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Sometimes I travel so much at the speed of light, I tend to forget that light is also one of the most important parts of a cannabis plant’s life. Not only does light regulate photosynthesis, but it also dictates when it’s time for your plant to flower those precious buds that make it all so worthwhile. Home growers may be familiar with the vegetative and flowering phases of cannabis cultivation and growth; however, the 12 hours of light, 12 hours of darkness schedule goes a little bit deeper.
This 12/12 light phase of the weed plant’s life follows the vegetative cycle, which has it primed for the eventual weight and density of flower buds. Photoperiod seeds and plants need a period of darkness to promote specific hormones and compounds that are critical to producing buds.
For outdoor growers, the shift in daylight from summer to autumn naturally provides a full period of darkness each night. Indoor growers must mimic the internal signals of a photoperiod plant during dwindling daylight to a 12/12 split. During the vegetative cycle, the recommended amount of light is 18 hours, with 6 hours of darkness. To flower, your weed plant needs a hormonal signal to initiate the flowering process, and simply allowing for 12 hours of light will achieve that!
Autoflower seeds and plants don’t rely on the same light cycle to trigger the signal to flower internally as their photoperiod counterparts. This quicker-to-harvest cannabis seed type instead relies on age to shift directly from vegetative to flowering phases. [1]
Autos flower after only about 2-4 weeks on average due to their unique cannabis ruderalis genetics. This trait has evolved in nature to cause flowers to bloom automatically after a few weeks, despite drastic shifts in the amount of daylight available in the wild. Indoor breeders have continued to breed this trait into their autoflower cultivars.
Growers of autos instead typically utilize 18 hours of light/6 hours of darkness or 20 hours of light/4 hours of darkness to maximize photosynthetic processes and grow bigger plants. It’s not unheard of to run continuous light 24 hours a day for autos, but rest, recovery, and hormonal balance from a few hours of darkness are important as well. It can also help your electric bill!
The main thing during flowering is photosynthesis. The ability of the weed plant to convert light energy into internal energy is critical for the continued growth of the flower buds during this time.
Proper light also ensures the plant can synthesize complex compounds and nutrients to continue developing. There are a few important things that take place right under the glow of the late summer/early fall sunshine or those bright grow lights in your tent or indoor grow room.
Among the main stem and secondary branches, preflowers begin to form. Growths known as meristems reproduce on the stems and branches. These growths form into individual preflowers. Each female reproductive part, known as the stigma, starts as a tiny white filament, eventually turning a reddish-brown during maturation. The flower buds develop along the stems and branches, becoming swollen with density and eventually cannabinoid-rich trichomes during the entire flowering cycle. [2]
Important cannabinoids and terpenes are produced and developed by the weed plant according to the strain. This is when primary cannabinoids like THC and CBD begin to fill the trichomes with their sticky resins, while other minor cannabinoids and less-present terpenes also begin to form. Cannabinoids cause the psychoactive and potentially medicinal effects of weed. Terpenes also contribute to the overall impact of certain strains while enhancing their flavor and aroma as well. [2]
Like the vegetative cycle of your weed plants, the flowering cycle needs to ensure that light quality and intensity are also maintained.
Your plants will continue to blossom with buds and get bigger as a result, as will the canopy too. While LED grow lights emit less heat than HPS lights, it’s still important that no matter what kind of lights you use when growing indoors, you maintain a healthy distance from the plant’s canopy. According to the folks at AC Infinity, your primary grow light should be hung at approximately 1.5” away from the canopy during flowering to avoid stress and light burn.
One of the most important ways to optimize bud growth during the flowering phase is to provide specific light intensities. Photon Flux Density (PPFD) determines the amount of usable light and active photons (in the PAR range) that reach the plant’s surface each second. However, as no surprise, too much light intensity can adversely affect and stress your plants. Growers should try to keep their light intensity levels in the following ranges, with gradual adjustments, according to researchers:
Sunlight contains a full spectrum of different wavelengths that naturally affect how a weed plant grows, manufactures terpenes and cannabinoids, and yields. Some of these wavelengths are great for vegetation processes early on during the outdoor grow season, while others help promote flower production.
When the days get shorter and extended periods of darkness (12 hours) begin in the late summer, it’s time for your outdoor plants to begin naturally flowering. If there isn’t sufficient darkness, your weed plants aren’t getting the right signal and are going to stay in a vegetative state, incapable of flowering.
The preflower starts to develop along branches and stems, forming the beginnings of your buds. Hormones called gibberellins and cytokinins kick off the flowering process, and the introduction of 12 hours of darkness is what sparks it. Trichome, terpene, and bud development also begin to take shape due to the right amount of darkness and stored energy from the 12-hour photoperiod that gets synthesized.
The dark period of 12 hours is also a time for your plant to recover from the light with increased respiration and growth. Don’t confuse your weed plant by interrupting the 12-hour cycle of darkness–even for a moment! This will not only confuse your plant into thinking it’s in veg again, but it’ll also delay and affect flowering and bud growth, while the stress of it can lead to undesirable hermaphrodite weed plants. Keep to a strict 12-hour darkness period, no matter what!
Keep to a few best practices for indoor growers when switching your cannabis plants to flower, once they are big and strong enough in veg. These simple practices can make the difference between a successful harvest and a major bummer.
The photoperiod of light and dark, as well as nighttime, regulates the light schedule required for outdoor plants. However, for indoor growers, the choice must be made once the plants are as ready as they can be in vegetation.
Flipping your indoor lighting schedule to 12/12 is a simple task that ultimately encourages both light and dark to start the party with dense and dank flower buds. Flipping to a 12/12 flower cycle is a simple task that ultimately paves the way to bring them to the table.
Mastering the 12/12 light schedule is one of the most critical techniques in cannabis cultivation, especially for those growing photoperiod cannabis strains. Whether you’re working with indoor grow lights or relying on natural sunlight outdoors, providing your plants with exactly 12 hours of uninterrupted light and darkness will trigger the hormonal changes needed to produce dense, trichome-rich buds.
From strain selection to lighting intensity, distance, and spectrum, every factor plays a role in shaping the quality of your final harvest. If you’re aiming for high THC levels, strong terpene profiles, or simply the healthiest plants possible, understanding and controlling the 12/12 cycle is non-negotiable. And if you’re growing autoflower seeds, knowing the difference in light response will help you get the most from their rapid life cycle.
Whether you’re dialing in your 12/12 light cycle or deciding between autoflower, photoperiod, or high-CBD cannabis seeds, our expert team at Multiverse Beans is here to help. We offer a carefully curated collection of premium cannabis seeds online, including rare genetics, fast-flowering strains, and high-yield varieties trusted by growers across the galaxy. If you have questions about lighting schedules, nutrients, or choosing the best strain for your setup, don’t hesitate to reach out. Contact us anytime to chat with one of our grow specialists and find the perfect seeds to match your cultivation goals. Your next harvest starts with the right beans and the right light.