How to


When to Switch a Cannabis Light Cycle to 12-12

Cannabis is a fun plant to grow. Watching growth cycles as they progress from seed to full-blown flower is a marijuana grower’s delight. One of the most enjoyable stages, for many, is the transition between vegetative growth into pre-flower bud formation as delicate white pistils begin emerging on the plant.

Indoor cannabis growers have the advantage of pre-planning for this remarkable transformation by manipulating light to ‘flip’ plants into the flowering phase. Known as the 12-12 light cycle, photoperiod cannabis plants naturally begin a different growth phase as they adapt to fewer hours of light.

While it is simple to change light output, knowing when to switch to a 12-12 light cycle requires a few other tactics to achieve maximum yield. Plant characteristics, training techniques, and nutrient recipe alterations also contribute to the ultimate success when growing cannabis.

Let’s take a look at how these maneuvers, working together synergistically, help pave the way to an abundant harvest.

How Does Switching the Light Cycle to 12-12 Initiate Growth Changes?

Photoperiod cannabis plant’s growth progression is dependent upon the amount of light received each day. Plants grown outdoors under natural light transform into the flowering period as the days become shorter after Summer Solstice. Indoor plants, given abundant light on an 18-6 or 24-0 cycle during veg time, require the switch to 12-12 to induce flower production.

Reduction of daylight hours redirects energy flow from expanding vegetative growth into bud formation during reproduction. As a plant can store more sugars during hours of darkness, redirected energy now supplies nutrition to create big, dense trichome-rich flowers.

Autoflowering cannabis plants, on the other hand, do not require light-cycle changes to progress into the flowering state. The Ruderalis genetics found in autoflower strains dictate the plant’s life cycle based solely on time. They grow for a certain period, vegging, and when their internal clock changes, they proceed into the flowering cycle.

Changing Light Spectrum Going into Flower

Science has proven parameters in light spectrum analysis to determine ideal conditions for cannabis growth. As noted, visible light spectrums range from 380nm – 750nm with cooler colour temperatures on the lower end of the scale and warmer temps in the upper range.

Marijuana plants growing in the vegetative phase flourish under cooler blue light in the 400nm-500nm range. Walking into a veg room in the morning reveals lovely ladies stretched high ‘praying’ to the light as they enter a new day. On the other side, energized plants in the flowering stage prefer warmer, higher light between 620nm – 780nm.

Traditionally, Metal Halide (MH) and High-Pressure Sodium (HPS) bulbs have been the cannabis industry standard. MH bulbs emit a cool blue light, providing ideal lighting conditions for plants to build a flourishing structure before flipping them to flower. As light availability decreases when switching to 12-12, changing the bulb temperature motivates marijuana plants into flower development.

A cannabis strain switched to a 12-12 light cycle
Reduction of daylight hours redirects energy flow from expanding vegetative growth into bud formation during reproduction.

HPS bulbs warm up the room. The bright, high-intensity lights shine hot, spinning photosynthesis into a frenzy. Many strains, especially sprawling sativa, can double in size during ‘the stretch’ in pre-flowering growth under HPS’s light spectrum. Many successful cannabis growers have used these light sources for decades.

However, recent LED grow light developments have opened new doors for achieving optimal light in all phases of cannabis growth – with just one fixture. Technical advancements in full-spectrum LED research produce lower energy demand with fewer electrical costs while creating a highly efficient means for light spectrum availability. The future’s looking bright for LED in the cannabis industry.

Size Matters in Determining When to Flip to 12-12

How large is the grow space? How many plants are in the room? Did you choose fast-growing sativa strains or luscious, dense indica genetics? How big are the plants going to get? These are all factors to consider when deciding to switch the light cycle to 12-12.

Determining plant density depends on the size of the grow room. Knowing a strain’s growth characteristics helps outline how many plants can be grown in a particular space to maximum potential. Add some proper training techniques to the equation, utilizing all available areas, and a good blueprint emerges to know when to flip to 12-12.

Training a Cannabis Plant to Explode in Growth When Switching to 12-12

As a young seedling emerges from the ground, a beautiful plant emerges and can be sculptured to grow in any fashion, trained to maximize a specific environment. It is exciting to watch how quickly a cannabis plant responds to bending, trellising, and clearing out lower unproductive growth. Preparing for rapid advancement when the lights are switched to 12-12 provides optimal controlled plant structure as a marijuana plant enters the flowering stage.

Low-Stress Training (LST) techniques positively expand growth in desirable directions. For instance, imagine a beautiful indica hybrid that fits perfectly into a funky corner in the grow space. Training the plant to maximize the awkward corner can be easily achieved by trellising (Scrog), staking, trimming, or super-cropping (bending the stems) in the desired direction. These methods encourage bud site expansion and future flower development.

Another option is the Sea of Green (SOG) environment where plants are grown close together, cropped to produce one bodacious large cola. Flipping younger plants into flower early, for instance, 2-3 weeks into vegetative growth, prevents them from growing very big. As plants are crowded together in a SOG, a micro-environment develops under a large canopy, energizing synergistic energy flow. The result is a plethora of easily manageable flowering plants, producing a substantial harvest, utilizing minimal space.

Change the Light Schedule…Change the Nutrient Recipe

Altering nutrient requirements when the light cycle changes to 12-12 supplies energy into flower development. Hungry vegetative growth, heavy in nitrogen (N), encourages abundant structural expansion. Stems and leaves multiply rapidly with high N amendments coupled with smaller doses of the equally important elements, phosphorus (P) and potassium (K).

Increasing phosphorus and potassium levels when the light cycle switches to 12-12 feed the insatiable demand of blooming flowers. Nitrogen is reduced at this point, signaling the change in sugar flow. Slowly introducing the new nutrient recipe incrementally over time reduces the possibility of stress during this transition period.

Choosing the Best Cultivars for Indoor Grow Spaces

Having general knowledge of a cannabis plant’s growth characteristics helps a grower determine which strains will grow best in their environment. Planning out which type of cannabis genetics to purchase provides a distinct idea of when to switch the light cycle to 12-12. Here at Marijuana Grow Shop, our in-depth plant descriptions deliver estimated plant size, growth features, typical yield, and so much more.

Post author
Charle Thibodeau
Charle’ Thibodeau is a freelance writer with almost a decade´s experience, specializing in cannabis content for the past two years. A strong motivation to educate, inform, and promote the culture surrounding this miraculous plant is her earnest mission.
See more from Charle Thibodeau

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