18 Hour Light Cycle

Growing with an 18 hour Day/Night Cycle

The theory behind the 18 hour Day/Night cycle is that during a normal 24 hour light cycle plants will usually achieve high growth rates peaking at 100% capacity during the first 50 - 60% of the day. The growth rates will then diminish rapidly and the last 20 - 30% of the day achieves minimal growth. So by reducing the length of the day we are triggering an increased growth mode where the growth rates are at their peak for the majority of the day. This effectively achieves a very fast growth cycle with full yield potential.

NOTE: To achieve these incredible growth rates it is important to provide maximum light intensities and CO2 enriched conditions. The recommended lighting is 600W per square metre.

The Cycles



The Benefits

The growth during an 18 hour cycle can be the equivalent to that achieved during a 24 hour cycle. So by running 18 hour cycles the same growth and yield can be achieved in 75% of the time. Reduced day lengths also mean reduced power consumption. Grow more and use less power. Who can argue with that?

For example, an average crop grown from seed using a 24 hour day/night cycle will have a 4 weeks grow cycle and an 8 weeks flower cycle. This equates to 28 days @ 18 hours a day and 56 days @ 12 hours a day = 1176 hours of light over 12 weeks.

An accelerated crop using an 18 hour day/night cycle will achieve the same yields using a 3 week grow cycle and a 6 week flower cycle. This equates to 21 days @ 14 hours a day and 42 days @ 6 hours a day = 546 hours of light over 9 weeks resulting in a 40% reduction in power consumption and a 25% reduction in crop time.


Crop rotations in 7.5 weeks (down from 16weeks)

Getting efficient output is what it’s all about. I have to be at least a little dubious. But try this for size... I have to think that there’s something to it.
All species of plant, insect or animal on earth have their "Day period" during which, their efficiency is greater for SOME of that time. While they can use up to 18hrs of light, maybe for 12 of those, they are at 100%, then drift off to only 60%-70%, then to 20%-30%. Consider... most days we wake up and are at 90% in 5 minutes, at 100% in 10 minutes. After 10/12 hrs, we’re flagging badly and while happy to stay up another 6-8 hrs, are just "laidback". A young kid on the other hand, is at 90% within 10 seconds of waking and 100% in 30 seconds. They hit the wall "like a bird flying into plate glass" after 14 hrs, then sleep like a brick for 10! Others however, take an hour to get to 60%, another hour to get to 90%.

Plants don’t think, they react to a genetic 'drive to survive'. So, I conclude that there is a case to examine where a short day cycle could trip a plant's genetic response into a "survive or die" accelerated mode to flowering and fruiting. The idea is to capture the hours that would give 90% or better, of the plant's "production" but maybe saving 25-30% of the daylight period (power being a special consideration).

• Set vegetative cycle at 14hrs day and 4hrs night for an 18hr day
• Run it for 7 days (about 10 "plant days") at 2000ppm CO2
• That period gives sufficient leaf development to proceed with the flowering phase.
• The "Fast Flower" 6hr days and 12hr nights regime lasts 45 days.
• Crop is better than EVER had in 16 weeks." Yields?, "I’d say 135-140% of previous best crops.
• 7weeks and 3 days Harvest. Impressive result. 6 great crops a year, low management of them and every one a winner!

The claims Leaf phase (light hrs) Flowering phase Total

1. Non CO2 4wks (28days @18hrs) ,12wks (84days @12 hrs) =16wks (1512hrs)
378 lights ON hrs/mth.

2. "Short Cycle" 7day (10cycle@14hr), 45day (60days @6 hrs) =7.4wks (500hrs)
Unbelievable result! Easy harvest and growth. 250 lights ON hrs/mth. Great result, especially that the grow room/crop management is also massively reduced.

3. Std CO2 cycle: 2wks (14days @18hrs), 10wks (70days @12 hrs) =12wks (1092hrs)
Fantastic crop. 364 lights ON hrs/mth. Easy harvest. Supporting evidence.
Indicated power usage for the short phase growing cycle reduces to about 250hrs/mth, from 378hrs/mth, a drop of 33%!
Power usage dropped to 66%, a saving of 34% (from $500/mth to around $328/mth).
This had a consequential effect of reducing residual heat in the room, nutrient and water tanks, with profound effects reducing mould and fungus problems.

These figures just blow me away. The total "light ON" period of 500hrs to get a harvest is amazing. That is only 2/3rds of the time and 50% of power that a regular CO2 equipped growing operation is using. In just 1/2 of the time at 1/3rd of the power, a better harvest than ever achieved with a 16week method.
For the flower seeds he is cropping, he has hit a crop rotation and growing efficiency that would make most people's eyes water.
Good luck to him.


Hope that this is of interest to all in Home Growing Heaven

I've not tried this method but it sounds really interesting. The only downside appears to be that 18 hour timers seem few and far between - you either need to get a mega expensive CO2 controller or an Australian company makes them for about £100 each.

Any other sources for timers would be great to hear about.

Find me a Dutch grow house incorparating these tech`s....... or a commercial operation with this cycle on vegtables ???? Ive looked you wont....... Sounds great in theory........

plants need the period of 24hrs because of hormone production...... over thousands of yrs plants have learnt to grow over 24hrs.... plants cells dont stop dividing nor hormones flowing and production of some hormones are done in the late hrs of the `OFF` period.
So i guess this is my justification in theory for why it would not work....
But please try, We hear talk of it all the time but no one has reported back on a result!
Good luck......

I know I know... but it's got me hooked & wanting to try it out. I'm a sucker for this sort of snake oil... LOL

I was thinking maybe running 2 seperate identical DWC buckets, using same cuttings in each one and see what happens. It should be quickly evident if there's anything in this 18 hour theory...

Unfortunately it's very hard to find the timers which is probably why so few people have attempted it. I can't find any for sale apart from the Australian company

Manual - would be a nightmare..... but do`able if you think your going to save that much.... just do it for a month see what happens - set an alarm on your phone or watch.....

Just use a 24 hr digital just do your MATH.....

Ive got one that has 90 programable slots for on off periods over 7 days...

Is it true that roots only grow at night, or in dark? Thanks

budder said:

Is it true that roots only grow at night, or in dark? Thanks

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Wives tail.......... cell division happens 24/7 in plants which is growth.... in both directions!......

yes, I'm going to bump this old thread up...Ive been thinking about this and with the added info, via Ed Rosenthal, that a "spray of far red light will kickstart the switch to active flowering hormone at "dusk"(without the far red light, ie: in a normal turn off the lights situation it can take up to 2 hours for the switch to happen) I wonder about even more shortening... and it only makes sense that exactly 12 hours dark is not neccessarily needed, logic implies that like most everything , it follws the bell shaped curve, where some do fine on 11 hours(for example) and some need 13 .. so it seems to me that in flowering anyway, you could put the plant on a 16 hour day perhaps, 10 hours dark and 6 hours light, cutting flowering by 1/3 ... that coould cut a 12 week to 9 or a 9 week to 6, yes I assume some loss in yield, but in some situations you have to play with the cards youre dealt. Would it increase chance of hermies? again, probably , but from waht to what percentage idk.
It's somethign to think about anyway, I happen to believe plants are pretty tough, from gardening for many many years and have seen plants grow to fruition,literally (oh.... is that where that word came from? light bulb moment ) in the worst situations
Ideas , Comments? suggestions?

That is a lot to digest all at once...lol. There are I would think many many things to consider, IMO the first and most important what stress would the plant suffer switching from a 24 hour day to a 16 hour day. The plant's life cycle is based on a 24 hour day by and large, it phyisology is set up this way. I would tend to think that this could truly stress a plant. I have been reading the past couple of hours trying to find how this might interupt the natural cycle of the plant. While plants do most of their growing during the night cycle and produces most of its sugars during the light I don't know what to think. I am not an indoor grower so I don't know this for a fact indoors but outside anyway you want more intense light for the tighter denser flowering. Seem like one of the draw backs could be a looser or more fluffy bud.
So, one thing I don't understand in your reasoning is the time you expect to cut flowering time? To me and granted I may not be the sharpest knife in the drawer but if you are cutting the light cycle so much you are cutting the time your plant has to produce the energy to grow your flower. I think that you would also tend to have a stretchy pale plant, but idk. Just from the reading I don't think it would be a benefit but would love to see the results.
Here is one of the sites I was reading and on what I have based my opinions.
http://www.hcs.ohio-state.edu/hcs300/

well, look at it this way, obviously everyone wants to be able to grow full size plants to completion but sometimes you just dont have that option and its a choice between not as much as possible really good smoke or none at all ... plus its fun to think about... so say you give a plant 14/4 for 3 weeks, then switch to 8 light/10dark for flowering ... would it just decrease yield or would it hasten the flowering process itself? Im speaking here of non-autoflowers... for autos, since recommend is 20/4 what would running them on 10/2 do? some say you can run autos 24/0 ... so would the quickened pace of physical light/dark periods (essentially running at 2x normal dalight speed) do anything?

Harbor Freight has a Digital 24 hour timer but you could do some math and time it for 14 hours on and 4 off it has a hell of a lot of on/offs. The hydro store harvest moon used to buy em at harbor freight (wich is right next door) for $9 and sell em for $25. I ended up seeing the harvest moon owner buying a bunch and she told me what was going on.

its interesting certainly, wish it was legal growing so I could do some fiddling around with it, someday ... at thispoint in my grow, I figure ihave maybe 6 weeks to go, so it would save me only 10 days , maybe... which is a lot to me now, but maybe not so much as to endanger what crop I do have, if I could do it to just one, I would definitly consider it

depending on how my soil plants do i will do this method to see if it really cuts back i would go from 16/8 for 1 week then 14/10 for 1 week then 10/14 so it begins to think its really becoming fall/winter for the one 1 week of 14/10 so it would be a gradual decrease or go down from 21/3 for 3 days 20/4 for 3 days 18/6 for 3 days 16/8 for 5 days thats a 2 week veg cycle and then 12/12 for 3 weeks then 10/14 for the last weeks of the flower may seem like alot of light stress but may be the key to fast gowing who knows ima go right this down before my high ass forgets

drpeppersolo said:

depending on how my soil plants do i will do this method to see if it really cuts back i would go from 16/8 for 1 week then 14/10 for 1 week then 10/14 so it begins to think its really becoming fall/winter for the one 1 week of 14/10 so it would be a gradual decrease or go down from 21/3 for 3 days 20/4 for 3 days 18/6 for 3 days 16/8 for 5 days thats a 2 week veg cycle and then 12/12 for 3 weeks then 10/14 for the last weeks of the flower may seem like alot of light stress but may be the key to fast gowing who knows ima go right this down before my high ass forgets

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in my area this is a slightly alterd scale of daylight hours we have so if i was to do a outside grow it would go from 18/6 to start 14/10 late summer then 12/12 for fall and 10/14 winter so it would be about the same if i set my lighting similar to that 2 weeks 18 1 week 14 then 4 weeks 12 and 4 weeks 10 this would save you slightly on the total coast ill have to dig deeper into this thank you for the brain food

I've read a bunch about this and it seems to work in theory. Im setting up a small test grow soon that I'm going employ this technique on. I'll be sure to post the link here.

well if you think about it your just acting more like the natural sunlight it should grow faster

This seems to have pros and cons, as all methods do. Obviously we all need to try it openly and see if the pros outweigh the cons. My bet is that if the plant is only used to a 18hr cycle from seed, it may adapt and grow as such, if its genetics predetermine a 24hr cycle than it will probably grow hermaphrodites. There are strains that are equatorial like lots of Thai strains, that veg on a 12/12 cycle and have a hard time flowering outdoor in the northern or southern Hemispheres because there is not enough darkness. These might be better for the 18hr cycles since they are used to less daylight to begin with.

This seems to be one of those genetics vs environment or a combo of both. I believe that if started from seed they will be ok and may work out with the OP theory, but clones carry the age of the mother, so if they are used to 24hr cycle and it changes to 18 they will probably hermie. Ill try some at my warehouse, we grow quite a bit here in CO-thank god for legality

Think what im going to do is start a mom from seed at an 18 hr cycle, cut come clones, and put 1 of the clones to a 24 and 1 to an 18. Trying to think of a way to start seeds in 24 and 18 but phenos and plant strength is variable. no way to get a control from the beginning, so this is my best idea. any suggestions or corrections to this?