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

Q: 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.

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 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 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.

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 larvae ^[2] and Pythium root rot ^[3]. Night-time humidity adds bud-rot risk in flower.

Drip-fed coco 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, 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

Check pots at the same time every day, just after lights-on.
Water the ones that are light; leave the heavy ones.
Bring water to room temperature, 18–22 °C (64–72 °F).
If you only get home in the evening, shift the light schedule so lights-on matches when you're there.
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 overnight.
Mistaking normal night-time leaf droop for thirst.

Full article: Drooping: overwatering vs underwatering
Answers: how often to water · how much water per day · leaves droop at night
Systems: crop steering
Terms: dryback

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

Douglas R, 2022. Ways to calculate water use and transpiration rates for indoor cannabis cultivation. Greenhouse Grower. greenhousegrower.com/?p=157955

UConn IPM. Managing Fungus Gnats in the Greenhouse. ipm.cahnr.uconn.edu/managing-fungus-gnats-in-the-greenhouse/

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. doi.org/10.1080/07060661.2018.1535466

Greenhouse Grower. The details behind crop steering in cannabis production. greenhousegrower.com/production/the-details-behind-crop-stee