Cannabis Grow Light Schedule 18/6: Veg Cycle Guide

An 18/6 cannabis grow light schedule means the light stays on for 18 hours and stays off for 6 hours every day. It is the standard veg cycle for photoperiod cannabis, used before the plant is flipped to flower. The 6-hour dark window lets the plant run respiration, move stored sugars, and build roots.

Why do growers use 18 hours of light for veg?

Cannabis is a photoperiod plant. Long days tell it to stay in vegetative growth, and short days trigger flowering. Eighteen hours of light sits in the sweet spot: enough photosynthesis to stack nodes and bulk stems, without the power bill of 24/0.

Growth: 18 hours of light delivers more total photons per day than 12 or 16 hours.
Cost: A 6-hour dark window cuts light runtime by 25% compared to 24/0.
Recovery: Stems and roots grow in the dark hours, when the plant burns sugars made in the light.
Heat: Lights-off hours let a tent cool, which matters in small rooms and closets.

Does the dark period matter?

Six hours of dark is not wasted time. Plants ship the sugars made during lights-on to root tips and new shoots after the light goes off. Respiration also runs in the dark, and that process drives stem thickening.

18/6 compared to other light cycles

18/6: The veg default. Strong growth, 6 dark hours, moderate power use. Best all-around choice for photoperiod plants in veg.
20/4: More light, faster veg, higher draw. Common with autoflowers.
24/0: No dark period. Fast growth, hottest room, biggest bill. Works, but adds little for most growers.
16/8: A power-saving veg cycle. Slower growth than 18/6.
12/12: The flowering trigger. Do not run it in veg unless you want buds to start.

How do you switch from 18/6 to 12/12?

Wait until the plant reaches the size you want. Stretch in the first 2 to 3 weeks of flower adds 50% or more to final height.
Set the timer to 12 on and 12 off. Pick a start time you can live with, since the schedule locks for 8 to 11 weeks.
Keep the dark window dark. Tape over power strip LEDs, seal zippers, and check for streetlight or room light leaks.
Hold the cycle steady. A timer failure that leaves lights on for one night can stall flower development.

What light cycle do autoflowers need?

Autoflowers flower on their own age, not on day length, so they never need 12/12. Most auto growers run 18/6 or 20/4 from seed to harvest. A few run 24/0, but the extra yield seldom covers the extra power.

Do hours matter more than intensity?

Hours and intensity work together, and the number that combines them is DLI (daily light integral). DLI is the total light a plant receives in a day, measured in mol/m²/day. To find it, multiply PPFD by hours by 0.0036.

Vegetative target: 20 to 40 DLI
Flowering target: 30 to 45 DLI
Example: 500 PPFD for 18 hours equals 32.4 DLI, a solid veg number

A plant on 18/6 at 700 PPFD takes in more light than a plant on 12/12 at 700 PPFD. That is why veg plants handle long days: each extra hour adds photons to the daily total.

Watch the leaves, not just the meter. Bleached tips, curled edges, and taco-shaped leaves mean the fixture sits too close or the PPFD is too high. Raise the light or dim it. If stems stretch long and nodes sit far apart, add intensity or hours.

Common 18/6 mistakes

Light leaks in the dark window. Even a small LED can delay flowering or trigger male flowers.
Cheap timers that drift or fail. Use a digital timer and check it after power outages.
Flipping to 12/12 too late. Stretch pushes tall plants into the light or the tent wall.
Running 18/6 on a photoperiod plant in flower. It revegetates or stalls.
Ignoring heat. Six dark hours help, but a hot tent still needs airflow.

FAQ

Can you run 18/6 for the whole grow?

Only with autoflowers. Photoperiod plants need 12/12 to flower, and 18/6 during flower stalls bud development. Autoflowers can run 18/6 from seed to harvest with no light change.

Is 24/0 better than 18/6 for veg?

Not by much. 24/0 gives more photons and faster growth, but it costs 33% more power than 18/6 and removes the dark period roots use. Most home growers see no payoff from the extra hours.

Does 18/6 save money?

Yes. Compared to 24/0, the cycle runs the light 25% less. A 400W LED at $0.16 per kWh costs about $46 a month on 24/0 and about $35 a month on 18/6, a savings near $11.

Do plants need complete darkness for those 6 hours?

Yes. Zero light is the goal. A tiny glow from a timer, a cracked zipper, or a phone screen can interrupt the dark cycle and slow flowering.

What time of day should the 6-hour dark window run?

Any time, as long as it stays consistent. Some growers run lights at night to fight summer heat. Others run them during the day to keep a cold room warm. Pick one window and hold it.

Bottom line

Use 18/6 for photoperiod plants in veg, switch to 12/12 to flower, and let autoflowers run 18/6 the whole way. Keep the dark window dark and track DLI instead of guessing from the wattage on the box.