Cannabis seedlings do best on an 18/6 light schedule: 18 hours of light and 6 hours of dark. Hang the fixture so the seedling canopy receives about 100 to 300 PPFD, which works out to a daily light integral (DLI) of roughly 6 to 19 mol/m²/day depending on where you sit in that band. In practice that means 18 to 24 inches above the plant for a dimmed LED panel, 2 to 6 inches for a T5 fluorescent, and 24 to 36 inches for a metal halide HID lamp.
marijuana grow light wattage for 4x4 tent
What is the best light schedule for cannabis seedlings?
18/6 is the standard. Seedlings are still building root mass and leaf area, so they do not need maximum light, and a dark period supports normal respiration and keeps stretch in check. Autoflower seedlings can run 18/6 to 20/4 with no downside.
marijuana grow light wattage for 4x4 tent
Can cannabis seedlings handle 24 hours of light?
Yes, plenty of growers run 24/0 through seedling and veg with healthy plants. The tradeoffs are higher electricity use, more heat in a small tent, and no measured yield gain over 18/6 in most home setups. If you do run 24/0, drop intensity by about 20 percent so DLI stays in range.
marijuana grow light wattage for 4x4 tent
Is 12/12 ever right for seedlings?
No. Twelve hours of uninterrupted darkness is the flowering trigger for photoperiod cannabis, and starting seedlings there stalls growth and can push young plants into early flower. Keep 18/6 until you are ready to flip.
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How far should the light be from a cannabis seedling?
Distance depends on the fixture, not on the plant's age alone. Use the manufacturer's hang chart as a starting point, then adjust by watching the seedling for 48 hours.
- Dimmable LED panel or bar: 18 to 30 inches at 40 to 60 percent power.
- T5 or T8 fluorescent: 2 to 6 inches above the canopy.
- CFL bulbs: 3 to 6 inches.
- Metal halide or HPS: 24 to 36 inches for 400W, 36 to 48 inches for 1000W.
What PPFD and DLI do seedlings need?
Aim for 100 to 300 PPFD across an 18 hour photoperiod. Late seedling stage can push toward 350 PPFD if leaves look flat and happy rather than cupped. A PAR meter, or a phone app paired with a diffuser, gives you a real number instead of a guess.
How do you read seedling stress from light distance?
The plant answers within two days. Change one variable at a time and wait before adjusting anything else.
- Long stems, small leaves, wide node spacing: light is too far or too weak. Lower the fixture or raise power by 10 to 15 percent.
- Leaves cupping or tacoing, edges curling up: too intense. Raise the light or dim it.
- Bleached or yellow-white tips on new growth: light burn. Move the fixture up several inches.
- Flat horizontal leaves, short internodes: dialed in. Leave it alone.
Does heat from the light change the plan?
Seedlings like roughly 70 to 80°F and 60 to 70 percent relative humidity at canopy level. An LED driver adds far less radiant heat than an HID ballast, so a small seedling tent often needs a heat source rather than more exhaust. Measure temperature at the canopy, not near the ceiling.
What are common seedling light mistakes?
- Running the fixture at full power on day one.
- Hanging at the flowering height printed on the box instead of a seedling height.
- Changing distance every day instead of waiting 48 hours for a response.
- Using 12/12 on photoperiod seedlings to save power.
- Trusting a wattage number instead of measuring PPFD at the canopy.