Sunburn
Light & Radiation Abiotic disorder
Last updated
This profile contains basic abiotic disorder data. Regional field notes and expert review are in progress.
What Causes It
Sunburn damages plant tissue when solar radiation loads the leaf or bark surface faster than transpirational cooling and pigment protection can dissipate the heat. On leaves: direct midday sun on tissue not acclimated to full exposure drives leaf surface temperatures above the thermal tolerance of chlorophyll and membrane lipids, bleaching the center of the leaf blade to tan or off-white while veins and shaded tissue remain green. On bark: strong sun on smooth thin-barked trunks raises cambium temperature beyond tolerance, killing narrow strips of cambium that later show as sunken bark wounds. The damage is usually worst on south and southwest exposures and intensifies when soil moisture is limiting, because stressed leaves close stomata and lose the evaporative cooling that normally keeps them below ambient air temperature.
Quick Reference
Symptoms
On broadleaf plants: bleached or tan patches in the center of the leaf blade between large veins, most intense on the south or southwest side of the plant. Affected tissue may dry and crumble; edges often remain green. On rhododendrons and similar broadleaf evergreens, the center of the leaf bleaches out to tan or off-white. On tree trunks: sunken, cracked, or discolored bark on the sun-facing side, often followed by bark slough. Sunburn damage itself is not usually fatal but provides an entry point for opportunistic fungi and bacteria; saprophytic fungi often develop in sunburned areas and make the injury look like a primary disease. Cultivars with heavy indumentum (fine hair on the lower leaf surface) lose less water during heat events and sunburn less readily. (Source: PNW Plant Disease Management Handbook, Rhododendron - Sunburn.)
Sunburn tracks sun exposure directly: damage concentrates on south and southwest facing surfaces, top of the canopy, and the side of a plant facing a reflective wall or pavement. It is most severe during heat events over 90F combined with limited soil moisture. Unlike scorch from drought or salt, sunburn bleaches the center of the leaf rather than the margin, because the margin continues to cool by evaporation while the center overheats.
Timeline: Acute: bleaching appears within hours to days of a heat event, most commonly during the first summer over-90F period. Another acute form appears on new spring growth during unusually bright cloudy weather before the leaves have hardened. Chronic: repeated mild sunburn each summer on the same exposure produces progressive leaf loss and canopy thinning.
Triggers & Conditions
Direct solar exposure above the plant's acclimated tolerance, especially on tissue that developed in shade or during cloudy weather. Amplified by air temperatures over 90F, low soil moisture, reflected heat from walls or hardscape, and sudden weather transitions from cloudy to clear. Newly planted or recently moved specimens are the most vulnerable because their leaves formed at the nursery under different light conditions. (Source: PNW Plant Disease Management Handbook, Rhododendron - Sunburn.)
Midsummer heat events and early spring new growth flushes during unusually bright weather. Plants recently moved from shade to sun are vulnerable for their first full season at the new exposure.
The Puget Sound lowlands are cloudy and cool enough most of the year that sunburn is a minor concern, but the 2021 heat dome (temperatures over 110F in Seattle) produced region-wide sunburn on plants that had never before shown damage: western red cedar foliage bleaching, Japanese maple leaf scorch and burn, western red cedar and hemlock needle browning on exposed south faces, and trunk sunburn on thin-barked young trees on recently cleared sites. The 2024 summer heat waves added cumulative damage on the same plants. Shade-preferring broadleaf evergreens (rhododendron, camellia, hydrangea, Japanese maple) are the most consistently susceptible, especially on west and southwest exposures near reflective walls. Cloudy spring weather followed by a sudden clear high-sun day is a common local trigger for sunburn on new growth that has not yet hardened to full light.
Management
Prevention
- Match plant species to site light exposure
- Maintain soil moisture through heat events
- Provide afternoon shade for vulnerable species
- Avoid pruning during summer that opens previously shaded interior wood
- Protect young thin-barked trees from trunk sunburn
Mitigation
- Provide emergency shade during forecast heat events
Place shade-loving broadleaf evergreens on east or north exposures, under deciduous overstory, or in courtyards protected from direct afternoon sun. Avoid west and southwest exposures near reflective walls or pavement. Use tall drought-tolerant species as windbreak and shade overstory on exposed sites. Consider temporary shade structures for specimens during the establishment phase.
Plant Tolerance
Shade-origin broadleaf evergreens and woodland understory plants are the most vulnerable. Thin-barked young trees with smooth trunks are susceptible to bark sunburn. Full-sun adapted species, conifers with heavy waxes and needle anatomy, and plants with thick cuticles or indumentum are more tolerant.
More Tolerant
- Quercus species (most)
- Arctostaphylos species
- Cistus species
- Pinus nigra
- Pinus ponderosa
- Mediterranean-origin shrubs generally
More Sensitive
- Rhododendron species and hybrids (many cultivars)
- Camellia species
- Acer palmatum and cultivars
- Hydrangea macrophylla
- Cornus florida
- Corylus avellana
- Young thin-barked trees generally
Origin habitat light regime (sun vs shade), leaf cuticle thickness and waxes, presence of indumentum or leaf hair, canopy density that provides internal shading, bark thickness and reflectivity, and acclimation state at the time of a sun event. Plants grown in shade nurseries and planted into full sun are at much higher risk than field-grown plants acclimated to direct sunlight.
Secondary Effects
Sunburned tissue is colonized by saprophytic fungi and can appear as a primary fungal disease on casual inspection.
Bark sunburn on young trees creates entry points for opportunistic cankers and wood decay fungi.
Repeated sunburn on the same plants indicates site mismatch and produces progressive canopy decline.