Cannabis Seedling Care: The First 3 Weeks Done Right
Seedlings need less than you think. Almost every seedling problem is caused by a grower giving too much — too much water, too much light, too much nutrient — rather than too little.
Light: 18/6, dimmed, and hung high
Run 18 hours on and 6 off. Dim your LED to 25-40% and hang it 24-30 inches above the canopy. A seedling under full-power light will bleach, stall, or grow tight and yellow.
Humidity and temperature
Target 65-70% relative humidity and 72-78°F. Seedlings drink through their leaves as much as their roots at this size. A humidity dome for the first week helps, but vent it daily so the surface is not permanently soaked.
Watering: a shot glass, in a ring
Water in a small circle a couple inches out from the stem, not directly on it. That encourages roots to reach outward. A shot glass worth every day or two is plenty in a small pot.
Overwatering is the number one seedling killer. Drooping with wet, heavy soil means too much water, not too little.
Nutrients: wait
In living or lightly amended soil, the seed and the medium carry the plant for 2-3 weeks. Start at quarter strength only when the third or fourth set of serrated leaves appears and the plant is clearly hungry.
Frequently asked questions
- Why is my seedling stretching?
- The light is too far away or too dim. Lower it a few inches at a time and watch the internode spacing tighten.
- When does the seedling stage end?
- Once the plant has 3-4 sets of serrated leaves and visible new growth every day, it is in vegetative growth.
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