# Top leaves or bracts bleached white or yellow nearest the light: likely causes and fixes

> Top leaves or bracts bleached white or yellow nearest the light on cannabis is most often Light bleaching at the canopy top, Heat or light stress foxtailing on the top colas or Light burn: too much light for the plant's climate and CO2. How to tell them apart, the fix for each and what recovery look

Also called: light bleaching, bleached tops, white tops, top colas turning white, sun bleached leaves.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=new&s=bleached-top-leaves

## Likely causes, most likely first

### Light bleaching at the canopy top (Possible)

- Kind: Light; urgency: Deal with it this week
- 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.
- Confirm: 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.
- Confirm: 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.
- Article: [Heat and light stress: taco leaves, bleaching and foxtails](https://genetixbay.com/knowledge/environment/heat-light-stress.md)

### Heat or light stress foxtailing on the top colas (Possible)

- Kind: Light; urgency: Low urgency
- 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.
- Confirm: 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.
- Confirm: 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.
- Confirm: 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.
- Article: [Foxtailing buds on cannabis: genetics or stress?](https://genetixbay.com/knowledge/diagnosis/foxtails.md)

### Light burn: too much light for the plant's climate and CO2 (Possible)

- Kind: Light; urgency: Deal with it this week
- 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.
- Confirm: 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.
- Confirm: 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.
- Article: [Heat and light stress: taco leaves, bleaching and foxtails](https://genetixbay.com/knowledge/environment/heat-light-stress.md)

## Note

Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.