Sunburn on san pedro cactus
What's Happening
Columnar cacti like San Pedro develop shade-adapted chloroplasts when grown indoors behind window glass, which filters out 50%+ of UV radiation and reduces light intensity to 100-500 µmol/m²/s. When abruptly moved to unfiltered outdoor sunlight exceeding 1500 µmol/m²/s, the photosynthetic apparatus cannot process the energy surge. This causes photooxidative damage where reactive oxygen species destroy epidermal and chlorenchyma cells, creating yellow, bleached, or brown necrotic patches within 24-72 hours of exposure.
How to Fix It
- 1
Immediately relocate to bright indirect light or dappled shade to halt further damage—do not gradually reduce, move completely
- 2
Withhold water for 2-3 weeks to prevent secondary rot in compromised tissue; resume only when soil is bone dry
- 3
Expect permanent scarring: damaged tissue will not heal but will become less noticeable as new growth emerges
- 4
Monitor for secondary fungal infection in scorched areas; treat with copper fungicide if orange spores appear
- 5
Resume gradual outdoor acclimation only after 4-6 weeks recovery, starting from indirect light only
What You'll Need
How to Prevent It
Acclimate gradually over 14-21 days: begin with 30 minutes of morning sun (7-9am), increasing by 30 minutes every 2-3 days. Use 50-70% shade cloth during peak UV hours (10am-4pm) in the first week. Maintain this schedule even for 'full sun' cactus species—the issue is transition speed, not ultimate light tolerance.