# Should I water when the lights come on or before they go off?

> Water cannabis soon after the lights come on, ideally within the first 1–3 hours. Plants transpire mostly in the light, so a morning watering is used during the day and the pot dries back overnight. Watering just before lights-off leaves a wet, cool pot in the dark, which slows dryback, raises humidity and favours fungus gnats and root rot. Drip-fed coco stops irrigating 2–6 hours before lights-off.

- Canonical: https://genetixbay.com/knowledge/answers/water-at-lights-on-or-off.html
- Topic: Systems · Type: How-to
- Evidence: Grower practice · Status: Draft: awaiting expert review
- Full article: [Drooping cannabis: overwatering vs underwatering](https://genetixbay.com/knowledge/diagnosis/drooping-overwatering-vs-underwatering.md)
- Also asked as: What is the best time to water cannabis? | Can I water at night? | Is it bad to water at lights-off? | Water in the morning or evening indoors?
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Short answer

Water soon after the lights come on, ideally in the first 1–3 hours. The plant uses most of that water while the lights are on, and the pot dries back overnight. Watering right before lights-off leaves a wet, cool pot in the dark. *Grower practice*, based on how [transpiration](https://genetixbay.com/knowledge/glossary/transpiration.md) works.

## Why

Plants move water mainly by transpiration, and nearly all the water they take up leaves through the leaves [1]. Transpiration runs with light, when [stomata](https://genetixbay.com/knowledge/glossary/stomata.md) are open, and almost stops in the dark.

A morning watering matches supply to demand. The plant draws the pot down through the day, and by lights-off the mix is already drying and pulling air back into the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md).

An evening watering does the opposite. The pot sits saturated through a cooler night when the plant barely drinks, and the extra evaporation raises tent humidity in the dark period.

Constantly wet media favour [fungus gnat](https://genetixbay.com/knowledge/glossary/fungus-gnat.md) larvae [2] and [Pythium root rot](https://genetixbay.com/knowledge/glossary/pythium.md) [3]. Night-time humidity adds bud-rot risk in flower.

Drip-fed [coco](https://genetixbay.com/knowledge/glossary/coco-coir.md) follows the same logic with more precision: growers stop irrigating hours before lights-off so the substrate dries back overnight [4].

## The numbers

| System | When to water | Notes |
|---|---|---|
| Soil, hand-watered | First 1–3 h after lights-on | Lift the pot first |
| Coco, hand-watered | First 1–3 h; second watering by midday if needed | Never let it dry fully |
| Coco or [rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md), drip (vegetative steer) | First shot 0.5–1.5 h after lights-on; last 2–3 h before lights-off | Grower practice [4] |
| Coco or rockwool, drip (generative steer) | First shot 2–3 h after lights-on; last 4–6 h before lights-off | Grower practice [4] |
| Emergency wilt | Any time | Small amount, then normal routine |

## What to do

1. Check pots at the same time every day, just after lights-on.
2. Water the ones that are light; leave the heavy ones.
3. Bring water to room temperature, 18–22 °C (64–72 °F).
4. If you only get home in the evening, shift the light schedule so lights-on matches when you're there.
5. On drip, set the last shot well before lights-off.

## Common mistakes

- Watering at lights-off because that's when you remember.
- Leaving pots in saucers of [runoff](https://genetixbay.com/knowledge/glossary/runoff.md) overnight.
- Mistaking normal night-time leaf droop for thirst.

## Related

- Full article: [Drooping: overwatering vs underwatering](https://genetixbay.com/knowledge/diagnosis/drooping-overwatering-vs-underwatering.md)
- Answers: [how often to water](https://genetixbay.com/knowledge/answers/how-often-to-water.md) · [how much water per day](https://genetixbay.com/knowledge/answers/how-much-water-per-day.md) · [leaves droop at night](https://genetixbay.com/knowledge/answers/leaves-droop-at-night.md)
- Systems: [crop steering](https://genetixbay.com/knowledge/systems/drain-to-waste-crop-steering.md)
- Terms: [dryback](https://genetixbay.com/knowledge/glossary/dryback.md)

## Follow-up questions

**Is it bad to water just before lights-off?** Watering just before lights-off isn't an emergency, but it is the worst routine time. Plants close their stomata in the dark and use little water, so the pot stays saturated through the cool night period, roots get less air and humidity in the tent rises. Over many weeks that favours fungus gnats, root rot and bud rot. Move the watering to the first hours after lights-on.

**Can I water during the dark period if my plant is wilting?** Yes. If a plant is badly wilted from a dry pot, water it now, whatever the light schedule, using a small amount slowly and topping up at lights-on. Use a dim green light or open the tent briefly; a short look won't harm flowering. Then move your routine so the pot is checked and watered in the first hours of the light period.

**When should the last irrigation be in coco?** In drip-fed coco or rockwool, the last irrigation of the day usually comes 2–3 hours before lights-off for vegetative steering and 4–6 hours before for generative steering, so the substrate dries back overnight. These timings are grower practice adapted from greenhouse crop steering. Hand-watered coco should simply be watered early in the light period.

## Sources

1. Douglas R, 2022. Ways to calculate water use and transpiration rates for indoor cannabis cultivation. Greenhouse Grower. https://www.greenhousegrower.com/?p=157955
2. UConn IPM. Managing Fungus Gnats in the Greenhouse. https://ipm.cahnr.uconn.edu/managing-fungus-gnats-in-the-greenhouse/
3. Punja ZK, Rodriguez G, 2018. Fusarium and Pythium species infecting roots of hydroponically grown marijuana (Cannabis sativa L.) plants. Canadian Journal of Plant Pathology 40(4):498–513. https://doi.org/10.1080/07060661.2018.1535466
4. Greenhouse Grower. The details behind crop steering in cannabis production. https://www.greenhousegrower.com/production/the-details-behind-crop-steering-in-cannabis-production/