Top leaves or bracts bleached white or yellow nearest the light

Likely causes, most likely first

Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.

1. PossibleLight bleaching at the canopy top

   LightDeal with it this weekRead: Heat and light stress: taco leaves, bleaching and foxtails

   Fix: Raise or dim the light by 10-20% so top-canopy PPFD sits around 800-1,100 µmol/m²/s at ambient CO2 (up to ~1,500 with CO2 and good climate). Keep the canopy even so one cola doesn't sit in a hotspot. Bleached tissue won't re-green.

   Recovery: No new bleaching within 3-5 days; new bracts and leaves at the top stay green.

   How to confirm it

   • Measure PPFD at the canopy top. Use a quantum (PAR) meter, or a calibrated phone app as a rough guide, at the top leaves in a 5-9 point grid. Compare with the stage target: seedlings 100-300, veg 300-700, flower 800-1,200 µmol/m²/s at ambient CO2. Bleaching and taco at the top above ~1,100 point to light; stretch below ~300 in veg points to too little.
   • IR leaf temperature at the canopy top. Point an IR thermometer at the top leaves under the light (5-10 cm away, emissivity 0.95) and at shaded leaves. Leaf temperature above about 30 °C (86 °F) at the top only points to heat or radiant load; leaf VPD can be calculated from leaf temperature and RH.

2. PossibleHeat or light stress foxtailing on the top colas

   LightLow urgencyRead: Foxtailing buds on cannabis: genetics or stress?

   Fix: Lower top-cola PPFD and leaf temperature (leaf under ~28-30 °C / 82-86 °F); even out the canopy. Evidence is mostly grower observation.

   Recovery: No new spires on top colas over the next week.

   How to confirm it

   • Measure PPFD at the canopy top. Use a quantum (PAR) meter, or a calibrated phone app as a rough guide, at the top leaves in a 5-9 point grid. Compare with the stage target: seedlings 100-300, veg 300-700, flower 800-1,200 µmol/m²/s at ambient CO2. Bleaching and taco at the top above ~1,100 point to light; stretch below ~300 in veg points to too little.
   • IR leaf temperature at the canopy top. Point an IR thermometer at the top leaves under the light (5-10 cm away, emissivity 0.95) and at shaded leaves. Leaf temperature above about 30 °C (86 °F) at the top only points to heat or radiant load; leaf VPD can be calculated from leaf temperature and RH.
   • Compare with siblings or the same cut. Put the affected plant beside siblings or clones of the same cut in the same conditions. A trait shared by all of a cultivar (purple stems, foxtailing, pale colour) is genetic; a problem on one plant points to its roots, a virus or its genetics; a problem on every cultivar points to the environment or feed.

3. PossibleLight burn: too much light for the plant's climate and CO2

   LightDeal with it this weekRead: Heat and light stress: taco leaves, bleaching and foxtails

   Fix: Dim or raise the light 10-20%; ramp intensity over 7-10 days after a light change. Seedlings and clones need only 100-300 µmol/m²/s. High PPFD works only with leaf temperature, VPD, roots and feed all in range.

   Recovery: Top leaves flatten within 1-3 days.

   How to confirm it

   • Measure PPFD at the canopy top. Use a quantum (PAR) meter, or a calibrated phone app as a rough guide, at the top leaves in a 5-9 point grid. Compare with the stage target: seedlings 100-300, veg 300-700, flower 800-1,200 µmol/m²/s at ambient CO2. Bleaching and taco at the top above ~1,100 point to light; stretch below ~300 in veg points to too little.
   • IR leaf temperature at the canopy top. Point an IR thermometer at the top leaves under the light (5-10 cm away, emissivity 0.95) and at shaded leaves. Leaf temperature above about 30 °C (86 °F) at the top only points to heat or radiant load; leaf VPD can be calculated from leaf temperature and RH.

Look for

White or pale-yellow top leaves and bud tips directly under the fixture, sharp cut-off where a leaf shades another..