# Wilting under the lights that recovers at lights-off: likely causes and fixes

> Wilting under the lights that recovers at lights-off on cannabis is most often Heat stress, Air too dry / VPD too high or Low dissolved oxygen in the root zone. How to tell them apart, the fix for each and what recovery looks like.

Also called: droops in the afternoon, wilts midday, fine in the morning droopy at noon, midday droop.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=whole&s=midday-wilt

## 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)

### Air too dry / VPD too high (Less likely)

- Kind: Environment; urgency: Deal with it this week
- Fix: Raise RH or lower temperature to bring leaf VPD to 0.4-0.8 kPa (clones, seedlings), 0.8-1.2 (veg), 1.0-1.5 (flower). Humidifier, less exhaust when cool, fewer dry hot air inputs. Dry air also speeds spider mites.
- Recovery: Leaves flatten within a day.
- 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: 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: [VPD explained: vapour pressure deficit for cannabis growers](https://genetixbay.com/knowledge/environment/vpd-explained.md)

### Low dissolved oxygen in the root zone (Less likely)

- Kind: Root zone; urgency: Act within a day or two
- Fix: Restore oxygen: clean or replace air stones, size the air pump for at least ~1 L/min per 4 L, keep a 2-5 cm air gap under net pots once roots reach the water, cool the solution to 18-22 °C (64-72 °F). Aim for 6-8+ mg/L DO.
- Recovery: Fresh white root tips within 3-5 days; wilting stops.
- Confirm: Dissolved oxygen check: Measure DO with a meter in the root zone, or check the system: air pump running, stones bubbling evenly, return water splashing, no dead spots. Target 6-8+ mg/L. Without a meter: if stones are clogged or the pump is undersized for the volume, assume low DO.
- Confirm: Measure reservoir and root-zone temperature at the end of the light period: Measure the solution in the reservoir and in the root chamber or bucket, at the warmest time of day. Target 18-22 °C (64-72 °F). Above about 23-24 °C (73-75 °F), dissolved oxygen falls and Pythium risk rises; below 16 °C (61 °F) uptake slows.
- Confirm: Look at the roots: Hydro: lift the net pot and look at the root mass in good white light. Containers: tip the plant out with a hand over the medium and look at the edges and bottom of the root ball. Healthy roots are white to cream with fine fuzzy tips. Brown roots whose outer layer slides off between the fingers leave a bare thread: rot. Firm, evenly tan roots with white tips in coloured solution are staining.
- Confirm: Check pumps, timers, nozzles and drippers: Watch one full cycle: the pump starts on time, every nozzle mists or every dripper delivers, NFT channels have flow end to end, air stones bubble. Clear clogged emitters and filters. In HPA, a failed pump or blocked nozzle can kill exposed roots fast, so check this first when plants on one system wilt together.
- Article: [Deep water culture (DWC)](https://genetixbay.com/knowledge/systems/dwc.md)

### Reservoir or root zone too warm (Less likely)

- Kind: Root zone; urgency: Act within a day or two
- Fix: Keep solution at 18-22 °C (64-72 °F): insulate and shade the reservoir, move it off the warm floor or away from the light, use a chiller or frozen water bottles in an emergency, avoid submersible pumps that add heat, cool the room.
- Recovery: Solution holds below 22 °C through the light period; new white roots in 3-5 days.
- Confirm: Measure reservoir and root-zone temperature at the end of the light period: Measure the solution in the reservoir and in the root chamber or bucket, at the warmest time of day. Target 18-22 °C (64-72 °F). Above about 23-24 °C (73-75 °F), dissolved oxygen falls and Pythium risk rises; below 16 °C (61 °F) uptake slows.
- Confirm: Dissolved oxygen check: Measure DO with a meter in the root zone, or check the system: air pump running, stones bubbling evenly, return water splashing, no dead spots. Target 6-8+ mg/L. Without a meter: if stones are clogged or the pump is undersized for the volume, assume low DO.
- Article: [Brown roots in hydroponics (DWC, RDWC, NFT): a differential](https://genetixbay.com/knowledge/diagnosis/root-browning-hydro.md)

### Root-bound in a pot too small (Less likely)

- Kind: Root zone; urgency: Deal with it this week
- Fix: Up-pot photoperiods into a container about 2-3x larger, loosening circling roots gently. Autoflowers: don't transplant; increase watering and feeding frequency instead. Small pots can still grow big plants with high-frequency fertigation (several feeds a day).
- Recovery: New white roots in the fresh medium within 5-7 days; daily wilting stops.
- Confirm: Look at the roots: Hydro: lift the net pot and look at the root mass in good white light. Containers: tip the plant out with a hand over the medium and look at the edges and bottom of the root ball. Healthy roots are white to cream with fine fuzzy tips. Brown roots whose outer layer slides off between the fingers leave a bare thread: rot. Firm, evenly tan roots with white tips in coloured solution are staining.
- Confirm: Lift the pot and check moisture at depth: Lift the pot right after a full watering to learn its wet weight, then lift before each watering. Water when it is clearly lighter (about half the added water gone). Push a finger or a moisture probe 5 cm (2 in) down. Heavy for 3-4+ days with droop: overwatering. Very light with limp papery leaves: thirst.
- Article: [Cannabis root problems: the master decision tree](https://genetixbay.com/knowledge/diagnosis/root-problems-decision-tree.md)

### Underwatering (Less likely)

- Kind: Root zone; urgency: Act within a day or two
- Fix: Water slowly in 2-3 passes until 10-20% runoff, so the whole root ball wets. Water when the pot is clearly lighter, before the plant droops. Mild late-flower dry-back is a house-endorsed practice for some cultivars (Caplan 2019), but wilting is not.
- Recovery: Turgor returns within 1-6 hours of watering.
- Confirm: Lift the pot and check moisture at depth: Lift the pot right after a full watering to learn its wet weight, then lift before each watering. Water when it is clearly lighter (about half the added water gone). Push a finger or a moisture probe 5 cm (2 in) down. Heavy for 3-4+ days with droop: overwatering. Very light with limp papery leaves: thirst.
- Article: [Drooping cannabis: overwatering vs underwatering](https://genetixbay.com/knowledge/diagnosis/drooping-overwatering-vs-underwatering.md)

### Salt build-up in the root zone (Less likely)

- Kind: Root zone; urgency: Deal with it this week
- Fix: Root-zone flush: water slowly with pH-adjusted, quarter- to half-strength feed (not plain water in coco) to 20-30% runoff, about 2-3x the pot volume, until runoff EC is within ~0.3-0.5 mS/cm of input. Then water to 10-20% runoff each time. This treats the medium, not the plant; it is not a pre-harvest flush, which is a myth.
- Recovery: Runoff EC tracks input at the next 2-3 waterings; new leaves stop burning.
- Confirm: Measure runoff EC against input EC: Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.
- Confirm: Pour-through extraction for container pH and EC: Water normally, wait 30 minutes to 2 hours, pour enough pH-adjusted water on top to collect about 50 mL of leachate (roughly 75 mL for a 15 cm pot, 150 mL for a 4 L pot, 350 mL for 11 L), and measure pH and EC of that 50 mL. Less disruptive than full runoff and more repeatable.
- Confirm: Slurry (1:2) test of the medium: Take a sample from the root zone (not the top 2 cm), mix 1 part medium with 2 parts distilled water by volume, stir, wait 30 minutes and measure pH and EC of the liquid. Use it for soil and living soil where runoff is unreliable. Slurry EC reads lower than pour-through EC; compare like with like.
- Article: [Runoff testing: how to measure root-zone pH and EC and what the numbers mean](https://genetixbay.com/knowledge/ph-ec/runoff-testing.md)

### Dry, water-repellent medium (water runs straight through) (Less likely)

- Kind: Root zone; urgency: Deal with it this week
- Fix: Rewet slowly: small amounts every 10-15 minutes, or bottom-soak the pot for 20-30 minutes; a drop of plant-safe wetting agent helps peat. Don't let peat mixes go bone dry again.
- Recovery: Pot gains its normal wet weight and holds it for a day or more.
- Confirm: Lift the pot and check moisture at depth: Lift the pot right after a full watering to learn its wet weight, then lift before each watering. Water when it is clearly lighter (about half the added water gone). Push a finger or a moisture probe 5 cm (2 in) down. Heavy for 3-4+ days with droop: overwatering. Very light with limp papery leaves: thirst.
- Article: [Why does water run straight through or pool on top of my pot?](https://genetixbay.com/knowledge/answers/water-runs-through-or-pools.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.