The short answer
Run photoperiod cannabis on 18 hours of light and 6 hours of dark during veg. Autoflowers do fine on the same 18/6, and many growers push them to 20/4 or even 24/0 without trouble. If you only remember one thing, make it this: 18/6 works, it is cheap to run, and it gives the plant a dark window for the processes that light cannot drive.
how to shorten cannabis vegetative stage
Everything below is about the exceptions and the fine tuning.
How to Shorten the Cannabis Vegetative Stage: A Comprehensive Guide
Why 18/6 became the default
Cannabis is a photoperiod plant. It measures night length, not day length, to decide when to flower. Give it fewer than roughly 10 to 12 hours of uninterrupted dark and it stays in vegetative growth. Give it 12 or more and it commits to flowering. Veg schedules sit well above that threshold, which is why 18/6 keeps plants building leaves and stems instead of buds.
cannabis vegetative stage training techniques
The 6-hour dark window is not wasted time. Plants respire in the dark, moving sugars out of the leaves and into roots and new growth. Roots tend to stretch during that window too. Canopies also cool off, which matters in a tent where leaf surface temps run a few degrees above air temp under a hot fixture.
cannabis vegetative stage leaf problems
Other veg schedules and when they make sense
20/4
More hours, more daily light, faster veg growth in most setups. I reach for this when plants are behind schedule, when I am running low-intensity LEDs and want to hit a DLI target without cranking the dimmer, or in winter when the room is cold and extra light hours help hold temperature. Downsides: a shorter dark window, a slightly higher power bill, and less recovery time after heavy training or a nutrient push.
24/0
Continuous light produces fast early growth in some cultivars. It also costs the most to run and removes the dark period entirely. Research on cannabis photosynthesis has shown that continuous light can lift photosynthetic rates at first and then lead to chlorosis and lower net photosynthesis over weeks in some cultivars. If you run 24/0, watch older fan leaves for yellowing and be ready to drop back to 18/6.
16/8
A useful late-veg option. It stays safely above the flowering threshold for most photoperiod plants, it saves a bit of power, and some growers use it in the last week or two of veg to ease the plant toward the 12/12 flip. Anecdotal, but it does not seem to hurt.
Skip anything at or below 14 hours of light during veg. You are gambling on early flowering or a revegetation mess.
Hours matter less than total light delivered
Daily light integral (DLI) is the number worth tracking. It is the total photons a plant receives per day, measured in mol/m²/day, and it folds intensity and hours into one figure.
DLI = PPFD × hours of light × 0.0036
Rough targets for veg:
- Clones and seedlings: 100 to 200 PPFD, DLI around 8 to 12
- Established veg plants: 300 to 600 PPFD, DLI around 20 to 40
At 18 hours and 500 PPFD you land near 32 mol/m²/day, which is a solid veg number for most home setups. At 24 hours and 300 PPFD you get about 26. Same ballpark, different tradeoffs. This is why arguing 18/6 versus 24/0 misses the point: two rooms with the same DLI tend to grow at similar rates.
Indoor cannabis trials have found yield and potency climb as DLI increases, then plateau somewhere in the 30 to 40 mol/m²/day range for flowering. Veg tolerates less. Past a certain point, extra light is heat and cost, not growth.
Reading the plant
Flat, horizontal leaves and thick stems between nodes mean you are in range. Signs you have overshot: leaf edges curling up, bleached or pale new growth, tacoing, and leaves angling away from the fixture. Signs you are under: long gaps between nodes, thin stems, and slow leaf expansion with a pale green tone.
Distance depends on the fixture. Follow the manufacturer's veg height, then adjust based on what the canopy does over the next 24 hours. Move the light in small steps, not big ones.
Dark period hygiene
If your dark window is not actually dark, you are not running the schedule you think you are. Timers fail, tent zippers leak, and a phone screen at the wrong moment can be enough. This matters less in veg, since a stray hour will not flip a plant, but it matters a great deal the day you switch to 12/12. Fix the leaks during veg so the flip is clean.
When to flip to 12/12
Photoperiod plants roughly double in height during stretch, sometimes more for sativa-leaning lines. Flip when the canopy is about half the final height you want. If you are on 18/6 and want less stretch, some growers step down to 16/8, then 14/10 for a few days, then 12/12. There is no strong data behind it, but it does not seem to hurt.
Autoflowers do not need a flip. Keep them on 18/6 or 20/4 from sprout to harvest.
Veg light schedule checklist
- 18 hours on, 6 off for photoperiod plants
- 18/6 to 24/0 for autoflowers, chosen on heat and power cost
- PPFD 300 to 600 for established veg, DLI 20 to 40
- Set the timer, then verify it after any power outage
- Keep the dark window sealed and consistent
- Flip to 12/12 when the plant is about half its target height