Continuous Light On Marijuana Plants During Vegetative State

Cannabis continuous light vegetative stageI’ve seen a lot of people say that uninterrupted light during vegetative state is ideal for marijuana plants. In biology class, I was told that plants need both light and dark periods. Energy is produced when it’s light out and the plant uses that energy when it’s dark. Lab experiments have indicated that plants have a reduced growth rate under uninterrupted light.

Plants photosynthesize during the light period, but they also continue performing metabolic functions like respiration and growth. Observations of similar marijuana gardens on 18-hour light cycles and 24-hour light cycles indicate that marijuana itself does not need a dark period. The plants grow about 33% faster with continuous light than an 18/6 light regimen.


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i guess there will never be a definitive answer on this one.

for what its worth,…on my current grow, i left lights on 24/0 for 5 weeks of veg.
growth rate seemed normal to my somewhat beginner eyes.
in week 3 of flower now and i am very impressed with production.
im using diff lights and nutes compared to last grow so its hard to make actual comparisons.
but i certainly have not noticed any slowdown in growth.

ive read from lots of different sources that the only downside to 24/0 is more $$ on your elec. bill.

but the experts here (and elsewhere) claim the plants need to sleep.

ive got 4 more white widow just starting now. im going to do 18/6 and see if any difference in growth rate.
but i suspect there will not be anything noticeable.

Many grow 12/12 from seed to finish with some very,very good yields - really unbleiveable -i’m a believer 12/12 works well and one save’s money all so - perfect !!! Auto’s even do very good on 12/12 cycle - don’t knock it if you haven’t done it

The 18/6 should be much better for you. Your plant needs some darkness. I think this is when the plant will produces most of the sugars that it needs. I’m sure MaG or Lakewood will weigh in on this.

I’ve seen very slow growth in seedlings with 24hr.

I agree with Robert, I’ve always used a 24 hour light cycle to veg, and from what I’ve read I’m thinking when I do autoflowers I’ll probably try a 20 / 4

i wasn’t asking for advice,…simply pointing out contrast in many professionals’ opinions.

like i said 4 months ago…apparently there will never be a definitive answer on this one.

my thinking is,…it really doesn’t matter one way or the other. if it did,…there would be a “rule of thumb” and not this ambiguity.

Interesting, there is so many factors at play besides the light cycle. My cheap cfl lights in my closet grow works best at 6 hours dark with my old school seed sprouts. With my budget" i "mix soil and cheap Alaska fish fert. There is actual growth i notice during dark period. That’s my experience with what i have. Great read guys thanks.

Question from a newbie: If you do entire grow with a photo period of 12/12 how do you keep girls from going into flower? (non-auto)?


Still learning (2nd grow): I leave lights on 24/0 until seeds pop dirt, then I cut the photo period to 20/4 until veg starts (about 2-3 weeks) then go to a 18/6.for remainder of veg - then 12/12.

Thanks for all the info I get from you guys…really helps find what works best for my set-up (all from seeds and in soil)


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Many breeder’s use a 20/4 light from start to finish (Buddha Seeds) Many use an 18/6 from start to finish. The only issue with going to 12/12 from start to finish is smaller buds. So one has to use 5000K in veg stages (week 1-3) and 2400K in flower stages week 4-10. No matter what the plant has it’s own genetic time clock - no matter of light cycle.

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I just started a batch on 12/12 straight from seed just to sex them early. One thing I noticed is that they grow faster than the last which I had on an 18hr period. The stems thicken pretty fast too.

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You can grow from 12/12 seed to harvest with good yields, I mean tall plant’s also - use 5 gallon buckets, Some sativa strain it is recommended to veg for only 3 weeks and then flower. Some say that your bud are a bit smaller but never found that true (12/12)

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That’s good to hear. I have 18 growing on 12/12 and decided to put my mother in the tent with them because I’ve been so unsuccessful at cloaning her, now wasted what probably could have been a pound of bud and it’s got me so depressed.

forgive my confusion, but how did you waste bud?

by not blooming your mother plant sooner?
or by removing cuttings from mother for cloning?

By removing cuttings for cloning which never rooted.

thats my next thing, havent tried yet. doing tons of reading though.