How much water does a cannabis plant use per day?

Q: How much water does a cannabis plant use per day?

A cannabis plant uses roughly 50–250 mL of water a day as a seedling, 0.5–1.5 L in veg and about 1.5–2 L a day as a large flowering plant indoors. Plan the room at up to about 10 L per m² of flowering canopy per day (1 L per ft²) under strong light and heat. Nearly all of it is transpired, so light, temperature and VPD drive use.

Short answer

An indoor cannabis plant uses roughly 50–250 mL a day as a seedling, 0.5–1.5 L in veg and about 1.5–2 L a day as a large flowering plant. For planning, allow up to about 10 L per m² of flowering canopy per day (1 L per ft²) under strong light and heat ^[1]. Grower practice: cannabis water use hasn't been measured in many controlled trials, and plants vary widely.

Why

Almost all the water a plant takes up leaves through its leaves as transpiration; one estimate puts it at about 95% ^[1]. Very little stays in the plant.

Transpiration follows the climate. More light, higher temperature, lower humidity (higher VPD) and more airflow all raise it. A cooler, dimmer, humid room can use half the water of a hot, bright one.

Leaf area, and with it water use, climbs fast from mid veg to mid flower.

Water use is also a health signal. Roots that are rotting or short of oxygen take up less. A sudden fall in daily use with no climate change is worth investigating.

Deliberately giving less is a separate tool: a controlled, mild drought in late flower raised dry weight and cannabinoids in one study ^[2], but that is steering, not rationing.

The numbers

Stage	Typical water use per plant per day
Seedling	50–250 mL (given every 1–2 days)
Early veg, 1–4 L pot	0.2–0.5 L
Late veg, 11–19 L pot	0.5–1.5 L
Large plant, mid flower	1.5–2 L
Room planning maximum	Up to 10 L/m²/day canopy (1 L/ft²) ^[1]

Add 10–30% on top for runoff in coco and rockwool.

What to do

Weigh the pot after a full watering and again when it's due; the difference is what the plant used.
Log it for a week. You'll see use climb as the plant grows.
Size your reservoir or watering jug for the planning maximum, not the average.
Water by weight, not by a fixed daily amount ^[3].
If daily use drops suddenly, check the root zone and climate before anything else.

Common mistakes

Giving the same volume every day from seedling to harvest.
Assuming a thirsty-looking plant needs water without lifting the pot.
Ignoring a sudden fall in water use.

Full article: Drooping: overwatering vs underwatering
Answers: how often to water · water at lights-on or lights-off
Terms: transpiration · dryback · VPD

Follow-up questions

How much water does a grow tent use per day?

A grow tent in full flower uses up to about 10 litres of water per square metre of canopy per day, or about 1 litre per square foot, as a planning maximum. A 1.2 × 1.2 m (4 × 4 ft) tent can therefore need up to about 14–16 litres a day under strong light and heat, while cooler, dimmer rooms use far less. Add 10–30% for runoff.

Why does my plant drink more some days?

A cannabis plant drinks more on days with more light, higher temperature, lower humidity and stronger airflow, because almost all the water it takes up is transpired through the leaves. Water use also rises steeply as the plant grows from mid veg to mid flower. A sudden drop in water use with no change in climate is a warning sign: check the roots.

Should I give the same amount every day?

No. Give water by pot weight, not a fixed daily volume. Daily use changes with plant size, light and climate, so a fixed amount overwaters small plants and starves big ones. Weigh the pot after a full watering and again when dry, water at roughly the halfway point, and give enough for a little runoff in soil or 10–20% in coco.

Sources

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

Caplan D, Dixon M, Zheng Y, 2019. Increasing inflorescence dry weight and cannabinoid content in medical cannabis using controlled drought stress. HortScience 54(5):964–969. doi.org/10.21273/HORTSCI13510-18

Raviv M, Lieth JH, Bar-Tal A (eds), 2019. Soilless Culture: Theory and Practice, 2nd ed. Elsevier (container media physics and air-filled porosity). doi.org/10.1016/C2015-0-01807-5