The Importance Of The Dark Cycle In Cannabis Cultivation

Photosynthesis & Respiration

When cannabis plants, like other green plants, are in the light—be it sunlight or artificial light—they are photosynthesising. This is how they convert light into energy and release oxygen. They are also respiring. That’s right, contrary to the misinformation circulating elsewhere, plants, like all other aerobic organisms, are respiring all the time. We stop when we die, and we die if we stop. Cannabis plants are just producing a surplus of oxygen during the daytime.

During the dark cycle, cannabis plants are not photosynthesising, and continue to respire at a constant rate; they do not just begin to respire when the lights go out. The Calvin Cycle, which is light-independent, processes all that stored up solar energy into carbohydrates. Roots are also respiring, so it’s very important to grow in well-aerated substrates. Take advantage of air pots to keep your medium oxygen-rich.

Photoperiod Cannabis

Photoperiod cannabis is sensitive to changes in the hours of daylight it receives. In fact, photoperiod cannabis strains cannot transition from vegetative growth to flowering without long nights. 12 or more hours of darkness is the biological trigger for photoperiod cannabis to bloom. Outdoors, as the nights grow longer incrementally from the summer solstice onward, some strains can even begin flowering when receiving 14 hours of sunlight.

Autoflowering Cannabis

Only autoflowering seeds have a predetermined post-germination life cycle. The autoflowering trait is believed to be an adaptation to the almost continuous 24 hours of sunlight unique to Siberia. True autoflowering varieties will start to bloom after about 30 days of vegetative growth.

Vegetative Growth: 18-6 Or 24-0?

Photoperiod cannabis strains can essentially be kept in vegetative growth indefinitely, so long as they receive 14+ hours of light per day. The debate on what is the optimal light-dark schedule for vegging cannabis rages on. That being said, most indoor growers will agree that somewhere between 18–24 hours per day is perfect. Outdoor growers get started in spring/summer to make the most of the long days too.

The 24-0 schedule might speed up the vegetative growth process some. The downside is the higher power bills. 18-6 is more common because it’s a closer match for long summer days and a little cheaper. Higher success rates with cuttings on an 18-6 light-dark cycle than those receiving 24-0 is the only substantial difference we have discovered from our own experiments.

Alternative: 6-2?

Currently trending is the new 6-2 alternative vegetative growth lighting schedule. Three of these light-dark cycles per 24-hour period means the cannabis plant receives the same total 18 hours of light and 6 hours of darkness as a conventional 18-6. Moreover, this is a viable option as photoperiod cannabis needs a continuous 12 hours of darkness to bloom. The thinking being, those brief 2-hour intervals of darkness give the plant a chance to rest and process CO₂ more effectively. Also, 6-2 avoids light saturation and is potentially a more effective light-dark cycle for photosynthesis.

Flowering: 12-12, The Indoor Grower Sop

12-12, the even split of 12 hours of light and 12 hours of darkness, only occurs naturally close to the equator. Indoor growers rely on timers to artificially dial in this light-dark cycle. During the day or lights-on period, two receptors, phytochrome red and phytochrome far-red, are in balance. In darkness, the far-red change into red. It’s the increase in red that triggers flowering. Many growers leave plants in complete darkness for 36 hours before commencing 12-12 to ensure a high phytochrome red ratio.

However, even if you grow a photoperiod strain from seed with a 12-12 schedule, it will take 3–4 weeks before buds begin to develop. Photoperiod cannabis is an annual plant. Marijuana can survive with as few as 8 hours per day, and flowering photoperiod plants can even be re-vegged. The switch can really be flipped back to vegetative growth with the resumption of longer days.

Pitch Black, Do Not Disturb

Interruptions to sunlight come in the form of clouds. Cannabis plants can handle this. But anything short of a pitch black dark cycle with no light pollution for the duration will suffice for photoperiod strains. Light leaking into the grow-op when the grow lights are off will stress plants and disrupt flowering. This can stress a female plant into an intersex plant. Street lighting can confuse outdoor photoperiod cannabis plants and prevent them from flowering. Only a green bulb can be used if you must tend to plants or enter the garden during the dark cycle.

Grow tents and grow rooms need to be checked for cracks and tears. Greenhouses can be covered to make sure nighttime is completely dark as long as it needs to be. It doesn’t take much to patch a hole or buy a light-proof tarp. But interruptions to the dark cycle during flowering could cost you a whole harvest.

Do Autoflowering Cannabis Plants Need A Dark Cycle?

The short answer is yes with an “if”, and no with a “but”. If you want the highest potency and maximum yield, we suggest adopting either an 18-6 or 20-4 light-dark cycle. But should you run lights 24-0 indoors, your plants and all of your equipment like fans will be pushed to the limit as well. So far, 24-0 auto crops have failed to outperform those cultivated with dark cycles of 4–6 hours. There really is power on the dark side of the force.