Light Cycles and Flowering Cannabis

Rambo January 7, 2012 255
Light Cycles and Flowering Cannabis

In nature, cannabis plants (excluding ruderalis) begin to flower in the late Summer and early Fall. When the days start to become shorter and the nights longer, flowering signals are triggered by photoperiodism, beginning the flowering process. The increasing hours of darkness signal to the plant that Fall is just around the corner. The female marijuana plant shifts its energy from vegetative growth into flowering, in order to attract pollen and produce seeds.  By manipulating the hours of darkness, indoor and outdoor growers can delay flowering indefinitely or begin flowering whenever desired.

The traditional method used to sustain vegetative growth is to keep light on the plants between 18 and 24 hours each day. The key factor is not the hours of light, but the hours of darkness. With six hours of darkness or less, the plants will continue to put on vegetative growth as if they were experiencing an extended Summer.

When it is deemed time for the flowering process to begin, the light cycle is changed to a regimen of 12 hour on and 12 hours off.  The increasing hours of uninterrupted darkness cause the plants to respond as if Autumn is approaching, and begin to flower. By using these two light cycles, you can control when your plants flower.

Over the years I have taught a few people how to grow cannabis. In doing so, I have learned a few lessons myself. Simply telling someone what to do is generally not enough. Unless I also explain the reasons behind the practice, it is usually only a matter of time before they forget or decide to change things up. It also seems to be a lot easier for growers to diagnose their own problems when they have a firm grip on the reasons plants grow how they do.

The Science in Layman’s Terms

Cannabis plants are referred to as long night or short day plants, because they require a long period of darkness to trigger the hormones that tell the plant to switch from vegetative growth to flowering. These light receptors are color pigments in the leaves called Phytochrome Red (PR) and Phytochrome Far Red (PFR). These pigments get their names from the types of light they absorb. PR absorbs red light between 660 and 760 nm and PFR absorbs far red light between 760 and 800 nm. These two pigments chemically react to the light, and trigger the plant to flower or not.

This is where it gets a little confusing.

In cannabis plants, the normal presence of PFR switches off the flowering signal.  The level of PFR is what you can manipulate by adjusting the photoperiod.  PFR is quickly produced when plants are exposed to light that contains far red wavelengths. When there is light, the PFR and PR maintain a balance. When the sun goes down or the lights go out, the darkness gradually switches the PFR to PR. Because of this, PR levels gradually increase and the PFR gradually decrease during the dark period. The presence of PR is a neutral condition to the plants and essentially tells them nothing. When the light returns, or if a small amount of far red light interrupts the dark period, the PR immediately switches back to PFR. If the plant is without light long enough, the PFR will decrease past a tipping point. This decreased level of PFR signals the plants that Fall is approaching; and the plant begins flowering.

In short, the presence of PFR due to long hours of light and short hours of darkness keep the plants in the vegetative phase. If the plant experiences enough hours of darkness, most of the PFR turns to PR  ; and the low level of PFR signals the plant hormones to begin flowering.

Practical Applications

Enough with all of this scientific mumbo jumbo. Let us look at how we can take advantage of photoperiodism.

How to Induce Flowering

Because of photoperiodism, you can easily induce flowering in cannabis plants simply by changing your grow room light cycles–reducing the light from 18 hours a day or more, to only 12 hours each day. Again, what is really important to the plants is changing from six hours of darkness or less to 12 hours of darkness. Within two weeks of switching the light cycles to 12 on and 12 off, you should see small buds starting to form.

I need to stress the point that your plants are very sensitive to any light during the dark period. If you have any light at all leaking into your grow room, during the 12 hours of dark– even momentarily–the PR can change back to PFR.  This means that any light reaching your plants during the dark period may sabotage the flowering process.. For this reason,  you should never–not even for a moment–enter your grow room when the lights are turned off in the 12/12 cycle.

Before you make the switch to a 12/12 light cycle, you need to make sure your grow room has no light leaks. Enter the grow room while the lights are out and close the door behind you. Wait about 15 minute minutes for your eyes to adjust fully, then look around and make sure no light is entering the room. No light under the door, through a curtain, or shining from a CO2 generator or dehumidifier. I have made this last mistake myself. If any of your equipment produces light from the display, cover it with duct tape, making sure no photocells have been accidentally covered.  If you can see your hand in front of your face, you have a light leak that needs to be fixed.

While simply changing to a 12/12 light cycle will induce flowering, there is a trick to jump start the process. Between the switch from an 18/6 to a 12/12 light cycle, let your plants sit in total, uninterrupted darkness for 36 hours. This will cause the PFR to drop substantially, giving the plants a strong signal to flower. After the 36 hours of darkness, begin the 12/12 light cycle. In a side by side experiment, I saw significant results from this method. Be sure to flush out high nitrogen “grow” fertilizers from your growing medium and change to a high-phosphorus “bloom” formula. Also, adding high potassium supplements for the first two weeks can help increase rapid bud development

Light Deprivation For Outdoor

Through manipulation of photoperiodism, growers can induce early flowering in outdoor cannabis plants. It is much more difficult to make the great outdoors artificially dark than it is to turn off the lights–but not impossible. Many growers have perfected the art of light deprivation, and used it to harvest their outdoor crop in midsummer, or even multiple times each year. This can be achieved by building a garden that can be covered after the sun goes down, and then uncovered part way through the morning. If timed correctly, this can lengthen the natural night to a full 12 hours of darkness. The garden must be covered for several hours each day without exception, through the entire flowering period; but the effort can bring impressive early harvests.

Spring Clones

I can’t count how many times I have received urgent phone calls from alarmed acquaintances who planted clones outside too early. Despite what anyone says, clones can do amazing things if properly grown outside. Unlike seeds, however, you need to be aware of the hours of natural light when planting outside. Because clones are likely accustomed to 18 or more hours of light, they often begin to flower once placed outside in early spring. It will depend on your latitude and the strain; but at least in California, the nights are usually too long to plant clones outside before mid-May.

By using supplemental light to decrease the hours of darkness, you can plant your clones outside as early as you like. Simply clip a florescent light with an aluminum reflector onto a stake or cage around the plants. If the light shines on the plant for even a few hours after the sun goes down, it will usually be enough to prevent early flowering. If you are afraid of late frost, you can use an incandescent bulb instead, and also take advantage of the heat it generates.

Power Outages & Light Interruption

Even with light timers and a fully automated grow room, sometimes things go wrong. If the power goes out, or you need to change lights around, keep your light cycles in mind.

When your grow room is running on a 12/12 flowering cycle, a short power outage is not a major problem. A few extra hours of darkness will not really mess things up. Of course, the plants will not grow much without light; but they should be fine for at least two days. Any longer than two days, and they will start to suffer. This will not kill them, but may cause them to become stressed.

When running on a 18/6 vegetative light cycle, a power outage resulting in long hours of darkness can trigger the plants to flower. You need to find an alternative source of light for the grow room. An electric or gas camp lantern will be enough to prevent the PFR from dropping. If you need to change things around and interrupt your normal dark period, just leave the lights on until the following dark period. A few hours of extra light will not hurt anything. Remember, you do not need enough light to keep them growing–just enough to make it not dark.

  • To maintain vegetative growth, use 18-24 hours of light; six hours of darkness or less.
  • To promote flowering, alternate 12 hours of light with 12 hours of darkness.
  • When switching light cycles from vegetative to flowering, first give plants 36 hours of darkness.
  • To harvest outdoor plants early, cut the time light reaches the plants to 12 hours each day.
  • To plant clones outside before mid-May, supplement with artificial light at night to prevent early flowering.
  • During 12/12 flowering, additional dark hours are acceptable if necessary.
  • During 18/6 vegetative growth, additional hours of light will not cause problems.

Successful marijuana growers know how to manipulate the environment of their garden. Once you become comfortable with light manipulation and photoperiods, the sky is the limit.

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Rating: 4.4/5 (129 votes cast)

Light Cycles and Flowering Cannabis, 4.4 out of 5 based on 129 ratings



  1. ivyyt March 7, 2013 at 9:26 pm -

    i had my plants on 12on12of for 20days i was told to set them back to 6off18on wil it hert my plants. my seeds “last minutte” thank you

    • Rambo March 25, 2013 at 8:35 pm -

      If your plants have been on 12/12 for 20 days they should be starting to flower. Changing them to a 18/6 is going to freak them out. They probably won’t go back into full veg for quite some time but they probably won’t flower out right either. Why would you do this? As for “my seeds last minutte” I have no idea what you are talking about.

  2. WillyC April 10, 2013 at 2:36 am -

    I have heard I can use green light to go into the Flowering room. Is this true?

    • Rambo April 17, 2013 at 8:20 pm -

      Yes this is true. However, very few rooms are set up so that you can open and close the door without light getting in.

  3. Borobudz April 17, 2013 at 7:21 pm -

    To make a long story short, I have 2 northen lights autos that are about 7in and 4 weeks old and have been in a 12/12 cycle for 6days because of other plants. Is it too late to go back to veg or just continue with the 12/12 and see what I get? The plants already developed their pistils.

    • Rambo April 17, 2013 at 8:32 pm -

      After 6 days I would probably leave them in Veg. They will probably continue to veg to some degree under the 12/12 lights anyway.

  4. andre April 18, 2013 at 12:56 pm -

    hey noob here…Doing my first indoor grow with a bubble system…Trying white russian atm but im looking to aquire a strand that will produce the fastest…Can I please ask which strain you would recommend and where u recommend getting the seedlings from…It must still be a great herb…thx in advance for any input

  5. Foot Johnson April 26, 2013 at 2:56 pm -

    I just started inducing flowering, switching from 18 hrs of light to only 12. 2 weeks later and not very much has changed. i dont notice any buds starting to form. did i induce flowering too early?

    • Rambo May 1, 2013 at 11:00 pm -

      Give it another week, the plants will flower even if it is to early. Some strains take longer than others. It’s also possible you have an auto flowering strain that does not flower based on light cycles. In that case it will flower when it is ready regardless of hours of light. Uncommon, but possibly if you started from seed. Impossible if you started from clone.

  6. nunya May 1, 2013 at 3:27 am -

    is this post still being monitored? i have some crucial questions about starting indoor with some artificial light(flourescent) and then moving to outdoor


    • Rambo May 1, 2013 at 10:53 pm -

      Yes, Sorry for the delay, We get a lot of questions on here and unfortunately I have a million other things to do since this site produces nearly zero income and the bills don’t pay themselves. I might point out that while everyone seems to be using this as a place for questions and answers, the comments section of a blog post are meant for comments. While I don’t mind answering questions the forum is really the better place for it. That way others can post answers as well.

  7. Harry May 6, 2013 at 8:44 am -

    Thanks Rambo for a very informative site. Been growing many years outdoors in nor cal sierra nevadas, 3500′ elev. This year, I started a little earlier than normal, got good clones from Harborside, went outside into a greenhouse. Yep, you guessed it, I’ve got a lot of plants showing flowers. The blue dreams and amnesias are OK, but others not so much. After reading your info, I should have had a light on for a couple of hours in the evenings in my greenhouse. I will start tonight! How many hours do you think I should add now to reverse flowering back to veg, and would flourescent be better than incandescent, or does it matter that much. Thanks so much!

    • Rambo May 7, 2013 at 11:25 pm -

      Since you are trying to draw them out of flowering and back into veg, I would probably give them 24 hours of light until about June if possible. Fluorescent or incandescent both work floros are probably less expensive but make sure you are using ones that emit some red light.

  8. inandout May 11, 2013 at 10:26 am -

    Need some advise from someone urgently!!!!!

    i’ve got 10 nice ones going. they’re original skunk started from seed but i have a bit of an urgent dilemna.

    they’re about 16-20 inches tall. i have been keeping them on 18/6 with flourescents, obviously indoors. I put them outside during daylight hours to get sun.

    but now i have to sex them and reduce the number down to 2 or 3 for the sake of space. what is the best way to do that? keep using artificial light and reduce to 16/8 maybe?

    or can i stop flourescent altogether, and put them outside and go with normal daylight hours? nights are mild and right now a day lasts 14 hours here. so if i put them outside the days will be getting longer for another 5 weeks, and maybe they won’t show their sex.

    thanks in advance for quick reply. if somethings not clear let me know!

  9. Janis May 18, 2013 at 8:08 am -

    I’ve heard that once you switch to 12/12 that the plant needs to have 12 hours of uninterrupted darkness and that even a crack of light can throw them off flowering. Is this just because they are indoors or are we to assume moonlight doesn’t affect the flowering process when growing outdoors? Or is moonlight just different from a standard house light?

    • Rambo May 22, 2013 at 11:38 pm -

      This is a good question. Moonlight is reflected sunlight so the color spectrum should be about the same as sunlight. This means it should produce far red light which should change PR to FPR telling plants not to flower. I suspect that there is so little far red light reflected that it has a negligible effect on the plant. This would be a good questions for a botanist.

  10. prickles May 19, 2013 at 5:37 pm -

    my timer screwed up and gave my week 7 flowering plants an additional 9 hours of dark before i discovered this and turned them on.
    i have gone back to the regular cycle and am assuming that this minor hiccup will do no harm!
    your opinion?
    regards, prickles

    • Rambo May 22, 2013 at 11:28 pm -

      I think you’ll be just fine.

  11. AKA moneyman May 23, 2013 at 7:52 am -

    when you start the seeds what is the light cycle and how long be for you change to 12/12 cycle.This is my first time growing indoors and outdoors

  12. Dan Murphy June 6, 2013 at 5:21 pm -

    If my clones do start flowering early, is there any way to return them to vegetative state?

    • Rambo June 10, 2013 at 10:48 am -

      24 hours of light and a prayer

  13. jimi June 10, 2013 at 7:56 pm -

    skunk #1 how long do i need to veg before switching to 12 12 i grew back in the early 80s for 3 or 4 years and grew northern lights and nl#5x haze with great results in dirt and some in a ebb & flood in rockwool also. back then the genral consenses was 2 months veg and flower till finished now a days i see claims of much lower veg times im using deep water culture and im impressed after only 10 days i have alot of really healthy roots and incredibly fast growing plants from seed so i need to sex them but my goal is to cut 10 to 20 clones per plant but at this growth rate 2 months of veg im going to run out of space height and width i started with 10 skunk #1 seeds regular i want one male to grow my own seeds in case i need to start over and then start with all clones main question will 1 month of veg be enough for the plant to mature enough to start flowering with this strain. i have always been under the impression that when starting from seed 2 months of veg was manditory thank you

    • Rambo June 17, 2013 at 11:24 am -

      This involved questions requires an involved answer and probably some discussion. Please post it again in the forum.

  14. Tewks June 24, 2013 at 4:14 pm -

    i love your web sit,let me get to it” I am a first time grower and i started out with 50 plants… not sure what kind ? they look great. But I started putting them in darkness for the 12 /12. But it seems like every 3 to 4 days i get a male or 2.” Should i be worred? Or is this commen??

    • kram July 7, 2013 at 11:21 am -

      I suggest asking this question in the forum where moderators can help you.

  15. Gary Rose July 25, 2013 at 11:53 am -

    Can u have 18 hours of darkness n 6 hours light in budding stage

    • Rambo July 31, 2013 at 7:25 pm -

      Sure, but why would you want to do this? The plants need light and with only 6 hours your plants won’t reach their full potential.

  16. buddie July 30, 2013 at 11:23 am -

    Plants currently in 12/12 from 7 pm to 7 am. Because of job, need to change photoperiod from 9 pm to 9 am. What is the best way to transition, or will I lose my plants? (Plants in various stages of flower, one new plant each week).

    • Rambo July 31, 2013 at 7:19 pm -

      Just make the change. You don’t need to do anything special. The plants will be fine.

  17. Jennifer August 3, 2013 at 6:11 am -

    After the 36 hours do i go back to light for 12 hours? Then dark for 12 hrs??

  18. cmsierra September 3, 2013 at 9:41 pm -

    Question, in a hydro grow, we were vegging under a 400 light until we get ready to switch them into a different room. The new room has a 1000 light. We lost 4 plants the first time and thought it was due to improper rinsed coolers but this new round is starting to look the same yellow wilting leaves. Is it possible taking them one light to the other is causing this “shock”. Our first 2 grows we vegged under the 1000 and then bloomed under the 1000. the first crop was okay but the second was amazing. Just wondering if you have known of other indoor growers having this issue. The strain in Thai OG and the room is temp and moisture controlled.

    • Rambo October 2, 2013 at 9:20 pm -

      Sounds like some kind of shock. you could ease into the new room by raising the lights as high as possible for the first few days.

  19. planebill2453 September 8, 2013 at 9:22 am -

    Best Forum I’ve seen!!!
    I’m growing two plants outdoors since April, in NJ. They are 3-4′ high and starting to bud. They get a lot of light and it seems we are on a 12/12 cycle now naturally. How can I increase bud production and growth? I have about 4 weeks till first frost, and would like agood harvest. I’m a chronic pain type from serious accident, so its important to me.
    Also, is it safe to buy seeds online? Thanks a bunch.

  20. sperk October 25, 2013 at 5:39 am -

    Hey. This is my 1st grow(hydro). Im confused alittle bit on a few things. Im using, rockwool cubes and ph is perfect. I know pretty much about he ligehting from. recnt comments. After how many weeks do i change from18-6 to 12-12 the strain is indica and do i changr lighting again?. Also have nutrients do not have a clue wjat to do and how much 2 put in? I have root juice bio bloom and bio blossom can someone help out on wich weeks to do. and how much if any? 600w pellets and soil, do i use just pellets or soil and pellets? Hope to heer asap cheers!

    • Rambo October 29, 2013 at 7:01 pm -

      We would love to help you out but this is pretty involved so please take your questions to the forum. Thank you.

  21. Steph November 13, 2013 at 11:21 am -

    Hi, all this information has been so helpful so far. My plant is 5 weeks after first appearance and she is on 24/0 cycle. Im going to start flowering her now and was wondering should i move her straight to the 12/12 or try 18/6 for the first week? she’s about 22″ now and gorgeous bright green. She’s white widow.

    • Rambo November 19, 2013 at 5:46 pm -

      straight to 12/12

  22. Taylor fArms December 3, 2013 at 1:53 am -

    hey there. I i have a quick question on the subject of artificial light for flowering. have you ever encountered a problem with the 13 hour light cycle on, causing an overload of Prf that the plant cannot convert within thee 11 hours of night? i have read another article of yours on sativa strains needing more darkness than light. i believe my hybrid strain is sativa dominant. if you can help me at all it would be appreciated. i don’t have any of these to experiment with or i would. any tell tell signs of over lighting that you know of would be a great help. im going to revedge a few to try it my self but you could help prevent some loss my way.

    anything would be appreciated

  23. martin December 17, 2013 at 2:31 pm -

    Hey so my friends got a nice cycle goin BUT he over did it on the dark cycle during seed formation anyone know if that’s bad???are the seeds still viable

  24. brett December 31, 2013 at 1:26 am -

    Just woundering first tym grower,Can i cover my plants in complete darkness to make them flower early if growing out doors.Just woundering as I know u can do this in doors but using 12/12 after ? cheers

  25. Average Joe January 6, 2014 at 1:31 pm -

    Im using small 20 watt xenon bulbs and my plant flowered without a change in the light cycle (16/8)
    I looked into it and the light in the red and far red is very low with xenon.
    Do you think I could just leave the lights on for a straight 24 hours during my budding since the chance of production of pfr is very low?

  26. Sparky January 14, 2014 at 8:43 am -

    I’m a bit paranoid about my smart meter and considered flowering with a photosensitive switch, the kind that turn on at dusk and off at dawn. This would conceal my electrical consumption to look as though I have a 300W safety light outside.

    My concern is that as I induce flowering at this current date in January, I’d have longer periods of day light. There would always be at least 12 hours of dark but it would gradually be increasing, probably from 13/11 to 12/12 by harvest.

    Any ideas what this might do to my plants?

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  28. Dominik February 12, 2014 at 10:21 pm -

    Is it true that urine helps bud production? I’m worried that my neighbors are going to catch me on my balcony with my pants down, so to speak. I did have enough sense to add my manure to the soil inside where they can’t see me but the plants are too large now to easily bring them in for urination.

  29. mikehay March 14, 2014 at 11:37 am -

    I just want to thank Rambo I am a newer grower and your spot on my harvest was great thanks to your advice

  30. Dan March 15, 2014 at 3:33 am -

    Ive got 4 babys right now under flourecent lights and ive had umm for about a week and there growing but there not producing any more leaves theres still only 4 and the leaves arnt getting any bigger. can someone tell me if this is normal and if not what should i do.

  31. Cris March 24, 2014 at 11:43 am -

    How do I know what strain I have?

  32. Pineapple x Buddha April 2, 2014 at 9:08 pm -

    You got a lot of things wrong and a lot of things right. I am developing new ways of cultivation. The end results are fabulous. Never ever bud a plant when it is only 20″ tall. You are robbing it of its glory. Go big or go home. Graft vigorous root stock on your clones and put them in no less than 30 gallon pots. Let them get big. Then pop then into veg. Also prune your shit constantly. If you wanna enter your grow during the dark period use a dim head lamp, it will not affect a thing. Go completely organic as well. When they start to develope buds girdle the main stem. Never used LED lighting and use 1000 watts per plant.

  33. Deborah April 9, 2014 at 12:02 am -

    Hi, new grower with quick question. My girls got started on there flowering accidentally on a day 12/12 schedule , now. My room gets way to hot, so is it going to hurt them if I leave there lights off long enough to put them on a light at night schedule ? Like 19 hours not 12 ? Just to switch them to on at night time, so just a one time trauma. I’ve been stressing trying to find out what is right, so I don’t hurt them in any way.
    Thank you so much !

  34. Pete May 3, 2014 at 10:33 pm -

    For the first time, this season I obtained small rooted cuttings about 2 weeks ago. I put them into 3 gallon bags and onto an 18/6 light cycle. They will be transferred into 100 gallon grow bags outdoors to then carry on with just natural light until harvest. In previous seasons I obtained quite large plants which were put into the outdoor grow bags towards the end of May, blissfully unaware of how these plants had been grown, or of anything to do with the plant response to day length, and had quite good results with yields of up to 4 1/2 lbs per plant. My clones this year are rapidly outgrowing their current home with the 18 hours of light, and although I’m not wanting to transfer them outdoors quite yet, the natural light where I am (39.11N) is never even close to 18 hours and I certainly don’t want these plants to go straight into flowering mode when I put them out, with the significant reduction in light that will mean. Should I start now gradually reducing the light from the current 18 hours, or what do I need to do to ensure uninterrupted vegetative growth continues until the plants know that the natural autumn has arrived?

  35. bob May 5, 2014 at 9:00 pm -

    how often should I water when flowering

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  40. Higgins July 2, 2014 at 6:38 am -

    New grower here! I found a see in one of my bags so I planted it and it is two weeks old today and looking bright green about 3 inches tall. When will I be able to tell if it is male or female? how much longer? or will it need to go into flowing before I can tell?

  41. Gmac July 8, 2014 at 2:58 am -

    I have already started my 12-12 cycle, going on day 5
    then i read about the 24 or 36 hr lights out
    can i still do it ? or is it to late ?
    new grower and wanting to learn it the right way, the first time
    plants are 32″ tall and hella healthy
    7x5x8 room under a 600 watt air cooled light
    would love some thoughts on this

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  43. brucie86 July 16, 2014 at 6:31 pm -

    I have a question I have 2 growing beautiful I just switched them to the 12/12 cycle an I was readin
    And it says to leave in dark for 36 hours now it wont hurt them being in the dark for 36 hrs n do I water them during the 36 hr dark cycle

  44. Jore August 6, 2014 at 1:22 am -

    Yo! So when to change from 12/12 to 11/13? Im harvesting in about three weeks from now.

  45. Dale White August 9, 2014 at 9:13 am -

    I have a 8 ft tall blue dream plant outside grow. I live in a condo that has outdoor lights at night will this affect my flowering