# Leaf edges rolling under, leaves curling downward without darkening: likely causes and fixes

> Leaf edges rolling under, leaves curling downward without darkening on cannabis is most often Heat stress, Calcium deficiency (often a delivery problem) or CO2 set far too high. How to tell them apart, the fix for each and what recovery looks like.

Also called: edges curling down, leaves rolling under, leaf margins curl under.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=old&s=margins-roll-under

## Likely causes, most likely first

### 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)

### Calcium deficiency (often a delivery problem) (Less likely)

- Kind: Nutrient; urgency: Deal with it this week
- Fix: Rule out mites with a 30-60x lens first. Then fix delivery: leaf VPD 0.8-1.5 kPa (lights-on RH not stuck high), airflow under the canopy, root-zone pH not below ~5.8 in coco. If supply is short (RO water, unbuffered coco), bring Ca to about 150-200 mg/L with calcium nitrate or a Cal-Mag at label rate. LEDs do not create a Cal-Mag need; that is a myth.
- Recovery: Leaves that open 7-10 days after the fix are flat and spot-free; old spots stay.
- Confirm: Magnifier check of leaf undersides and new growth: Use a 10-20x loupe for spider mites, thrips, aphids and whiteflies, and a 30-60x lens or clip-on phone macro for broad and russet mites. Check the undersides of 10 affected leaves across several plants and the newest shoot tips, in white light. Look for moving dots, eggs, shed skins, webbing, slender insects with black specks, and glassy specks on new growth.
- Confirm: Measure input pH and root-zone (runoff or reservoir) pH: Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
- Confirm: Get a water report (or test your water): Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.
- 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.
- Confirm: Watch new growth for 5-10 days after one change: Make one correction at a time, mark the top node with a twist tie and photograph the same leaves every 2-3 days. Damaged leaves never recover; judge only new growth and whether the symptom stops spreading. Mobile-nutrient symptoms stop climbing within 5-7 days; immobile ones show in new leaves within 7-10 days.
- Article: [Calcium (Ca) in cannabis: deficiency, transport, targets and sources](https://genetixbay.com/knowledge/nutrients/calcium.md)

### CO2 set far too high (Less likely)

- Kind: Environment; urgency: Deal with it this week
- Fix: Hold 800-1,200 ppm; above that there is little further gain in cannabis. Leaf rolling and chlorosis at very high CO2 are documented in tomato and other greenhouse crops. People: keep occupied rooms under the 5,000 ppm 8-hour limit and never enter a room above 30,000 ppm.
- Recovery: Symptoms stop on new growth after CO2 is lowered.
- Confirm: Measure CO2 during the light period: Read a CO2 monitor at canopy height through the light period. A sealed room without enrichment can fall below 300-350 ppm and stall growth; enriched rooms should hold 800-1,200 ppm. Keep occupied spaces below the 5,000 ppm workplace limit.
- Article: [CO₂ enrichment: when it pays, how much, and how to stay safe](https://genetixbay.com/knowledge/environment/co2-enrichment.md)

### Copper deficiency (Less likely)

- Kind: Nutrient; urgency: Low urgency
- Fix: Very rare with a complete micro mix. Supply about 0.03-0.1 mg/L Cu; do not overshoot (excess Cu damages roots).
- Recovery: New leaves regain size and turgor in 10-14 days.
- Confirm: Magnifier check of leaf undersides and new growth: Use a 10-20x loupe for spider mites, thrips, aphids and whiteflies, and a 30-60x lens or clip-on phone macro for broad and russet mites. Check the undersides of 10 affected leaves across several plants and the newest shoot tips, in white light. Look for moving dots, eggs, shed skins, webbing, slender insects with black specks, and glassy specks on new growth.
- Confirm: Leaf tissue analysis: Send 20-30 most-recently-matured leaves (no petioles) from affected plants, and separately from healthy ones, to an agricultural lab. Compare with cannabis and hemp sufficiency ranges (for example N 3.3-4.8%, Mg 0.40-0.81%, K 1.8-2.9% dry weight in recent mature leaves). Results take 1-2 weeks; use it to confirm a chronic problem, not an emergency.
- Article: [Copper (Cu) in cannabis: limp, distorted new growth and root-burning excess](https://genetixbay.com/knowledge/nutrients/copper.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.