Cannabis Vegetative Light Hours: 18/6 Guide

How Many Hours of Light During Cannabis Vegetative Stage?

Most indoor growers use 18 hours of light and 6 hours of dark. That schedule is the standard for vegetative growth. Some growers use 24 hours of light, 20/4, or 16/8. The plant stays in vegetative growth when the dark period is shorter than 12 hours.

how long is the cannabis vegetative stage

Common Vegetative Light Schedules

• 18/6: 18 hours light, 6 hours dark. Most used schedule. Balances growth and energy cost.
• 24/0: 24 hours light, no dark. Fast growth. Higher electric cost. No dark period for respiration.
• 20/4: 20 hours light, 4 hours dark. Middle ground.
• 16/8: 16 hours light, 8 hours dark. Closer to natural summer light. Slower growth than 18/6.

Outdoor Light Hours

Outdoor plants get light from the sun. In the US, spring and summer provide 14 to 16 hours of daylight in most states. Northern states get more hours near the summer solstice, up to 16 hours. Southern states get 13 to 14 hours. Plants stay in vegetative growth until daylight drops below 12 hours in late summer or fall.

how many hours of light during cannabis vegetative stage

Light Intensity for Vegetative Stage

Hours are one part. Intensity is another. For vegetative growth, use 300 to 600 PPFD at the canopy. The daily light integral (DLI) target is 20 to 40 mol/m²/day. An 18/6 schedule at 400 PPFD gives about 26 mol/m²/day. A 24/0 schedule at 400 PPFD gives about 34 mol/m²/day. More light does not mean more growth. Too much light causes leaf bleaching and stress.

related article

Does the Dark Period Matter?

Yes. Cannabis uses dark hours for respiration and root growth. A 24/0 schedule can work, but some growers report slower root development and more stress. A 6-hour dark period gives the plant time to move sugars and repair tissue. The dark period also keeps the plant from flowering. Flowering starts when the dark period reaches 12 hours or more.

Temperature and Light Schedule

Vegetative stage temperature range: 70 to 85°F (21 to 29°C). Keep the difference between day and night temperature small. A large drop at night slows growth.

Common Mistakes

• Switching to 12/12 before the plant fills the space.
• Using 24/0 with high PPFD, which causes light burn.
• Ignoring DLI. 18/6 at 600 PPFD gives 38 mol/m²/day, near the top of the range.
• Light leaks during dark period in veg do not cause hermies. Light leaks in flower do.