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What are light cycles and how do I use them?


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The amount of time your garden should be exposed to lighting depends on what 'cycle' your garden is in:

The 'Vegetative Cycle' of your garden starts with the sprout of the seedlings and can be continued indefinitely. In the veg cycle your garden will require a minimum of 16-18 hours of light and 6-8 hours of darkness daily. Since a given amount of light can only do so much, equal production can be realized in a smaller space with less plants, where the light is concentrated and the plants can grow more efficiently. Using more light helps additional co2 uptake.

Since a plant can be kept in the 'Veg cycle' indefinately, many growers cultivate 'Mother' plants. This plant is used for clone starts and never produces buds, only new growth.

The 'Flower Cycle' or 'Bud cycle' is typically equal amounts of light and dark, 12 hours on, 12 hours off or 12/12. This produces a change in the plants metabolism simulating Fall, shorter days....less light.

This is the cycle that the plants will show their sex. Usually, you'll be able to determine the sex within the first 2 weeks of 12/12. By the 3rd week most plants have developed healthy bud sites or pollen sacks.
The plants will continue on the 12/12 cycle until harvest.

When someone 'Re-vegges' a plant that has been in the flower cycle, they're switching the light cycle back to 18/6 to stimulate new vegetative growth.

Author: Mr.Zog


New Member
My comments are meant for "light tight" indoor grows only. Somewhere in the myriad of threads I read about running your lights in the coolest part of the day. This gave me a reason for "which" 12 hours of the day I'd run my 400 w. HPS.
With summer coming soon, I'm training my flowering plants to a 12/12 grow cycle, starting at 9:00 pm each evening till 9:00 am each morning. I figure I won't be fighting so much heat as in the daytime.
Is this some obvious fact I've just become aware of or does everyone run their light cycles this way? Naturally 18/6 & 12/12 light cycles are normal, but I have been running them according to my daily schedule. Changing the schedule just seems logical and more economical.


New Member
I think Gadgetman is on to something here. I believe my plant went male simply because of the temp. I can't think of any other reason. We too have had our plants on our shedule, however, this grow was during the summer, and things went differently this time. I'm starting a new grow, and going to have the lamps running during the cooler night hours and see what happens this time.


On Vacation
i am also new and what i am trying is 24 for 2 day and 23.5/.5 for 2 days and so on and so on untill the 50th day when it is 12/12 and leave it at 12/12 good idea or just simply idiotic?


New Member
Never heard of this. why would you do this?
"plants take advantage of dark times to ramp up the manufacture of plant sugars that their roots store for use in making new cells for growth. It's kind of like a factory where the raw materials are being unloaded all day (i.e. water from the soil, light from above and carbon dioxide from the air), and then when the truck leave, the workers have time to actually produce the goods."
I got this quote from a plant growing sight. you should google why plants need darkness to fully understand the necessity of darkness for your babies.


New Member
Plants need dark to build up Florins (hormones the plant uses to flower)
I've only on my 1st grow but from experience & all my research, I think the safest bet is to: 24 hrs first 2 weeks max, then drop to 18/6


New Member
Light schedules are not complicated, so don't make them just that, the three main schedules are as follows

a) 24/0 or sometimes known as 24/7.......this refers to the light being on for 24 hours a day / or 24 hours a day for 7 days of the week for the desired number of weeks that plant needs to mature enough to be flowered,or to your desire, this is known as the "vegatative stage."

b) 18/6 this means on for 18 hours and off for the remaining 6, and as above till the plant has matured enough to be flowered, or the height you desire, this is also a vegatative light schedule.

c) 12/12 this means on for 12 hours and off for 12 hours, this scheduule is used to force the plant to flower, by mimicing the on-set of winter, the female plant will flower in the hope of receiving pollen from the male plant, and produce seeds....hence not having any males polllinate your females means you have "sensemilla" or seedless buds,this schedule is used in the "flowering" phase.


New Member
Just read your artical about lighting for my mmj. just picked up some clones and war clueless as to how to take care of them. I also read some of the helpfull comments. I have a greenthumb for plants, but, was fearful of the care and feeding of the herb. Thank you and the people who wrote comments, for the heads-up:thumb:


New Member
I have had my plant now for about 3 to 4 weeks,The main stem in middle is looking real good,There are leaves growing in the corners of the main stem(middle) and the branches.should I start 12/12?or wait till I get it about a foot taller.Its about 8 to 10" tall now.:smokin:I am trying to post pics,But till then if someone could give some info,would appriciate it.:reading420magazine:

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New Member
awesome. so when the plants are seedlings, they need at the very least, 16 hrsof light or more and the dark period is important as well? On one of these threads, it said that for the first 2-3 weeks they should hav light 24 hrs a day. we ll c i guess!!! :D


New Member
Thanks for your info MRS. Lonely. I have a green thumb , but when it comes to growing a weed, I temporarly out of my comfort zone.:thanks::thanks::thanks:


New Member
ive been veg'n for 5 weeks now and will continue to veg untill it gets warmer.
so ill be running 20/4 due to cold temps. veg time will then go to 18/6.
total veg time will be 14 weeks for wonder woman, and master kush.
they're around 7 to10 inches now w/ nodes right on top of each other.
so thick they're like green leather bushes. i run a 5200k MH 400w, and 400w hps w/4- 6500k fluors on clamp reflectors.
if the lights are'nt on i have to run an electric heater 1350watts. may as well keep the lights on. id use less electricty.
so im going to try n grow some trees. we'll see how it goes. peace


New Member
:bravo::thanks::thanks::bravo::thanks::thanks::thanks:thanks for the info. i am growing sour diesel, golden goat, and romulin, of the three of them the sour diesel is nom 29" high. the others are 20" and very bushy. i do welcome any growing advise. thankx again.


New Member
Hey gang, thought I would add another light cycle that I think needs a mention.

I have a small 1 plant grow under 6 240volt, 24watt CFL lights that I have raised from seed. I just went into the flowering stage last week but up until then I was using the 12/1 light cycle. This cycles save power by only having your lights on for 13 hours a day and off for 11 but it also keeps your plants out of flower.

Heres the idea.

12 hours on
5.5 hours off
1 hour on
5.5 hours off

From what I have read, the plants do a lot of their growing in the dark, and this boosted dark time is just what the Dr. ordered.
Also, the 1 hour of light breaking up the two 5.5 hour periods of darkness stop it from going into flower early by tricking it into thinking its a new day (for an hour).

So as I said at the start, I am just going into flower now in my first grow and my plant is looking really good. I have had several friends who are seasoned growers comment on how healthy she is. (looking at how the bud sites are developing we are estimating about 2.5/3oz from her. Not bad for a 800x600x350mm grow box)

My next plant I am going to grow on the 18/6 cycle in the veg just to see if there is a difference but so far I think 12/1 is the way to go just because of the power saving benefits...

Has anyone else had any experience with the 12/1 cycle?


New Member
How did this method workout for you?

I am about 4 weeks from harvest in an 8 week flowering cycle and just looking at how the buds are developing we (friends who have grown more than a few crops and my self) have estimated I should get anywhere from 3oz to 6 or even 8oz. I grew this plant from seed in a box in my room about the size of a LARGE computer box using CFL bulbs (6x 24 watt 6500k). Once the plant was ready to go into flower I moved it into a bigger box (small wardrobe) with a 90watt UFO LED grow light and 2 LED bars I took out of a night club lighting system as side fill lights.

This is the strain I am growing and according to this I should expect 350g per sq/m. I am growing in about half that and I'm expecting more yield. So I would def. recommend growers try this lighting cycle. Unfortunately this is my first grow so I have nothing to compare its progress with.
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