# Foxtails: new calyx spires growing out of mature buds: likely causes and fixes

> Foxtails: new calyx spires growing out of mature buds on cannabis is most often Heat or light stress foxtailing on the top colas, Heat stress or Cultivar foxtailing (genetic). How to tell them apart, the fix for each and what recovery looks like.

Also called: foxtailing, buds growing towers, fox tails, spiky buds, new calyxes on top of buds.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=buds&s=foxtails

## Likely causes, most likely first

### 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)

### Heat stress (Possible)

- Kind: Environment; urgency: Act within a day or two
- Fix: Bring canopy leaf temperature under about 30 °C (86 °F): more exhaust (one tent air exchange every 1-3 minutes), raise or dim the light, add canopy airflow (0.3-1.0 m/s), keep RH so VPD stays 1.0-1.5 kPa, water well. Above ~30 °C photosynthesis falls unless CO2 and light are high.
- Recovery: Leaves flatten within a day; no new foxtails or crisping.
- 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: Check temperature and RH at canopy height with a calibrated sensor: Put a calibrated sensor (salt test: a sealed bag with damp table salt reads 75% RH after 8-12 hours) at canopy height, out of direct light, and log for 24 hours including lights-off. Look for lights-off RH spikes and dew point.
- Article: [Heat and light stress: taco leaves, bleaching and foxtails](https://genetixbay.com/knowledge/environment/heat-light-stress.md)

### Cultivar foxtailing (genetic) (Less likely)

- Kind: Genetics; urgency: Low urgency
- Fix: Nothing to fix if it appears evenly through the canopy, including shaded buds, in a cultivar known for it. Record it on the cultivar's GB-ID journal.
- Recovery: Same pattern in siblings or the same cut in other grows.
- 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.
- 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.
- Article: [Foxtailing buds on cannabis: genetics or stress?](https://genetixbay.com/knowledge/diagnosis/foxtails.md)

### Light bleaching at the canopy top (Less likely)

- 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)

### Light leak or interrupted dark period (Less likely)

- Kind: Light; urgency: Act within a day or two
- Fix: Seal the dark period: pinholes, zips, vents and equipment LEDs (tape them), and check the timer. Even dim night light can delay flowering in sensitive cultivars and reveal herm-prone genetics. Use a green headlamp only for brief checks.
- Recovery: Flowering resumes within 1-2 weeks; no new single-blade leaves.
- Confirm: Check for light leaks in the dark period: Sit inside the tent or room for 5 minutes after lights-off until your eyes adjust. Look for pinholes, zips, vents, indicator LEDs on equipment and light under doors. Also confirm the timer actually switches off and stays off.
- Article: [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.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.