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When to Switch To 12 12 Light Cycle

When to switch to 12/12 light cycle

Indoor cannabis farmers can plan for this great change in advance by regulating light to ‘switch’ plants into the blooming phase. Photoperiod cannabis plants automatically begin a distinct development phase when they adjust to fewer hours of light, which is known as the 12/12 light cycle. While changing the light output is straightforward, understanding when to switch to a 12/12 light cycle requires a few more strategies to maximize yield. Plant traits, training tactics, and nutritional formula changes, all contribute to the final success of cannabis cultivation. Let’s look at how these transformations, when combined, assist prepare the way for a bountiful harvest.

How to identify if your cannabis plants are ready for a 12/12 light cycle

Before going from the 18/6 veg to the 12/12 flowering light cycle, you must first evaluate whether your plants are ready. To assess whether your plants are ready, you should establish goals and objectives for what you hope to achieve with this flowering cycle. Consider the following questions to begin:

  • Do you wish to blossom a large number of plants or just a few?
  • Is it your goal to optimize production per plant or yield per batch?
  • Do you want to maximize the number of cycles you can run in a year?
  • Or are you simply blooming a crop less regularly, say, twice a year?

All of these elements will play a role in determining when to move to the 12/12 light cycle. The reason for this is that, as you are aware, there are two photoperiods (which means two light cycles). Indoors, a light cycle of 18 on and 6 off is used to achieve the vegetative period. Additionally, indoors, a 12 on, 12 off light cycle is used to achieve blooming.

How does changing the light cycle to 12-12 impact your cannabis growth?

The development of a photoperiod cannabis plant is determined by the quantity of light received each day. Growing weed outdoors in natural light, plants undergo blooming when the days shorten following the Summer Solstice. Indoor plants that have been exposed to abundant light on an 18/6 or 24-0 cycle during the vegetative phase require a transition to 12-12 to trigger bloom formation. When daylight hours are reduced, energy flow is redirected from growing vegetative development to bud formation during reproduction. Since a plant may store more glucose throughout the night, diverted energy provides nutrients to produce large, dense, trichome-rich blooms.

However, autoflowering cannabis plants do not need light-cycle modifications to enter flowering. Ruderalis genetics discovered in autoflower strains control the plant’s life cycle exclusively via time. They veg for a long time before entering the blooming process, which occurs when their internal clock shifts.

Shifting light spectrum moving into flower

Light spectrum analysis criteria have been established by science to identify perfect conditions for cannabis development. Visible light spectrums vary from 380nm to 750nm, with colder color temperatures at the bottom of the scale and warmer temperatures at the top. Cannabis plants thrive in the vegetative phase under cooler blue light from 400nm to 500nm. Walking into a veg room first thing in the morning reveals attractive plants stretched. Conversely, plants at the blooming stage prefer warmer, higher light between 620nm and 780nm.

Light cycles for weed in flower

Metal Halide (MH) and High-Pressure Sodium (HPS) lights have traditionally been the cannabis industry standard. MH bulbs generate a cool blue light, which provides optimum lighting conditions for plants to develop a thriving structure before flowering. Because light availability reduces when going to 12-12, altering the bulb temperature encourages cannabis plants to bloom. HPS lamps warm the room. The high-intensity lights blaze brightly, whipping photosynthesis into a frenzy. Many strains, particularly sprawling Sativa strains, can double in size during ‘the stretch’ in pre-flowering development when exposed to the HPS light spectrum. For decades, many successful cannabis cultivators have relied on these light sources.

However, recent LED grow light advancements have opened new avenues for getting ideal illumination in all stages of cannabis growth – with only one fixture. Full-spectrum LED research advances have resulted in decreased energy demand and cheaper electricity expenses, as well as a highly efficient way of light spectrum availability. LED has a promising future in the cannabis industry.

Factors to consider when switching from veg to bloom

Plant’s age

Although “cuttings” can be exchanged whenever desired, it is normally advisable to wait until the clone develops a robust root system before switching (typically 2-3 weeks old). If room (and time) allow, a 6-8 week veg phase can provide higher yields for “seedlings.” You can also swap them immediately after germination; however, they are physically incapable of budding until around three weeks of maturity.

Plant’s height

Cannabis plants will often double in size during blooming, depending on the strain. So, if ‘room’ height is restricted, the transition to 12/12 must be made as soon as the plant has achieved 50% of the ‘available’ height (remember to consider the minimum gap distance between lamp and foliage). Topping and LST are excellent ways for maintaining plants short and bushy, allowing for a longer veg (6-8 weeks) if desired.


Low-Stress Training (LST) approaches to promote growth in desired directions. Consider a gorgeous Indica hybrid that fits wonderfully into a quirky area of the grow space. Trellising, scrog, staking, clipping, or super-cropping in the desired direction are all simple ways to train the plant to optimize the difficult corner. These techniques promote bud site growth and subsequent flower development.


Another alternative is the Sea of Green (SOG) setting, where plants are cultivated in close proximity and clipped to form one massive cola. Early flowering of younger plants, for example, 2-3 weeks into vegetative development, stops them from getting too large. Also, a micro-environment emerges beneath a huge canopy of plants in a SOG, sparking synergistic energy flow. The result is many readily handled blooming plants that provide a big yield while taking up little space.

Nutrient formula

When the light cycle shifts to 12/12, the nutritional requirements alter, supplying energy to flower growth. Hungry vegetative development, high in nitrogen (N), promotes structural expansion. With strong N amendments combined with lower dosages of the similarly crucial elements phosphorus (P) and potassium (K), stems and leaves proliferate fast. When the light cycle shifts to 12-12, phosphorus and potassium levels rise, feeding the insatiable desire for blossoming flowers. At this stage, nitrogen decreases, signifying a sugar flow shift. The likelihood of stress during this transition phase is reduced by gradually introducing the new nutrient combination over time.

General principles of the bloom phase (powering the flower)

As a general rule, the plant will continue to increase in height (known as “flowering stretch”), and buds will form throughout the first 2-3 weeks of blooming. Following that, floral growth will take over, while veg growth will decelerate and eventually halt. Some general guidelines:

  1. Lights are turned “off” for 12 hours every day! Use high pressure sodium (HPS) bulbs rich in the yellow-red light spectrum.
  2. Increase the pace of air exchange. Flowering plants enjoy a steady decrease in relative humidity to approximately 40-60%. (high humidity can cause rot and mildew). The ideal temperature range is 68-82 degrees Fahrenheit (20-27 deg C). Furthermore, carbon dioxide consumption is nearing its peak and is thus frequently the weakest link.
  3. Avoid light burn by keeping foliage at a safe distance. However, keeping plants too wide apart will induce “stretching.”
  4. Pruning during flowering should be avoided unless there is overcrowding or damaged growth. ‘structural’ pruning should be done during veg.
  5. Don’t get complacent about pests or illnesses like powdery mildew! Check both sides of the leaves on all your plants regularly, especially those that go missing in the corner.

Tips for preventing your cannabis from overstretching

Many gardeners report that their plants ‘stretch’ too much, particularly during the first 2-3 weeks of flowering. Lower foliage will be more difficult to light efficiently, while top foliage will be at risk of being burned if lights cannot be lifted any higher. Plants can also be skinny and fragile, making them prone to breaking if loaded with buds. Although “flowering stretch” is biologically unavoidable, there are strategies to minimize it:

  1. Find a strain that is prone to being short.
  2. Use an MH lamp for the first 2-3 weeks of flowering, then move to an HPS lamp. Stretching is encouraged by the orange or red spectrum of HPS and other “flowering-specific” lights. Conversely, MH’s “blue” spectrum seems to prevent stem stretch.
  3. Maintain the night (lights off) temperature colder but within 9 degrees Fahrenheit (5 degrees Celsius) of the daytime temperature. Maintain a daytime temperature of fewer than 82 degrees Fahrenheit (27 deg C).
  4. Ensure enough airflow. This promotes shorter, stronger plants.
  5. Topping and LST are excellent for altering the plant so that all foliage receives adequate light. The majority of this should be completed when on veg.

A broad understanding of a cannabis plant’s development characteristics might assist a producer in determining which strains will thrive in their area. Choosing which cannabis genetics to buy gives you a good indication of when to change the light cycle to 12/12. Our detailed cannabis plant descriptions at Premium Cultivars include anticipated plant size, growth characteristics, usual harvest, and much more.

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Marcus Smith

Marcus is a relative newcomer to the cannabis world. Though it may seem that his youth wouldn’t allow for a wealth of knowledge, this is untrue. Marcus Smith has close relationships with many cannabis breeders and grow owners which have allowed him to sample the best cannabis across the US and beyond while also gaining valuable insight into how different strains grow and develop. About this Author

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