How Much Water Does a Cannabis Plant Need? A Simple Guide

How much water does a cannabis plant need?

Short answer: in soil, plan on roughly 5 to 10 percent of the container volume per watering, then wait until the top inch or two of medium goes dry. A plant in a 5-gallon pot drinks about 1 to 2 quarts (1 to 2 liters) every 2 to 3 days by mid-flower. A seedling in a solo cup needs a few tablespoons. The spread is wide on purpose. Pot size, plant size, light, and room humidity move the number more than any rule of thumb.

cannabis watering schedule for hydroponics

Ballpark amounts by stage

• Seedling, weeks 1 to 2: 2 to 4 ounces (60 to 120 mL) every 2 to 3 days. Pour in a small ring around the stem so roots chase the moisture outward.
• Early veg, 1-gallon pot: 1 to 2 cups (250 to 500 mL) every 2 to 3 days.
• Late veg, 3 to 5-gallon pot: 1 to 2 quarts (1 to 2 liters) every 2 to 3 days.
• Flowering, 5 to 10-gallon pot: 2 to 4 quarts (2 to 4 liters) every 2 to 3 days. In coco, or under a strong light with low humidity, that can turn into once a day.

Use those as a starting dose, not a calendar. Water, then watch how long the pot takes to dry. If it dries in a day, give more next time. If it is still heavy after four days, you gave too much or the room is too cool and humid.

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What moves the number up or down

• Container size. A big pot holds more water per fill but dries slower. Small pots dry fast and need more frequent, smaller pours.
• Stage of growth. Water use climbs through veg and peaks in mid-flower when buds are packing on weight.
• Temperature and VPD. Warm, dry air pulls water out of the leaves fast. The same plant in a cool, humid room can sip half as much.
• Medium. Coco coir and rockwool drain fast and often want daily feeding. Peat mixes and living soil hold water longer.
• Light intensity. More light means more photosynthesis, more transpiration, more water.
• Root health. A root-bound or damaged root ball drinks less than it looks like it should.

How to tell if you got it right

Lift the pot. That is the whole trick, and it beats any schedule. A freshly watered 5-gallon pot feels like a bag of wet sand. Three days later it should feel light, almost hollow. Water when it feels light, not when the top looks dry.

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A finger or a cheap moisture meter pushed 2 inches down tells you the same thing. If the medium still clumps on your finger, wait a day.

Signs of overwatering

• Leaves droop and stay drooped after the soil dries
• Lower fan leaves yellow and soften
• Fungus gnats hanging around the top of the pot
• Slow growth with a heavy pot and no dry-down

Signs of underwatering

• Leaves curl up at the edges and feel crisp
• Medium pulls away from the sides of the pot
• The pot is feather light and the plant wilts by mid-day
• New growth is small and slow

Water quality and pH, briefly

In soil, aim for 6.0 to 7.0 pH. In coco, 5.5 to 6.5. If you water with hard tap water, pH drift shows up as spotting and nutrient lockout within a couple weeks. Letting tap water sit out overnight does nothing for chloramine, which most US cities use, so do not count on it. Straight tap water is fine in most cases.

Runoff matters in coco and hydro: pour until 10 to 20 percent comes out the bottom so salts do not build up. In a healthy living soil, you can skip runoff most of the time.