How often should I water my cannabis plants?

Q: How often should I water my cannabis plants?

Water by weight, not by calendar. In soil, water when the pot feels about 40–50% lighter than just after watering, usually every 2–4 days for established plants. Coco and rockwool want watering once to several times a day with 10–30% runoff. Seedlings in big pots need small amounts around the stem. The pot's weight and the plant's size set the schedule, not the day of the week.

Short answer

Water cannabis by pot weight, not by the calendar. In soil, water when the pot is about 40–50% lighter than just after watering, usually every 2–4 days for an established plant. In coco and rockwool, water once to several times a day with 10–30% runoff. Grower practice, backed by basic root physiology.

Why

Roots need air as much as water. A saturated pot fills the pore spaces with water, roots run short of oxygen and the plant droops even though it is wet. Constantly wet media also favour Pythium root rot ^[1] and fungus gnat larvae ^[2]. Letting the pot dry back between waterings pulls fresh air into the root zone. A mild, controlled dry-down in late flower has even raised yield and cannabinoid content in one trial ^[3], but that is deliberate steering, not neglect.

The numbers

Medium	When to water	How much
Soil, established plant	Pot 40–50% lighter, top 3–5 cm dry	To light runoff, ~10–20% of pot volume
Coco coir	1–6 times a day as the plant grows	10–30% runoff every time
Rockwool	Several times a day in flower	10–30% runoff, steer by dryback
Seedling in a large pot	Every 1–3 days	A small circle around the stem only

What to do

Weigh or lift the pot right after a full watering, and again when it is dry. Learn both.
Water when the pot is roughly halfway between the two.
Water slowly, in two passes, until a little runs out of the bottom.
Empty the saucer so the pot doesn't sit in runoff.
As the plant grows and drinks faster, shorten the interval rather than drenching harder.

Common mistakes

Watering on a fixed schedule ("every Tuesday") without lifting the pot.
Putting a seedling in a huge pot and soaking the whole thing.
Daily small sips in coco with no runoff, which builds salts.
Treating every droop as thirst: an overwatered plant droops too.

Full article: Drooping: overwatering vs underwatering
Media: growing in soil · growing in coco
Terms: field capacity · dryback · watering to runoff

Follow-up questions

Should I water every day?

Water every day only in coco, rockwool or small containers where the plant drinks the pot dry within a day. In soil, daily drenching keeps roots short of oxygen and is the most common cause of drooping, slow growth and fungus gnats. Lift the pot first: if it is still heavy, wait.

How much water should I give per watering?

In soil, give enough to wet the whole pot with a little runoff, roughly 10–20% of the pot's volume for an established plant. In coco and rockwool, water to 10–30% runoff each time so salts don't build up. Seedlings need far less: wet a small circle around the stem.

How do I know when my plant needs water?

Your plant needs water when the pot is clearly lighter than just after watering, the top few centimetres are dry and leaves are just starting to soften. Learn the weight of a freshly watered pot and of a dry one, then water at roughly the halfway point. A moisture probe 5 cm down helps in big pots.

Sources

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

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

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