# Light Cycles - 24/0 vs 18/6 Photoperiods



## GrowinGreen (Mar 29, 2009)

*I found this useful-*

"It's been established for many many many years now that cannabis is a C3 plant. It does not need a dark period.

C3 plants gather CO2 only during the light period when they are photosynthesizing. As long as the light is on, C3 plants gather and use CO2 for photosynthesis.

Some growers practice a version of anthropomorphism with their plants. They believe that since people need rest, plants do as well. Concerning cannabis, this is not true.

Every grower can make a personal choice about light cycle. They can save on their electric bill or prolong ballast/bulb life. 18/6 can be less of a "shock" when changing over to 12/12 for flowering than 24/0 or heat issues can be addressed by fewer hours of light, but basic botany has proven long ago that cannabis needs no dark period.

Ed Rosenthal, Mel Frank and Robert Clarke all have covered this extensively over their careers.

hXXp://www.mellowgold.com/grow/mjbotany-removed/marijuanabotany1.html  Marijuana Botany Chapter 1 - Sinsemilla Life Cycle of Cannabis
hXXp://www.cannabisculture.com/articles/3127.html Need the dark
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"This is a direct quote from Ed Rosenthal whom most of you know is a marijuana growing guru:

_Marijuana plants photosynthesize as long as they receive light as well as water, air, nutrients and suitable temperature. Photosynthesis is the process in which plants use the energy from light (primarily in the blue and red spectrum's) to combine carbon dioxide (CO2) from the air and water (H2O) to make sugar while releasing oxygen to the air.

Plants use sugars continuously to fuel metabolic processes (living) as well as for tissue building. The plant combines nitrogen (N) with the sugar to make amino acids, the building blocks of proteins. They are the substance of plant tissue. When the light is off, the plant's metabolic processes, respiration and growth, continue.

The plant can photosynthesize continuously so it produces the most energy and growth when the light is on, continuously. Continuous light does not stress the plant, which reacts somewhat mechanistically to it.

Plants under an 18-6 light-dark regimen are producing sugar only three quarters of the time. They are thus growing at only 75% of their potential. Leaving the light on continuously will result in bigger plants, faster, which leads to higher yields."_

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"The following information is straight from Greg Green's "The Cannabis Grow Bible"

Cannabis is a light demanding plant. Professional growers keep the light on their plants using the 24/0 photoperiod for this reason. Plants that grow under 24/0 flourish and do not need a quantity of darkness in order to rest and perform photosynthesis properly. Plants that are grown in optimal conditions under 24/0 light regime grow vigorusly and the benefits of a 24/0 photoperiod can be seen actively in the results. More nodes are formed, more branches are created, leaf numbers increase, the plant is growing at its finest.

Some growers opt to use 18/6 as their photoperiod. This is 18 hours of light, six hours of darkness light regime. Under these conditions the plant will grow quite naturally but not as vigorously as the 24/0 photoperiod.

The 18/6 photoperiod expels 3/4 the amount of light that a 24/0 photoperiod does. Although this does not mean that a plant produces 1/4 less leaves,branches and nodes under the 18/6 photoperiod, it certainly does show the correlation between light and cannabis growth. As we have said already, cannabis is a light demanding plant. There are no problems associated with 24/0 and although some have attributed cannabis sexual dysfunction (the hermaphrodite conditon) to 18/6 photoperiod these problems are actually the result of heat stress.

A 24/0 photoperiod requires that your grow room temperature be kept well monitored. The 18/6 option is cheaper to run. You use a quarter less electricity and this will have an impact on your electricity bill. Also the 18/6 photoperiod will generally extend the bulb's lifespan. During the 6 hours of darkness the grow room is allowed to cool down for this period but a well maintained good grow room setup should not require a cooling down period.

24/0 and 18/6 both share the same problem though. *Once you start the photoperiod you should keep that way especially when the plants near maturity (the preflowering stage). An irregular photoperiod can cause more males than females to develop.* It can also cause sexual dysfunction to appear. Whether you choose 24/0 or 18/6 as your vegetative photoperiod try to keep that photoperiod unitl your plants are mature enough to express their sex."

*Sooo what light cycle do you use? And why?*


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## valleyboy (Mar 29, 2009)

24/0, couple of hours of sun exposure in there, the rest under the flouro's


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## Alistair (Mar 29, 2009)

Great bits of info.  Thanks.

One thing though, it was stated that the temperature should be carefully monitored.  I've always believed that it was necessary for the "night" period to be cooler by about 10-15 degrees F.  What if the temperature is within a reasonable range, but varies only by 5-7 degrees or so throughout the day?

Thanks again for the information.


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## Rockster (Mar 29, 2009)

Very interesting but I'll go by field trials not theory and stick to 20/4 in veg ta very much.

Had a chat with Ed last year at Spannabis the hempfair in Barcelona.

He was dressed in a wizards outfit.

I didn't think he was broke but there ya go?


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## Vegs (Mar 29, 2009)

I roll with 18/6 then 12/12 for just about every strain other then auto's which 18/6 for the first month then 16/8 for the last 5 weeks is my preference to strike a fine balance of saving on money and equipment life matched with the overall yield.

There is more then one way to skin a cat. =)


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## GrowinGreen (Mar 29, 2009)

Alistair Young said:
			
		

> Great bits of info.  Thanks.
> 
> One thing though, it was stated that the temperature should be carefully monitored.  I've always believed that it was necessary for the "night" period to be cooler by about 10-15 degrees F.  What if the temperature is within a reasonable range, but varies only by 5-7 degrees or so throughout the day?
> 
> Thanks again for the information.




Hey no problem- but with the temps I'm sure you are fine. Mine vary as well. During the day they can be 75-85 and during the night they can get down to 70-75. I don't think it is a big deal- I think it is most important that the temps don't get too high. I try to never let mine get above 85.

Someone around here might have some more info about this though.


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## GrowinGreen (Mar 29, 2009)

Rockster said:
			
		

> Very interesting but I'll go by field trials not theory and stick to 20/4 in veg ta very much.


So what did you find different between 24/0 and 20/4 with these field trials? Because I think of it this way- if it is known that marijuana is a C3 plant, meaning it only does its work when its getting light- why not leave the lights on 24/0? The only time I wouldn't do 24/0 is if I had money constraints, and this would only be true if I had a MH.

"Plants under an 18-6 light-dark regimen are producing sugar only three quarters of the time. They are thus growing at only 75% of their potential. Leaving the light on continuously will result in bigger plants, faster, which leads to higher yields."

"Some growers practice a version of anthropomorphism with their plants. They believe that since people need rest, plants do as well. Concerning cannabis, this is not true."


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## thedonofchronic (Apr 23, 2009)

thanks alot gg
great info


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## BuddyLuv (Apr 24, 2009)

I have done 18/6 and 24/0 during veg. The 24 hrs straight will give you tighter bushier plants but will also create lots of side branching. For someone like me that like short, straight, tightly spaced main cola type plants this makes it difficult to find a equilibrium when pruning, recovery time then flower. I just turn half my bulbs in my flouro's off now for 6 hours.


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## BBFan (Apr 24, 2009)

Good post GrowinGreen.

But I'm not a big fan of Greg Green- in his Cannabis Grow Bible he suggests that male plants of certain strains contain more THC than the female plants of less potent strains (not talking hemp here).  I just don't get that.


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## 7greeneyes (Apr 24, 2009)

I was under the impression that dioecious woody herbaceous (or however you spell that word) annual growing plants required dark period for root growth. Was I uberbaked or somethin when I read my horti books?


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## BuddyLuv (Apr 24, 2009)

7greeneyes said:
			
		

> I was under the impression that dioecious woody herbaceous (or however you spell that word) annual growing plants required dark period for root growth. Was I uberbaked or somethin when I read my horti books?


 
nope they grow just fine with 24/0


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## 7greeneyes (Apr 24, 2009)

When I first got into growin my own, I learned from "The Old Hippy", a ganja guru of the utmost. We all know one...and he had educated using old school methods. Strictly 18/6 veg and 12/12 for bloom...but now that I think bout it...I keep my clones on 24/0 then switch to 18/6 under my hid...wow....I gotta either start smoking less of this stuff...or alot more! lol!


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