (most)
- Pinus nigra and P. ponderosa
- Succulents and xerophytes
More Sensitive
- Acer palmatum and cultivars
- Hydrangea macrophylla
- Rhododendron species (many)
- Tsuga heterophylla
- Cornus florida
- Container plants of any species
- Newly planted stock
Species origin (hot climates vs cool maritime), leaf anatomy (waxy cuticles, reflective surfaces, indumentum), transpirational cooling capacity (dependent on soil moisture), rooting volume and soil depth, and exposure to reflective hardscape and wind. Well-watered plants in deep soil tolerate heat much better than stressed plants in containers or thin soil.
Secondary Effects
Heat-stressed plants are predisposed to secondary pests, sunburn, drought stress, and opportunistic pathogens.
Flower and fruit abortion during heat events reduces yield on edible plants and bloom on ornamentals.
Severe events can kill containerized plants and newly planted stock within hours.
Repeated heat stress events across multiple growing seasons (2021, 2022, 2024 pattern on Puget Sound) create cumulative canopy decline that may exceed plant recovery capacity.