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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Light Cycles and Flowering Cannabis, 4.4 out of 5 based on 127 ratings



  1. bobby August 23, 2012 at 7:34 pm -

    Hi guys,
    I’ll need to change my light schedule. Right now the light is ON from early in the morning to the night. for this weekend, I’ll need to put the light ON during the night and then get it back to “normal” cycle. Do you think it will be bad for the plant? By the way, i’m in vegetative mode and the plant is 9″ tall.

    • Rambo August 24, 2012 at 2:20 pm -

      I’m not certain of what you are trying to do but it sounds like you want to keep the plants in veg but you want to switch from running the lights during the day to running them at night and then back to running them during the day after the weekend is over? This won’t hurt the plants as long as the transition doesn’t give them periods of darkness over about 12 hours.

  2. gore August 25, 2012 at 1:30 pm -

    This concerns me, an abrupt change like this could create stress, and stress can create hermaphrodites. I think it may be safer to simply leave the lamps on through the plants “old” night, and when the plants “new” night comes around, begin the new photo-period. When plants are in veg, it’s always safer to burn lamps for more hours, rather than less.

    Furthermore, because of the way Marijuana plants photosynthetic metabolism works, the plants does not benefit from a dark period at all. Many people argue that their plants need to “rest” but this is a misunderstanding. Instead, throughout veg, periods of darkness for up to 6 hours serve to mitigate heat generation and reduce electrical consumption.

  3. chaz August 30, 2012 at 1:40 am -

    Hello have some plants that started to bud last week when do I harvest theres no male present looks to me like 2 different types of weed thanks 4 any information

    • Rambo September 1, 2012 at 2:47 am -

      You probably have between a two and three month wait ahead of you. I know it’s hard to be patient, but if you keep giving them plenty of light, food and water, they will take care of you as well. Once the buds have filled in, keep an eye on the trichomes and look for mushroom heads on them that are turning from clear to milky or amber. This will mean that the cannabinoid production is at its peak… time to harvest.

  4. Jimmy August 30, 2012 at 2:27 pm -

    How do I make buds more dense or longer life span to get bigger …..already got buds started small and want them to fill out as much as I can let them what’s the cut off

    • Rambo September 1, 2012 at 2:44 am -

      Jimmy, Just keep giving them good food and water and keep them on a 12/12 light cycle for another solid two months. They should do the rest on their own. You’ll know they are ready to harvest when the trichomes under magnification start to balloon up at the ends like mushrooms and look a little cloudy white or amber colored.

  5. Jimmy August 30, 2012 at 2:29 pm -

    Do I leave the lights at 12 12 or do I add more sunlight to let buds grow better?

    • Rambo September 1, 2012 at 2:42 am -

      Keep it at 12/12 through the end of harvest. While monkey business with light cycles is not totally foreign to me, and a solid grow technique can accommodate slight variations, keep it simple. 12/12 will get you where you want to go every time.

  6. Jimmy September 1, 2012 at 3:41 am -

    Any certain Food or fert I should give them ? What is the number for the fert ….got some 19-19-19 would be good for it or no?let me kno what ur advice is for flowering ferts

  7. Jimmy September 1, 2012 at 5:54 am -

    Also should I dim my light I have dim able blasts there is a 50% a 75% and a 100% what setting would be the bests for the flowering stage

  8. Jimmy September 1, 2012 at 6:19 pm -

    Would it bother plants that were grown outside if I bring them inside and finish the flowering with my indoor plants?

  9. gore September 4, 2012 at 7:10 am -

    @ jimmy

    Please bring your questions to the forum. The blog roll is intended to be kept on the topic of its hosting article.

  10. Jimmy September 4, 2012 at 12:51 pm -

    K sorrry

  11. zach September 6, 2012 at 5:51 pm -

    hello ,i have a qustion about my plants. today i was looking at my plants and i decideed i was going to pick 1 of them because it was looking pretty bad. when i went to cut the leaves around the bud i saw that there was like little bulb type looking thing and their was alot of seeds in most all of them, what is this and did i pick to early

  12. DENNY September 7, 2012 at 7:36 pm -

    ok this is my first run and i let my plant get about 2 feet them put it into flower heres the problem i ran both light cycle s 12-12 is there hope still and no i did not wait 36 hours before putting into flower please tell i am ok thanks

    • Rambo September 16, 2012 at 4:01 pm -

      Here is the problem? I don’t understand your problem. If your plants are two feet tall and you switched them to 12/12 then you did exactly what you were supposed to… 36 hours of darkness is optional. Its really not that complicated. If this does not answer your question, please clearly and articulately post your question in the forum

  13. tribud September 16, 2012 at 9:32 am -

    I am just now getting buds
    on mine is it to late will the produce still?

  14. kp September 16, 2012 at 5:34 pm -

    I have 4 plants that are in the flowering stage almost at week 6 their doing really well. Tall, green and nice juicy buds but I have a problem I live in an apartment and their doing inspections.. so my question is can I box my plants up during the lighting cycle and keep them in the dark a couple hours later to transport them and what’s the worst possible scenario when doing such things.

    • Rambo September 16, 2012 at 5:48 pm -

      It would be best to box them up while they are supposed to be in the dark period of the 12/12 but if you have to box them up during their scheduled light period it probably will not be a problem. Just be sure they don’t get to hot sitting in a car or something.

  15. Jason September 17, 2012 at 10:04 am -

    Your comment is awaiting moderation.

    Absolutely love the page, dedicating my whole grow op on your advice provided above, its my first indoor grow and im really excited with the results so far, only 2 weeks in (from germination) and they looking pretty. I just have 1 question; How soon is too soon to expose the plant to the 12:12 cycle? Growing in a attic (Roof), going to get a fan up there ASAP and not much to work with, read a comment where the guy said that by exposing the the plant thats at about 1 ft in height being exposed to a 12:12 cycle will cause it to double in length. Sorry if this comment doesnt make sense, quite high at the moment. looking forward to your comment

    • Rambo September 17, 2012 at 4:58 pm -

      Exposing the plant to 12/12 or beginning the flowering cycle will not in an of itself make the plants double in size. However, they will continue to grow in size after they have been switched to the 12/12 light cycle. Depending on the strain, the light intensity, fertilizer used, temperature, air flow and other factors, the plants very well may double in size or more by the time they are ready to harvest. With this in mind you should switch to flowering so they will finish with at least a foot of room between the light and the tops of the plants. Take the distance between the top of your growing medium and the max height of you bulb, then subtract a foot and divide by two. That should be about the max height the plants should be when you switch. You can always switch sooner, or wait longer and tie them down or top them for a wider plant with more tops.

      • Jason September 19, 2012 at 9:46 am -

        Great thanks for such a detailed answer, just about the “How soon is too soon for the plant to be exposed to the 12:12 cycle? I understand leaving it to become more mature yields better results but just want to know how soon i can let them “start” budding. Thanks again for all your advice. Regards from South Africa

  16. chaz September 22, 2012 at 4:54 pm -

    hi u talk about amount of light during bud stage does moonlight affect this ie. full moon. should I cover outside plants even with streetlights / neighbors lights etc

    • Rambo September 28, 2012 at 7:01 pm -

      Good question. Street lights and neighbors lights can definitely affect your flowering cycle. The moon on the other hand will not create problems with flowering. I assume this is because it reflects very little if any red light.

  17. killBill September 28, 2012 at 5:13 pm -

    No question just want to thank you Rambo and Mr Gore.Best advice I have found in all my studying.You guys ROCK!!!

  18. olly October 21, 2012 at 4:29 am -

    hi i have started flowering my plant 2 days ago and i didnt leave it for 36 hours i went to 12/12 is this okay.

    • Rambo October 21, 2012 at 9:29 am -

      Olly, Giving your plants 36 hours of darkness between the veg and flowering light cycles helps to induce flowering more quickly but it is not necessary at all. It is a “trick to jump start the process.” Simply changing from 18/6 to 12/12 will work just fine, it just may take a few days longer to notice a strong flowering response.

  19. Cunikeq187 October 22, 2012 at 2:39 pm -

    Hello there i might sound a little bit weird, i have started my little growing room, but i m stock at the lights can any one of you guys help me with this problem that i have, i wanted to know what kind of light i can use from the seeding light , till the end. HELP

    • Rambo October 22, 2012 at 3:25 pm -

      There are not weird questions. We are here to help if we can. If you want to use one light from seedling through the end of flowering then a High Pressure Sodium light is the way to go. These usually come in 400, 600, or 1000 watt sizes depending on the size of your room. HPS lights require a ballast to keep the light from drawing to much power and these ballast are available in two varieties. The older and less expensive style uses magnets while newer design is all digital. Digital ballasts produce less waist heat and make less noise and reportedly produce brighter a light. Go with the digital ballast if it’s in your budget. You may be able to find these used on Craigslist. As for the size of the light, it depends on your growing area. 1000 watts covers about a 4 foot by 4 foot area, 600’s can almost pull off that much growing area, maybe a bit less. The 400’s are probably best for a few plants, no more then a 3×3 area.

  20. Cunikeq187 October 22, 2012 at 3:36 pm -

    Thank you for responding back, k i got this , how about from the seed what should i use? Cfl are good for the first 3-4 weeks, and after that i can go with 600 watts Hps

    • Rambo October 22, 2012 at 3:40 pm -

      A CFL is fine from seed to the first few weeks. Your CFL’s will work fine if you keep them 6 inches from the tops of the seedling

  21. Cunikeq187 October 22, 2012 at 3:49 pm -

    one more question for today and i m done lol. Any kind of (led) can grow them right? and how about 120 watts led; are they good after i use Cfls for the first few weeks

    • Rambo October 22, 2012 at 3:53 pm -

      I have a feeling this won’t be the last questions. How about opening a thread in the forum

  22. uknome October 23, 2012 at 12:36 pm -

    I waited quite a bit longer than I perhaps should have to start flowering. (made switch at 21ins). and now i hav a plant quickly outgrowing its room. is there any way to slow growth down. It has been 2wks, im gettin sm buds forming but im worried it will outgrow the lights, which are maxed in heighth, before flowering is complete. any advice?

    • Rambo October 23, 2012 at 10:25 pm -

      I think most of us have been through this situation at least once or twice. Your best bet at this point is probably to try to tie the branches down. Here is an article I wrote that may help you out. You could also post your question in our forum and you might get some further ideas from the community.

  23. uknome October 24, 2012 at 5:14 am -

    Thanx alot for the link. I have already topped it about a wk and a half ago. it still seems to be stretching but seems lik it might be slowing down, i hope. If I pinch the tip now this far into flowering wil it cause a prob? And do u think the 4-12-4 fertilizer I’m using, mixed 75% strength is an ok food for flowering? It was the closest I could find in my area. There are no major grow stores near me. Sorry to ask so much, lol, iv just read most ur articles and they seem to be more benefitial, and accurate than most any site

    • Rambo October 24, 2012 at 9:25 am -

      The fertilizer you are using will probably work well for now, You might want to try and find something with a little K, or a K supplement. The plants probably will slow down the vegetative growth and no I would not suggest pinching tops this far into flowering. I think you best bet is to try to bent those branches down as much as you can. Happy to hear you have found our articles useful. We do our best.

  24. uknome October 24, 2012 at 8:12 pm -

    Is the reason u suggest not pinching tops so far into flowering because of losing weight/big bud @top or is it actually harmful/stressful for the plant? Not that I don’t trust ur advice, just curious about the reasoning behind it. Thanx alot. and also if u hav time, I was wondering if it was common in this type of plant, like other tall plants, to use some kind of stick/stabilizer rod stuck into the soil along the stem, or tied to a few inch away for support if it gets too top heavy? Sorry to ask for such detail, I’m just the kinda person who wants to learn all I can about everything. Thanx again

    • Rambo October 24, 2012 at 11:05 pm -

      Happy to help, but since your questions don’t have anything at all to do with the article, could you please ask them in the forum. Thanks.

  25. Lambsheep October 25, 2012 at 1:57 pm -

    I initially started growing my plant on a 12/12 lighting schedule. It has grown to be about 22inches and is starting to bud. Being I gave it no time to vegetate (18:6 ratio) I know it will affect the quantity, but will it affect the quality? In other words, am I still okay?

    • Rambo October 25, 2012 at 2:53 pm -

      Quality should not be effected, just yield. You should be fine

  26. Jason October 29, 2012 at 10:23 pm -

    Hi Rambo. i would just like to know, mixing lights is that a good idea? i have a gro-lux fluorescent light in the attic which gives off red spectrum its 2 40w bulbs as well as a LED which a mate of mine gave to me. i recently purchased a “Crop circle LED lights G73” will it be pointless in putting it up with the other lights will it prehaps mess with the spectrum of my new LED? your advice will be very much appreciated

    • Rambo October 30, 2012 at 12:52 am -

      Jason- Mixing light is not a problem and is even recommended in many cases. Florescent lights are often used for clones and vegging small plants. I have had great results using the newer high output T5 florescent arrays that run 8 of these bulbs. It should be noted that your “red” 40 watt bulbs are not actually producing only red light, but a mixture that contains more red then normal florescent bulbs I have honestly never used LED lights but the logic behind there use as grow lights is very solid. They reduce energy consumption by producing light in only very specific wavelengths. Because of this they may be able to produce as much useable Red and Blue light with 100 watts as an HPS bulb does with 1000 watts. Additionally HPS and MH lights waist a lot of energy producing heat, a problem that is minimized by LED semiconductors. Another property that LED and Florescent grow lights do not produce is UV light. There is some research that suggest that Trichome production on cannabis plants is a defense response by the plant to UV light. If this is true, you may find the potency of your harvest diminished. From the conversations I have had with several LED grow light makers, they just don’t produce the yield of an HPS or MH light. This may be in part because they are designed to hang above the plant canopy a foot or more and cover an area equivalent to a HPS light. 5 minutes of research into LED’s will inform you that LED lights are directional and that the light diminishes very quickly over distance. A proper LED array would be the same size as the canopy and placed no more then a few inches from the tops of the plants. I expect your yield will pay the price for not employing an HPS light but I would very much like to see your results. Please start a thread in the forum and keep us apprised of your findings.

  27. krisgottiboi November 4, 2012 at 9:36 am -

    hi there i was wondering i have my plant on 11 hrs lights n 13 hrs dark but can i switch to 12hrs on n 12 hrs dark will it effect anything are will it be ok ………..anyone with the answer will help ouyt lots

    • Rambo November 4, 2012 at 12:23 pm -

      An extra hour of light will make the plant grow faster and yield higher. You can probably switch without any problems but there is only one way to find out.

  28. Nick November 8, 2012 at 3:26 pm -

    I recently started to grow,I started from germination and the plant is standing at three inches tall when will i know the sex? Also if its turns out to a male plant what then?

  29. sneaks November 8, 2012 at 9:19 pm -

    hi. i just flipped to 12/12 to start flowering. i have been using a mh bulb. do i need to switch to a hps now or wait til i see signs of flowering. also, should i switch nutes for bloom now as well?

    • Rambo November 9, 2012 at 7:37 am -

      Yes. If you have an HPS bulb then now would be the time to switch to it and start using the higher P-K flowering formula fertilizers.

  30. julian November 24, 2012 at 10:32 pm -

    Question lot’s of them email me

    • Rambo November 25, 2012 at 10:01 am -

      Please post your questions in the forum so that I and the rest of the community can help you. This way others with similar questions can find help there and I don’t have to answer the same questions millions of times. Thanks

  31. Juju214Tx November 24, 2012 at 10:36 pm -

    I have 3 plants planted at different times one of which looks heart the second one looks real droopy the 3rd one looks okay but all of there first set if leaves done dried first time indoor experiment please E-mail me at the address above

    • Rambo November 25, 2012 at 10:03 am -

      It’s not uncommon for the first set of leave to dry up and fall off. This is normal. The plant will discard leaves as it no longer needs them. If you post some pictures in the forum we can help diagnose your plants problems.

  32. lee November 25, 2012 at 2:25 am -

    hi just wonderin if any 1 can help me. our plants have been up 2 week today they lookin good. the light as been on 24hour. wen do we change the time and do we feed them more food?plz get back to me asap.

    • Rambo November 25, 2012 at 10:06 am -

      when to switch from 24 hours of light to a flowering cycle really depends on how large you want the plants to be at harvest. Plants can as much as double in size so a 1 foot tall plant now could be 2 feet tall at harvest. Continue you to feed the plants but switch to a flowering fertilizer higher in P-K after you make the switch to 12/12. Hope that helps.

  33. pdot November 26, 2012 at 6:52 pm -

    do i keep the fan on in the flowering stage

    • Rambo November 26, 2012 at 7:30 pm -

      Yes. Air circulation is even more important while the plants are flowering.

  34. Islanderb December 11, 2012 at 8:58 pm -

    Is it too late to continue veg. state When buds are present. I had power issues and I’ve lost a lot of light time during vegetative state. Buds are now present. Since I’ve fixed the lighting/power issue. Can I continue 18/6 cycle. Since the plants are only 5″ to 8″ tall? Please help. Thank you

    • Rambo December 18, 2012 at 5:02 pm -

      You can try but to be honest the results probably won’t be great. If they are still so small you would be better off either continuing with flowering or starting over.

  35. Magic Dragon December 12, 2012 at 2:33 pm -

    Actually, I have some light filtering in my bud room from cracks in the door. Very little, not more than a 3/4 moon. As cannabis has been flowering in the great outdoors for longer than we have been cultivating indoors, it must have evolved to handle the light from a full moon. Either that or the moon is a relatively new phenomena. I also at times use a green light to access the darkroom. I have never had an issue.

    If just a brief flash can interrupt the plants, what happens in an outdoor garden that occasionally ‘sees’ car headlights?

    • Rambo December 18, 2012 at 5:08 pm -

      As for the moonlight, I suspect that it does not have an effect on flowering because if is not reflecting far red light. I suspect that the light leaking into your grow room will eventually cause flowering or herming problems. Outdoor gardens can be affected by car headlights as well as motion sensor lights often found on outbuildings.

  36. Ricky December 19, 2012 at 5:18 pm -

    Hey im a first time grower also and my 4 plants have started to bud but they are very short , I dont know If I should stay using the 18 hours so they can veg more or if I should just do the whole 36 hours without light then to 12 on 12 off

    • Rambo December 27, 2012 at 9:54 am -

      So you have plants that are flowering under 18 hours of light? This sounds like a strain that may not be reactive to light cycles but flowers based on the age of the plant itself. Was your plant grown from seeds or clones and do you know what strain it is?

  37. Rossta December 24, 2012 at 8:00 pm -

    Do you have any tips for growing in a greenhouse in winter? I am a seasoned grower but this is new for me. I put 25 bubba kush in my greenhouse with two 1000 watt hps. I have been vegging for three weeks and am going to start flowering next week. I plan on flowering for 8 to 10 weeks… Just before Spring starts. I live in a fairly mild client should I buy a heater or is 45to 55 degrees okay at night? Will the increased intensity of sun hurt budding. Any other issues I’ve missed? Growing outdoors at Christmas!

    • Rambo December 27, 2012 at 10:47 am -

      The low night time temperatures may stunt growth but the increased light from your HPS is a good thing, not a bad thing. Heating coils in the soil work great to help keep the roots warm. If your plants are in pots, heating pads will also do the trick. Keep an eye on your humidity when flowering and be on the watch for botrytis and mildew which are more likely in high humidity growing areas. Post some pics if the forum. I would love to see how you have it set up. Happy Holidays.

  38. baileybuds December 28, 2012 at 9:53 pm -

    I have my flower room that is approx 10’x20′ and right now I have 16 plants in flower slpit into two groups, the first group has about 5 weeks of flower time and i have had 2 1000 watt hps digital ballasts with vented hoods over them my second group I just completed a 24 hr dark and started thier first light cycle with the other group. Right now I just have a 600 watt hps digital over the first group. My question is what are the light requirments of plants as they progress through the flower cyle. when do the plants want and need the most light since i only have 2600 watts for the whole sixteen plants i want to knopw how best to get the most out of them. I also have light tracks and right now the 600 is on one over the second batch i just started flower the two thousands i have stationary since they fully light all the first eight with out being moved

    I hope this makes sense to you all
    thanks tons for your help.

    • Rambo December 29, 2012 at 2:40 pm -

      I really don’t know at what point in the flowering cycle the plants need the most light but I would assume that if you are mimicking natural outdoor seasons the plants would normally receive the most light at the beginning of flowering and it would decrease towards harvest when the days grow shorter. That being said, for indoor growing you want them to have as much light as possible the whole time. You did not indicate how big your plants are, but I hope you are not trying to fill you 10×20 room and then use only 2600 watts. Even with movers 2600 watts will only light an area of about 4×12 ft and I would prefer more like 3000 watts for that space.

  39. baileybuds December 31, 2012 at 7:43 pm -

    you are correct on both facts yes 2600 w would be way to small for total grow room space i provided, and i indeed neglected to say that that the two groups take up no more than one third the space(built big in hopes of law changes and also in hopes to grow larger and larger plants that being said my total flower are being used is about 6′ x 10′ } any way through further research I have found that your prior statement to make sense more light to start as nature provides so have repositioned so as first 8 get 1000 on track and second set have 1600 paired so as much more light per ft still would love all sugestions and comments, since i don`t think you can ever know enough when comes to growing pot and brewing spirits
    happy new year all
    again great site

  40. baileybuds December 31, 2012 at 7:49 pm -

    have also recently read that use of a tanning lamp during light and total blue during night at 100 watts per thousand watt grow light will help improve yield and potency have you or any one heard of this and is it worth it, also if worth it how about type and placement should it be close to canapy and tracked or higher so less intensity and more time on plants
    any and all help greatly appreciated

  41. baileybuds December 31, 2012 at 7:52 pm -

    just read post to put in forum area will repost there and look for reasponse there
    thanks for your time

  42. Mahayogi January 31, 2013 at 6:35 am -

    Hey there guys! I asked you a question a few months ago about photo-period interruptions, and now I have another similar question about the same thing.. So I had my flowering tent (located in the same room as my vegging plants) unzipped from the top to about ten inches down, to vent some extra heat. I accidentally fell asleep, and didn’t end up zipping it back up until just under a half an hour after lights out (the veg lights were still going outside the tent). My flowering plants are exactly one month into bloom at the moment… do I need to be concerned about this lapse in vigilance, or will they be fine as long as this does not happen again? I just need some reassurance; I’m sure you understand : ) Thanks – Mahayogi

  43. Rbz008 March 3, 2013 at 10:58 am -

    Gr8 helpful thread rambo. 1quick question. If a outdoor plant grown in winter was bought indoors to gain extra hours of light to stop early flowering, how much light is needed or does it just have to be not dark. Say 1 400w hps light be sufficient to do 12sqm

    • Rambo March 3, 2013 at 9:12 pm -

      In theory the plant being exposed to even a flash of red light should resit the plants systems and stop PFR from turning to PR. In practice, I don’t exactly know how much light is needed. It certainly doesn’t take much. A regular 60 watt incandescent bulb hung over an area about 4ft by 4ft is enough. I suspect your 400 watt would be fine, but consider rotating the plants furthest from the light to the center every few days for more even coverage.

  44. Rbz008 March 3, 2013 at 9:40 pm -

    Thank you very much for replying rambo. 1 more query, my outdoor crop been under rain for weeks now in and lots of leaves are goin yellow, not the normal yellowing. is it possible it could be myrtle rust. Using my mould formula of clove foliage spray and no improvement. I also sprayd catogry1 fungicide+ seaweed b4 this happened. Hope it didn’t weaken the plants defences. Wishi could show u photo. Myrtle rust is locally a problem atm in many commercial crops. Thanx 4your help

  45. Rbz008 March 7, 2013 at 5:08 am -

    Last question Mr rambo. I’ve had my winter crop veging under hps spread spectrum. Will changing to mh for remainder of veg stress them and if not would the results be noticeable. Thanx

    • Rambo March 25, 2013 at 8:32 pm -

      I’ve never tried this but I know a lot of people who switch from MH to HPS when they switch to flower with no ill effects so I am about 99.99% sure you should have no problems unless you strain is retarded.