Cannabis Light Schedules Explained: 18/6 vs. 12/12

Cannabis vegetates under long days and flowers under long nights. In veg, run 18 hours of light and 6 of dark (18/6). To trigger flowering in photoperiod plants, switch to 12 hours light and 12 hours uninterrupted dark (12/12). The dark period must stay truly dark, since light leaks during the night can stall or confuse flowering.

Veg light schedule

Photoperiod cannabis stays vegetative as long as it gets long days. The standard is 18/6, which gives strong growth while letting the room cool and the plants rest for a few hours. Some growers run 20/4 or even 24/0 for slightly faster growth, but the gains are small and 24/0 gives the plant no dark rest. We run a Colorado nursery and stick with 18/6 for our veg and mother plants: it grows fast, saves a little power, and keeps a dark cycle in the plant's rhythm.

Flower light schedule

To flip photoperiod plants into flower, drop the light to 12/12. That long, unbroken 12-hour night is the signal that fall is coming, and the plant responds by flowering. The most important rule here is that the dark period must be complete. Even a phone flashlight or a bright LED on your timer can interrupt it. Serious light leaks during the dark cycle can trigger stress, revegging, or hermaphrodite pollen sacs.

The schedule at a glance

Stage	Light / dark	Purpose
Seedling / clone	18/6	Gentle light while rooting and establishing
Vegetative	18/6 (or 20/4)	Fast structural growth
Flowering	12/12	Triggers and sustains bud production
Mother plants	18/6	Keep permanently in veg for cuttings
Autoflowers	18/6 to 20/4 whole life	Flower on age, not light

Autoflowers ignore the schedule

Autoflowering plants flower based on age, not light hours, so you do not flip them. Many growers run autos on 18/6 or 20/4 for their entire life, from seedling through harvest, since more light hours means more growth and the plant flowers on its own timeline anyway. If you are running photoperiod clones, though, the 18/6 to 12/12 flip is exactly what puts you in control of when they flower.

Getting the flip right

• Flip to 12/12 when the plant is about half its final target height.
• Make the dark period genuinely dark: seal light leaks, cover timer LEDs.
• Keep the schedule consistent; erratic timing stresses plants.
• Use a reliable timer, and check it after power outages.
• Do not open the tent during the dark cycle with room lights on.

Light timing works with your other flowering conditions, not alone. As plants move into bud, you also adjust temperature and humidity down. Our temperature and humidity guide and flowering stages guide cover how the whole environment shifts once you flip.

Light schedule and plant health

A clean, consistent schedule is part of keeping plants healthy from clone to harvest. New clones do best under gentler light on 18/6 while they root, then ramp up as they establish. Our clone care guide walks through those early days. Once rooted and vigorous, they veg hard under 18/6 and flip cleanly to 12/12 when you are ready.

Every technique here works better with strong genetics. Browse our rooted, HLVd-tested clones or start with forgiving beginner strains.

Frequently asked questions

Can I veg on 24 hours of light?

You can, and some growers do for slightly faster growth, but the benefit over 18/6 is small and the plant gets no dark rest. Many growers feel plants look healthier with a dark period. Running 24/0 also costs more power and adds heat. For most home grows, 18/6 is the practical, proven choice for veg.

What happens if light leaks into the dark cycle?

Light during the dark period is the most common cause of flowering problems. It can slow or stall flowering, cause a plant to revert to veg, or trigger hermaphrodite pollen sacs that seed your crop. Seal the tent, cover glowing timer and equipment LEDs, and never open up with room lights on during lights-off.

When do I switch from 18/6 to 12/12?

Flip when the plant reaches about half the height you can fit, since it stretches and roughly doubles early in flower. That usually lines up with a 4 to 6-week veg from a rooted clone. There is no fixed age; go by plant size and your space rather than a calendar date.

Do clones need a special light schedule?

Fresh clones root best under lower-intensity light on an 18/6 schedule while they establish. Keep the light a bit further away and humidity up until roots take hold, then ramp intensity as they grow. Once rooted and vigorous, they follow the standard veg schedule and flip to 12/12 with the rest of your garden.

Ready to start a run you can flip on your own schedule? Browse our cannabis clones for sale, all female-guaranteed and HLVd-tested.