Set your vegetative room to 18 hours of light and 6 hours of dark, then buy a 24-hour timer rated for your fixture and a dimmable full-spectrum LED sized to the canopy. Use the timer as the baseline, and adjust light intensity to reach a vegetative daily light integral (DLI) target instead of adding hours without a meter or manufacturer PPFD map.
How Big Should Cannabis Plants Be Before Flowering?
How many hours of light does cannabis need in veg?
For photoperiod cannabis, 18/6 is the standard vegetative schedule. It gives the plant a long day without the electricity cost and heat load of 24/0. Some growers use 20/4 for faster growth or 24/0 to simplify climate control, but 24/0 is optional and not a requirement. Autoflowering cultivars can also run on 18/6 or 20/4; many growers avoid 24/0 unless temperature and root-zone conditions stay stable.
When to Switch Cannabis Plants to Flower
Do not use the flowering 12/12 schedule during veg. Cannabis is a short-day plant, and the long dark period is the flowering trigger.
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What to look for when buying a veg light and timer
- Dimmable output: A dimmable fixture lets you match light intensity to plant size and avoid bleaching seedlings or clones.
- Full-spectrum white diodes: White full-spectrum LEDs make it easier to read leaf color and plant health than blurple-only fixtures.
- PPFD map and coverage: Buy a fixture with a published PPFD map for your tent size at a stated hang height. Coverage matters more than wattage claims.
- Timer rating: Check the timer's resistive and LED load rating. LED drivers can draw inrush current at startup, so a cheap timer may fail.
- Heat and driver placement: Removable or remote drivers help keep canopy temperature in range in small tents.
- Hang height and spread: A fixture that fits your tent footprint with even spread reduces the need to raise hours or intensity.
Parameter bands for vegetative growth
- Light schedule: 18/6 standard; 20/4 for more aggressive growth; 24/0 optional; 16/8 can work but may slow growth in some photoperiod cultivars.
- DLI: 20 to 40 mol/m²/day is a practical vegetative band, with 25 to 30 a common target. Use a PAR meter or the fixture's PPFD map to estimate DLI: PPFD × hours × 0.0036.
- PPFD: About 300 to 600 µmol/m²/s at 18 hours gives roughly 19 to 39 mol/m²/day. Start lower for clones and seedlings.
- Temperature and humidity: Keep canopy temperature in the mid 70s to low 80s Fahrenheit and use airflow to avoid leaf temperature spikes.
Pitfalls to avoid
- Adding hours to fix weak growth: Low PPFD, poor coverage, or root-zone problems will not be solved by 24/0.
- Ignoring timer load: A timer that arcs or sticks can leave plants on 24/0 or interrupt the dark period.
- Running 24/0 in hot rooms: Continuous light adds heat and electricity cost, and it can stress plants if climate control is marginal.
- Using flowering distance and intensity for seedlings: Start dim and raise the fixture or reduce output, then increase as roots fill the pot.
- Trusting wattage alone: Wattage is not a light output measurement. Use PPFD and coverage.
FAQ
Is 24 hours of light better for vegetative cannabis?
Not always. 24/0 can speed growth in some setups and simplify scheduling, but 18/6 is enough for most home grows and uses less power. Compare DLI and plant response before changing.
Can I use 16/8 in veg?
Yes, some growers do, but 16/8 gives less daily light than 18/6 at the same PPFD. If you choose 16/8, raise intensity within safe limits or expect slower growth.
Do autoflowers need 12/12 to flower?
No. Autoflowering cultivars flower based on age, not photoperiod. Many growers run them at 18/6 or 20/4 from seed to harvest.
What timer should I buy?
Buy a 24-hour digital or mechanical timer rated for your light's load, with a grounded outlet and enough capacity for LED inrush current. Test it for several on/off cycles before trusting a full crop to it.