Cannabis Watering Schedule: The Short Answer
Water a cannabis plant in soil when the top 2 to 3 cm (about 1 inch) of the medium feels dry. For a plant in veg in a 5-gallon (19 L) fabric pot, that cycle runs 2 to 3 days. In late flower the same plant can drain the pot in 24 to 36 hours. In coco coir, water once per day in veg and once or twice per day in flower. Target pH is 6.0 to 7.0 in soil and 5.5 to 6.5 in coco and hydro. Water temperature: 18 to 22 C (65 to 72 F).
How to Tell When the Plant Is Thirsty
Three tests, in order of reliability.
- Pot weight. Lift the pot before and after a watering. The difference is the water you added. A 5-gallon pot holds 6 to 8 L (1.5 to 2 gal) at capacity, which is 6 to 8 kg (13 to 18 lb) of water weight. Water again when the pot feels light.
- Finger or moisture probe. Push a finger 2 to 3 cm into the medium. Dry at the tip and damp below: water now. Wet at the tip: wait a day.
- Leaf angle. Leaves that sag at the petiole and recover a few hours after a watering were dry. Leaves that stay limp after watering point to root problems, not thirst.
Frequency by Growth Stage
Seedling, week 1 to 2
Roots are small. Water 30 to 100 mL around the stem every 2 to 3 days. Keep the medium damp, not soaked. A dome or 65 to 70% relative humidity slows evaporation.
Vegetative, week 3 to 8
A plant in a 5-gallon pot takes water every 2 to 4 days. A 1-gallon pot can need water every 24 to 48 hours. Root mass grows fast, so the interval shortens each week.
Flowering, week 1 to 9 and after
Water use peaks in weeks 4 to 7 of flower. Expect an interval of 2 days for a mid-size plant and once per day for a large plant in a small pot. Cut volume in the final week if you flush. Many growers water as normal until harvest.
How Much Water to Add
In soil with salt-based nutrients, pour until 10 to 20% of the volume runs out the bottom of the pot. This pushes old salts out of the root zone. In living soil or organic beds, water 5 to 10% of the pot volume with no runoff. A 5-gallon pot takes 1.5 to 2 L (0.4 to 0.5 gal) in one pass in veg, and 3 to 4 L in heavy flower.
Do not water in small sips all day in soil. Saturate the pot, then let it dry back. Roots need oxygen between waterings.
Water Quality and pH
Tap water at 100 to 300 ppm (EC 0.2 to 0.6) works for most plants. Chlorine at US municipal levels does not harm cannabis. Let cold tap water reach room temperature, or mix warm and cold.
Adjust pH after you add nutrients, not before. In soil, 6.2 to 6.8 covers the uptake range for nitrogen, phosphorus, potassium, calcium, and magnesium. In coco, 5.8 to 6.2.
Coco Coir and Hydro
Coco holds less water than peat or soil and holds it less tightly. Water once per day in veg and once or twice per day in flower, to 10 to 20% runoff each time. Coco also locks calcium and magnesium, so use a nutrient mix with a cal-mag supplement.
Signs of Overwatering
- Drooping leaves that do not perk up after a watering
- Dark green foliage with clawed tips
- Slow vertical growth in veg
- Medium that stays wet 3 or more days
- Algae, fungus gnats, or a sour smell at the pot base
Fix: stop watering, raise the pot on a rack for airflow, and wait. If the medium smells sour, the root zone may have rot. Root rot shows as brown, slimy roots and a plant that wilts between waterings while the pot stays wet.
Signs of Underwatering
- Leaves droop at the petiole and the tips point down
- Crisp, brown edges on lower leaves
- Pot feels light and sounds hollow when tapped
- Medium pulls away from the pot wall
Fix: water to runoff in one pass. A bone-dry pot of peat or coco can turn hydrophobic. Water in two passes 10 minutes apart so the medium reabsorbs.
What Changes the Schedule
Water use tracks temperature, humidity, airflow, and light. A room at 27 C (80 F) and 45% RH pulls water out of a pot faster than 22 C (72 F) and 65% RH. Two 15- to 30-watt oscillating fans on the canopy raise transpiration. A fabric pot dries faster than plastic. A 5-gallon fabric pot can dry a full day sooner than a plastic pot of the same size.
A 7-Day Example
Setup: one plant, 5-gallon fabric pot, soil, 600-watt LED at 60%, 25 C, 55% RH, mid veg.
- Day 1: water 2 L to runoff.
- Day 2: no water. Pot still heavy.
- Day 3: pot light. Water 2 L.
- Day 4: no water.
- Day 5: no water. Top 2 cm dry but the pot has weight.
- Day 6: water 2.5 L. Plant grew 4 cm since day 3.
- Day 7: no water.
Interval: about 2 to 3 days at this size. Track dates in a notebook. The pattern shifts each week.
Tools
- pH meter, calibrated every 2 weeks
- EC or ppm meter
- Moisture probe, or a kitchen scale for small pots
- Garden sprayer or a 2 L watering can with a rose head
- Runoff tray and a wet/dry vacuum
Common Mistakes
- Following a fixed calendar instead of pot weight
- Watering on a timer in soil
- Adding a small amount each day
- Mixing nutrients at full strength in a dry pot
- Leaving runoff in the tray
- Watering with cold water in a warm room
The numbers above assume a healthy plant, a pot with drainage, and a room at 22 to 27 C (72 to 80 F). Adjust from the plant, not the calendar.