has anyone tried an 18/12 light cycle?

[growsince79]

would it help/hurt to run an 18/12 light cycle during flower? would the plant "think" the days were 30 hours long and therefore take more actual days to finish? would the extra light hours help? would it still flower? i understand it has to have 12 hours of night uninterrupted, but does it necessarily need only 12 of light? i am just curious about how it works, and how much one could manipulate things.

has anyone here tried it?

You should try it and make a diary to show us what happens. Use a strain or clone that you know well.

 

[dannyhedges]

Photoperiod plants need 12/12 to start flowering, im running 12/12 from seedling but at the 4 or 5 week mark when it starts to bud im gonna switch it 18/6 for 3 weeks to increase bud size

You'll provoke the plants to reveg with 18/6, you'll have less, lower-quality flower.

 

[Backyard_Boogie]

This entire topic is blowing my mind! I never even though about going outside a 24 hour cycle the thought never even crossed my mine. I guess I wasn't thinking out of the box enough. My mind was stuck on earthly 24hr terms. 30 hours is straight crazy my mind thinks of massive wild cannabis plants growing on another planet. Like on planet Pandora or some shit! 3 moons in the night sky! Wow I gotta cut back on the reefer and Star Trek LOL!

 

[Ponky]

This entire topic is blowing my mind! I never even though about going outside a 24 hour cycle the thought never even crossed my mine. I guess I wasn't thinking out of the box enough. My mind was stuck on earthly 24hr terms. 30 hours is straight crazy my mind thinks of massive wild cannabis plants growing on another planet. Like on planet Pandora or some shit! 3 moons in the night sky! Wow I gotta cut back on the reefer and Star Trek LOL!

I've read down that rabbit hole. Conclusion was light cycle manipulation outside the 24 hr cycle can work. But no one sped anything up. No one gained yield vs. Wattage vs. Time.
If I can find the article I'll post it. Wasn't very long or compelling. They did the 10/10 they did 24/24. All sorts of experiments. The concluons was 12/12. With 14/10 working on certain cultivars.

 

[growsince79]

Photoperiod plants need 12/12 to start flowering, im running 12/12 from seedling but at the 4 or 5 week mark when it starts to bud im gonna switch it 18/6 for 3 weeks to increase bud size

Why? That's going to hurt yields big time. If it's just so you know the sex before growing big plants, you're better off to make clones at 3 weeks and flower them immediately.

 

[lvstealth]

well... there are some things to consider (now that i have read more and tried a few things). the plant has a daily amount of light that it likes. you can run a light longer, but at a lower level so the plant gets the right amount. it is not a line, it is more of a range that a plant does its happiest growing.

apparently stress is the big deal. it stresses them out to have too much or too little of most anything they need. light is one of those things.

it seems to work best if i use a 30 hour day from seed to harvest. they dont seem to like the change of hours (but i havent done it gradually)

it did not cut down on time much (in 30 hour days it still took 8 weeks for one i usually run 8 weeks. and that is 8 weeks of 30 hour days). if i increase the light they do not tend to like it, so if i calculate it out, i am running the same number of light in its day in either tent. what i mean is i have to dim the 30 hour tent so the total actual light hitting the plant is the same as running 100% for 24 hours. if i dont, it turns her yellow then cooks her.

they have done some experiments on humans and finding a "natural" day. fascinating! but in the end it all seems to work out mathematically. the magnetic pull doesnt seem to have the effect youd think.

btw, three moons creates totally different issues. look at what the moon does for us. the tides for one, but lots of other things and 3 moons fighting it out using the weapon gravity... well, i wouldnt want to be a "tri-moon-ian"

 

[diereckeddank]

Photoperiod plants need 12/12 to start flowering, im running 12/12 from seedling but at the 4 or 5 week mark when it starts to bud im gonna switch it 18/6 for 3 weeks to increase bud size

I wouldn't. End up with harvest time bananas in your bud.

 

[mstrpkl]

I was gonna bring up DLI but Aqua Man beat me to it.

No point in extending light times. Just provide the proper light to fill the plants DLI and you will be in a great place.

Obviously shortening the light schedule will make photoperiods switch to flower, but that is a hormonal change and has little effect on photosynthesis.

Look into Bruce Bugbee, he will answer all your questions with REAL science instead of bro science lol

 

[Gmix]

It’s not complicated
Most Plants need a minimum of 11hrs dark to flower
( I’ve never heard of 10 before being suffice still I won’t argue a hr )
18 hrs of light will not give you bigger buds it will only encourage more growth in leaves not buds.

You’ll actually stunt the growth changing from 12/12 to 18/6 as it tries to reveg its self

You cannot increase flower growth by adding extra light hrs

It’s just not how plants work

The plant as said will stop putting energy into producing flowers instead it will focus it’s energy into new growth that transition may also confuse the plant enough to start throwing out male flowers

Really it’s abc once you understand how flowers are produced

 

[Natep]

Bruce bugbee has a ton of videos on light and DLI. He explains this well.

 

[Coconutz]

Less light hrs encourages a faster flowering response, I never got anywhere near 8wk crops until I tried 11-13hrs, there's also the added bonus of less vegitive growth and less stretch making more plants faster an option.

Same age and different hrs of light, 12/13hrs will eventually be ok but you'll need weeks more to get a satisfactory product.

 

[Yamon]

Hi Y'all.
I have started an 18/12 schedule starting around day 17 of flower. I would have started at day 10 but hadn't thought of it yet.
Some thoughts to consider.

Extra Light in Flowering Doesn’t Revert Cannabis to Veg — Here’s Why​

Physiology-based explanation to debunk the “extra light = reversion to veg” myth:​

1. The real trigger: darkness, not light
Cannabis is a short-day (photoperiod) plant. Flowering is triggered and maintained by uninterrupted periods of darkness, not by the number of hours of light. As long as the plant receives the minimum dark window (usually ~12 hours for photoperiod strains), it will remain in flower.

2. Why people get it wrong
A common myth is that any light longer than 12 hours will force the plant back to veg. This misunderstanding comes from oversimplifying the 12‑12 schedule taught in textbooks. While 12‑12 is standard for flowering, extra light does not erase the dark signal. The plant senses darkness cues through phytochrome pathways, not by tallying light hours.

3. Natural variability
Plants evolved under variable conditions: cloudy days, shading, storms, or even volcanic dust can drastically change light intensity and duration. Cannabis has survived and flowered for millions of years under these conditions, which demonstrates that it is highly resilient and flexible in its light usage. What matters is the continuity of darkness.

4. Effects of extended light

More light hours = more energy for photosynthesis, allowing faster node stacking, taller stretch, and more biomass.

As long as dark periods are maintained, flowering continues normally.

Resin, trichome, and bud development are generally not negatively affected; in fact, extra energy may enhance overall growth.


5. Bottom line

Flowering depends on darkness continuity, not on capping light at 12 hours.

Extra light during flowering does not revert the plant to veg, it simply provides more energy to build bigger, stronger colas.

Reversion occurs only if the plant’s dark period is interrupted or shortened below the flowering threshold.


Conclusion: Growers can safely experiment with slightly longer light periods during flowering (e.g., 18‑12) to maximize stretch and energy capture, as long as the dark period remains uninterrupted.

 

[cpurola]

Hi Y'all.
I have started an 18/12 schedule starting around day 17 of flower. I would have started at day 10 but hadn't thought of it yet.
Some thoughts to consider.

Extra Light in Flowering Doesn’t Revert Cannabis to Veg — Here’s Why​

Physiology-based explanation to debunk the “extra light = reversion to veg” myth:​

1. The real trigger: darkness, not light
Cannabis is a short-day (photoperiod) plant. Flowering is triggered and maintained by uninterrupted periods of darkness, not by the number of hours of light. As long as the plant receives the minimum dark window (usually ~12 hours for photoperiod strains), it will remain in flower.

2. Why people get it wrong
A common myth is that any light longer than 12 hours will force the plant back to veg. This misunderstanding comes from oversimplifying the 12‑12 schedule taught in textbooks. While 12‑12 is standard for flowering, extra light does not erase the dark signal. The plant senses darkness cues through phytochrome pathways, not by tallying light hours.

3. Natural variability
Plants evolved under variable conditions: cloudy days, shading, storms, or even volcanic dust can drastically change light intensity and duration. Cannabis has survived and flowered for millions of years under these conditions, which demonstrates that it is highly resilient and flexible in its light usage. What matters is the continuity of darkness.

4. Effects of extended light

More light hours = more energy for photosynthesis, allowing faster node stacking, taller stretch, and more biomass.

As long as dark periods are maintained, flowering continues normally.

Resin, trichome, and bud development are generally not negatively affected; in fact, extra energy may enhance overall growth.


5. Bottom line

Flowering depends on darkness continuity, not on capping light at 12 hours.

Extra light during flowering does not revert the plant to veg, it simply provides more energy to build bigger, stronger colas.

Reversion occurs only if the plant’s dark period is interrupted or shortened below the flowering threshold.


Conclusion: Growers can safely experiment with slightly longer light periods during flowering (e.g., 18‑12) to maximize stretch and energy capture, as long as the dark period remains uninterrupted.

Fascinating! I'm gonna follow your progress, please keep us informed.

 

[Yamon]

I started a thread about this with some pictures and I'll be updating what's happening there. I'll try to update some info here.

Thread '18/12 Light schedule in Flower especially during stretch' https://www.thcfarmer.com/threads/18-12-light-schedule-in-flower-especially-during-stretch.173191/

 

[orggrwr]

The one problem i would have is the dark period will shift daily on a 30 hour schedule. I would definitely get high and open the tent during lights out at least a couple times a grow. Maybe rig an indicator light outside the tent so you know if your grow lights are on or off.

 

[chitownglass]

I was gonna try but I don't have a 30 hour timer lol. What does work with most plants is 13 light-11 dark- makes for bigger buds.

Does this really make bigger buds because there's more light?

 

[Coconutz]

The real trigger: darkness, not light
Cannabis is a short-day (photoperiod) plant. Flowering is triggered and maintained by uninterrupted periods of darkness, not by the number of hours of light. As long as the plant receives the minimum dark window (usually ~12 hours for photoperiod strains), it will remain in flower.

I can't see it working but I do hope that I'm wrong, #1 is a contradiction?

How do you give a minimum 12hrs dark period while giving 18hrs light?

 

[Yamon]

I can't see it working but I do hope that I'm wrong, #1 is a contradiction?

How do you keep give a minimum 12hrs dark period while giving 18hrs light?

18 hours of light then 12 hours dark. Repeat.
No contradictions

 

[amneziaHaze]

I can't see it working but I do hope that I'm wrong, #1 is a contradiction?

How do you give a minimum 12hrs dark period while giving 18hrs light?

tent doesnt have to live in 24h world

 

[Yamon]

tent doesnt have to live in 24h world

Exactly