Cannabis grow lights for autoflowering strains work best as full-spectrum LEDs run on an 18/6 to 24/0 schedule, hitting about 300-500 PPFD in veg and 500-800 PPFD in flower. You never shorten the day to 12/12 because autoflowers flower by age, not by photoperiod. Plan on 30-40 watts of quality LED per square foot of canopy.
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What light schedule do autoflowering strains need?
Autoflowers ignore the dark period when it comes to flowering. They start blooming about 3 to 5 weeks from seed no matter what the timer says. That freedom lets you pick a schedule based on power bills and room heat rather than plant biology.
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- 18/6: the common pick. Six hours of dark trim your electric bill and give drivers a rest.
- 20/4: a middle ground with more light hours and modest heat gain. Popular in cool basements.
- 24/0: the most light hours. Fast growth in veg, but heat and cost climb and some plants show stress.
Run one cycle on a new schedule before you commit to it. A watt meter and a thermometer tell you whether the extra hours pay for themselves.
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How much light intensity do autoflowers need?
Intensity drives yield more than hours do. Autos under weak light stretch, grow airy buds, and finish small. The trick is raising intensity as the plant ages instead of blasting seedlings from day one.
- Seedling (week 1-2): 150-300 PPFD
- Veg (week 2-4): 300-500 PPFD
- Flower (week 4 to harvest): 500-800 PPFD, up to 900 with added CO2
Track daily light integral (DLI) if you want one number to manage. Aim for 30-45 mol per square meter per day in veg and 40-55 in flower. A phone light meter app is close enough for a small tent.
What wattage and coverage do you need?
Match wattage to floor area, not to plant count. Two autos in a 3x3 need the same fixture as four autos in a 3x3 once the canopy fills the space.
- 2x2 ft tent: 100-150W LED
- 3x3 ft tent: 200-300W LED
- 4x4 ft tent: 400-480W LED
- 5x5 ft tent: 600W or two 300W fixtures
Buy a dimmable fixture. Dimming lets a 300W board run at 50% for seedlings and open up in flower without new gear.
LED vs HPS vs CFL for autoflowers
LED
The best fit for autos. Cool running, dimmable, and full spectrum, with efficiency near 2.5-3.0 µmol/J on mid-range boards. Higher upfront cost, lower power bill.
HPS
Cheap to buy and hot to run. A 400W HPS pulls 400W plus the cost of cooling it back down. It also needs more headroom, which hurts in a short tent. Bud quality is solid, but the spectrum leans orange.
CFL
Fine for the first two weeks, nothing more. Low output per watt and poor canopy penetration. Switch to LED before veg ends.
What spectrum works best for autoflowers?
Go full spectrum: 3000K to 5000K white diodes with extra 660nm deep red. Autoflowers run veg and bloom at the same time, so one wide spectrum beats swapping bulbs. Blue light keeps nodes tight and red light builds bud mass.
Ignore claims that autos need a special "auto spectrum." Any board rated for photoperiod flowering works.
How far should the light hang from the canopy?
- Seedlings: 24-36 inches, or dim to 40%
- Veg: 18-24 inches
- Flower: 12-18 inches
Read the plant instead of the tape measure. Leaves that cup, taco, or bleach at the tips mean too much light. Long gaps between nodes and thin stems mean too little.
Do autoflowers need a different light than photoperiod plants?
No. The same fixtures work for both. The difference is the schedule. Photoperiod plants need 12 hours of dark to trigger bloom, while an autoflower will flower under 24 hours of light and produce more weight with those extra hours.
That also means light leaks during dark hours will not send an auto back into veg. Sealing every zipper is less critical in an auto tent.
Does the light schedule change autoflower yield?
Yes, within limits. Moving from 12/12 to 18/6 adds six hours of light, a 50% jump in photons per day. More light means more bud mass when water, nutrients, and temperature hold steady. Past about 45 DLI without added CO2, extra light turns into heat and stress instead of weight.
Common autoflower lighting mistakes
- Flipping to 12/12 for bloom. This throws away light hours the plant can use and drops yield.
- Full power on seedlings. Light and heat stress stall early growth that autos burn weeks trying to recover from.
- Ignoring canopy temperature. Hitting 85°F or higher slows growth and thins terpenes.
- Hanging lights too high for the whole run. Extra distance cuts PPFD at the canopy and stretches plants.
- Skipping the timer. Hand switching produces results you cannot repeat.
Quick setup checklist
- Pick a dimmable full-spectrum LED sized to your tent.
- Set the timer to 18/6 for the first run.
- Start seedlings at 150-200 PPFD with the light 30 inches up.
- Raise intensity about 10% each week through veg.
- Hold 500-800 PPFD in flower and watch leaf temperature.
- Keep the canopy 12-18 inches from the light in late flower.
Autos reward simple lighting. One dimmable LED, a steady timer, and a meter to check intensity cover almost everything. Get those three right and the genetics do the rest.