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Light Cycles Explained

Many flowering plants, Cannabis among them, are what are known as "diurnal" plants—they use the cycle between day and night to trigger their growth patterns. This is important to understand when growing plants indoors so that your crop management takes this cycle into account.

There are two phases to the growth of most flowering plants: vegetative and flowering. The vegetating cycle focuses on establishing a solid root system, a strong main trunk, and ample foliage to absorb the light that’s essential to the photosynthetic process.

If you're starting from seed, you can get by with less light intensity until active growth starts; heat is more an issue. Keep the seed bed warm, but not hot, and give enough light to nourish the seedlings without scorching them. If you're starting seeds full-spectrum LED grow lights are a good choice because they give all the light needed without overheating concerns. Insufficient light will result in tall plants with long internodes, so don't use a weak light that causes the seedlings to reach for it, creating "stretch."

Once plants are established and in the vegetative phase, they require plenty of light in the right frequencies to stimulate growth. Outdoors, the sun provides more than enough light in all frequencies, but indoors, it’s up to the grower to give plants the quality of light they need. Leafy plants like Cannabis want a good amount of blue and red light in the proper wavelengths for optimal growth and bud production, and while mixing various light sources can approximate it, the simplest and ultimately most economical way is through the use of properly designed full-spectrum LED grow lights such as our LED grow lights featuring the Phyto-Genesis Spectrum™.

A lesser known and often overlooked process in plants is what is known as phototropism. It's the process by which a plant locates its light source and turns toward it to gather the most energy. It's triggered by blue light, and an insufficient amount will cause the plant to stretch toward the light source and become leggy and weak. This is an important consideration in your choice of lights because high pressure sodium (HPS) lamps don't emit enough blue light for proper phototropism. You can solve this with metal halide lamps during the vegetative stage, but here again, full-spectrum LED lights eliminate the need to change light sources during the growing cycle, which has other benefits as well.

While Cannabis plants don't have a "sleep cycle" per se, many growers feel that at least some time in the dark lets them relax and catch up on some other processes that improve plant quality such as root development. At the minimum, Cannabis plants require less than 12 hours of dark to stay in the vegetative cycle, so a good approach is 18 hours on and 6 hours off during vegetation. At the very least, it will save considerably on energy used for lighting and ventilation with very little effect on plant growth.

The flowering phase in Cannabis is triggered by the light/dark cycle. It is started by changing your cycle to 12 hours on and 12 hours off. During this phase, the plants will continue to grow vigorously and require even more light because of their size. Some growers switch to HPS at this time because the plants need more red light than they did previously, but with full-spectrum LED grow lights, the need to change sources is eliminated, saving grow light time, bother, and money as well as the need to control the large amount of heat produced by HPS lamps.

While it's possible to give plants too much light, it's not likely with an indoor grow room because you would likely overheat it and waste energy. Practically speaking, a minimum of 37 watts per square foot is a minimum, and 65-75 watts is your upper limit for our LED grow lights. Keep in mind you need more watts per square foot when using other lighting technologies such as HPS or MH, or even when using other LED grow lights that aren’t fully optimized with the Phyto-Genesis Spectrum™.

Rather than bogging down in botany, keep your growing simple by following three guidelines: use full-spectrum LED grow lights for all cycles, keep them on at least 18 hours during vegetative growth, and cut back to 12 on and 12 off for flowering. Then sit back and enjoy your harvest.

LED grow lights are the clear choice for serious growers of indoor vegetation of nearly any type. For 5 years Black Dog LED has set the standard for high-power LED grow lights, combining performance, design and innovation unmatched in the indoor garden industry. We are a 100% research-based company whose mission is to design the best indoor plant grow lights, outperforming all other types of HID, induction, plasma, fluorescent and LED grow lighting.

All Black Dog LED lights come with a Limited Lifetime Warranty, so why waste your time and money on outdated technology and multiple light sources when you can have industry-leading Black Dog LED grow lights and grow room accessories today? Simply use our store locator to find a Black Dog LED dealer near you.

18 thoughts on “Light Cycles Explained”

  • Karl

    how long are your lights guaranteed? what's covered and what's excluded?
    for how many hours will the emit their originally rated spectrum and intensity

    • Kevin

      Our PhytoMAX lights have a 5 year warranty, and the Universal Series lights have a full 3-year warranty, during which we will fix any failed parts for you according to our terms and conditions spelled out on our website.

      Like every other kind of light, LEDs degrade with time- they just do it slower than other lighting technologies, as long as they're in a properly cooled unit. We've been selling LED lights for over 5 years now and our experience has been that the lights will have a useful life of 3-5 years. Technology is always improving, and we're always making improvements to increase lifespan- but we don't want to exaggerate either.

      You can usually save enough money on electricity and bulb changes, not to mention cooling costs to have the light pay for itself compared to any HID light in 6-18 months. Combine the savings with bigger yield and higher quality and potency than HID lighting, and the 3-5 year lifespan, and our LED lights are the best choice you can make. With our 90-day satisfaction-guaranteed return policy you can give one a try for yourself and see just how much better grow lighting can be- at no risk.

  • Lee

    Hello do I keep the light in full spectrum mode all the way thru the plants life could you tell me please

    • Amanda Herman
      Amanda Herman June 27, 2018 at 9:24 am

      Hello Lee,

      Thank you for your question. Yes, use the same light, same spectrum the whole plant life; seed/clone, veg and flower. Here is a great article that goes a bit more into it!

  • Magpie

    Thanks for the helpful post with lots of good info! Using F/S LED's - Do you think the 18on/6off rule of thumb would apply to seedlings or would 12/12 work better while they are tiny babies?

    • Kevin

      Cannabis plants react hormonally to the length of their dark period- anything more than 6 hours of darkness starts to tell them they need to flower. Seedlings need the longer day lengths (18 hours on, 6 off) to ensure that they start growing vegetatively. If you give seedlings 12 hours of darkness and 12 hours of light, as soon as they are big enough to flower, they will- which will reduce your yield compared to giving them a time to grow larger vegetatively before you encourage them to flower. Seedlings get used to the light intensity they first encounter when they germinate, so it isn't a problem giving them 18 hours per day of bright light- they will do fine as long as it is what they first encounter when they come up.

      If you have germinated your seedlings with weak or no light, they will require some acclimatization to brighter, more intense light- slowly ramp up the intensity of light you give them over a period of a few days. However, it is still important to give them 18 hours of light and 6 hours of darkness to keep them from being stressed by the hormonal urge to flower when they are still so small and young.

  • Eric M

    Is 12-12 all that's needed or should I start lengthening the dark time as the flowering stage continues?

  • Travis

    When you very first plant the seed do you go ahead and do the 18 hours of light or do you wait a couple of days?

    • Kevin

      Some seeds need to see a little bit of light to germinate, but even for seeds that don't need light to germinate (like Cannabis), we still recommend turning the light on from the very beginning. Having the light on will help warm the soil surface and speed up germination, and when it does finally pop up through the soil, it is much better for it to see the light immediately so it doesn't grow "leggy".

  • Jack

    quite informative,, I am wanting to know, what level of light intensity must be provided to prevent triggering into the flowering phase during the shorter day length days please?

    • Kevin

      For cannabis plants it takes only 50 to 100 umol/m^2/sec of light intensity for 10-15 minutes at least every 6 hours to keep the plants "awake" enough to not trigger flowering. This level of light will not make the plants grow well at all, but it will keep them from flowering.

  • Brandon Whitford
    Brandon Whitford October 9, 2019 at 8:51 am

    When switching to flower should it be slowly switched to 12/12
    Or just go right to 12

    • Makenzie Salyer
      Makenzie Salyer October 31, 2019 at 7:53 am

      Hello Brandon,

      When you switch to flower you would want to change your light timer to 12 hours immediately. The quick light changes helps the plant recognize that it needs to begin its reproductive cycle.

  • Matt

    I have 8 week old durban poison clones i have them on 18/6 lighting 1200w led lites can any one tell me how long should i keep them in vegg before sending them in to flower and going in to 12/12 cycle.and at that age should i start feeding full strength and how often. Thanks guys

    • Makenzie Salyer
      Makenzie Salyer October 31, 2019 at 7:58 am

      Hello Matt,

      Thank you for the question. This is strain dependent as some strains are slower growers. The best way to determine how long you want to veg is the amount of time it takes your plants to completely fill in your growing space. We feed our plants full strength every time we water except for the very first feeding. Not feeding your plants each watering can actually cause more harm.

  • James

    Hi, I purchased a BlackDog 200. I have it connected to a timer ( tryed a different timer to see if it would solve problem ), when lights shutoff, some of the LEDS stay on during night mode. If i Unplug the plug they will shutoff. When it's in night time mode, It has barely any brightness, 0 Lux on my lux Meter and only the blue and red lights are on. I added a picture of the light during night time to see the problem. I was wondering is this something BlackDog do from Factory or is it a problem I have with my Light? Thank alot!

    I added picture on Week 13 of the light on my Timer Off.

    • Makenzie Salyer
      Makenzie Salyer January 2, 2020 at 7:00 am

      Please be assured there is nothing wrong with your light; if the timer were truly removing power to the light it would not glow. We do occasionally hear of this type of problem, and it is usually caused by one of two issues.

      The most common issue we see is that the hot and neutral feeds are reversed in the wiring of the outlet you are plugging into. When this happens, many timers do not stop power from getting to the light. Many home improvement stores have inexpensive outlet testers you can use to see if your wiring is reversed; if so, consult an electrician to rewire the circuit or outlet.

      Some timers, especially less-expensive digital timers, do not fully shut off power, but only restrict it with a transistor or similar technology. A higher-quality timer will usually fix the problem by fully shutting off power to the light.

      As much as we would like to give you a light that doesn’t use any energy while still growing your plants, we can’t break the laws of physics. If your LED grow light is glowing when the power is supposed to be off, it means the power hasn’t really been shut off. Our LED lights won’t provide light without power, but they are efficient at turning even small amounts of power into light.

      If these suggestions do not fix your problem, please let us know and we will be happy to further assist you. Let us know if you have any other questions.

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