Light Schedules for Flowering PlantsUpdated 7 months ago
The light schedule is one of the most crucial factors in triggering and maintaining the flowering stage for photoperiod-sensitive cannabis plants. During this stage, plants require a specific light-to-dark ratio to switch from vegetative growth to flowering, where they produce buds. An incorrect light schedule can prevent flowering, cause plants to revert to vegetative growth, or lead to hermaphroditism (the development of both male and female flowers), all of which can severely impact yield and quality.
For most photoperiod cannabis strains, the standard light schedule for flowering is 12 hours of light and 12 hours of uninterrupted darkness each day. This 12/12 schedule mimics the natural shortening of daylight hours as summer turns to fall, signaling to the plant that it’s time to start flowering.
Light Period: The 12 hours of light should be intense and full-spectrum, ideally from a high-quality LED or HID light, to maximize photosynthesis and support bud development.
Dark Period: The 12 hours of darkness must be completely uninterrupted. Even brief exposures to light during this period can confuse the plant and disrupt its flowering cycle.
Implementing a 12/12 light schedule is straightforward with the help of a reliable timer:
Use a Timer: Automate the light schedule using a digital timer. Set it to turn the lights on and off at the same time every day to maintain consistency, which is vital for a successful flowering phase.
Check for Light Leaks: Ensure that the grow space is completely dark during the 12-hour dark period. Light leaks, even from small sources like LEDs on equipment, can prevent the plant from fully entering the flowering stage. Seal any gaps or cracks that allow light to enter the grow space.
Consistency is Key: Maintain the 12/12 schedule consistently throughout the flowering stage. Any significant deviations from this schedule can stress the plants, potentially leading to issues like delayed flowering, hermaphroditism, or reversion to vegetative growth.
Interruptions to the dark period can have serious consequences for flowering cannabis plants:
Reversion to Vegetative Growth: If the dark period is interrupted or shortened regularly, plants may revert to vegetative growth, halting bud development and wasting valuable time.
Hermaphroditism: Consistent interruptions can stress the plant, leading to the development of both male and female flowers. This can result in self-pollination and the production of seeds in your buds, reducing their quality and potency.
Delayed Flowering: Any light interruptions can confuse the plant's internal clock, delaying the start of flowering or prolonging the flowering period, which can extend the time to harvest.
The type of light used during flowering plays a significant role in the quality and quantity of the final harvest:
LED Lights: High-quality LED grow lights are often preferred during flowering because they can be tailored to provide the specific wavelengths of light that cannabis plants need during this stage. Look for lights that emphasize red and far-red spectrums, which support bud development and flowering.
HID Lights (HPS and MH): High-Pressure Sodium (HPS) lights are traditionally used for flowering due to their strong red and yellow light output, which promotes flowering. Metal Halide (MH) lights, on the other hand, emit more blue light, which is better suited for vegetative growth. Some growers may use a combination of HPS and MH during flowering for a full-spectrum effect.
Light Intensity: Ensure that the light intensity is sufficient for the flowering stage. Insufficient light can lead to smaller, less dense buds, while too much light can cause light burn, leading to bleached or damaged buds.
Accidental changes to the light schedule can stress the plants, but the response depends on the nature and duration of the change:
Short-Term Changes: A one-time interruption or slight change in the light schedule may not cause significant issues if corrected immediately. However, it’s important to return to the consistent 12/12 schedule as soon as possible.
Prolonged Changes: Repeated or extended alterations to the light schedule can cause more severe problems, such as delayed flowering, reversion to vegetative growth, or the development of hermaphroditic traits.
Emergency Response: If you realize the light schedule has been disrupted, immediately correct it and monitor the plants closely for any signs of stress. If stress symptoms appear, such as leaf curling, discoloration, or unusual growth patterns, take steps to mitigate the stress, like adjusting environmental factors (temperature, humidity) and ensuring the correct nutrient levels.
Some growers may wish to extend the flowering period to allow buds more time to mature and increase in size:
Gradual Reduction in Light: Some advanced growers gradually reduce the light period to simulate the natural shortening of days as fall progresses. For example, moving from 12/12 to 11/13 or even 10/14 can potentially extend the flowering period, encouraging buds to ripen further. However, this approach carries the risk of diminishing returns if the plants do not respond well.
Strain-Specific Considerations: The success of extending the flowering period depends largely on the strain. Some strains naturally have longer flowering periods, and trying to artificially extend this period may not yield better results. Research your specific strain’s flowering time before attempting to manipulate the light schedule.
Different cannabis strains respond uniquely to light schedules during flowering:
Indica-Dominant Strains: These strains typically have shorter flowering periods and are less sensitive to light schedule variations. They generally respond well to the standard 12/12 schedule and may even finish flowering slightly faster with consistent lighting.
Sativa-Dominant Strains: Sativas often have longer flowering periods and can be more sensitive to changes in the light schedule. For these strains, maintaining a strict 12/12 light schedule is essential to ensure they fully mature and produce high-quality buds.
Hybrid Strains: Hybrids can vary depending on their dominant traits. It’s important to understand the specific needs of the hybrid strain you’re growing and adjust the light schedule accordingly to optimize flowering.
As flowering progresses and harvest time approaches, slight adjustments to the light schedule and environment can improve the final product:
Monitor Trichomes: The best indicator for harvest readiness is the development of trichomes. As they transition from clear to milky white and amber, it signals that the plant is nearing peak potency.
Adjust Light Intensity: Some growers reduce light intensity slightly in the final weeks to mimic the waning sunlight of late fall. This can also help prevent light burn on maturing buds.
Final Flushing: About 1-2 weeks before harvest, many growers flush the plants with water to remove any remaining nutrients from the soil, which can improve the taste and smoothness of the final product.
Extended Darkness: Some growers turn off the lights for the final 24-48 hours before harvest, believing that the extended darkness can increase resin production. While this technique is debated, it’s worth experimenting with on a small scale.
Autoflowering strains are not dependent on light schedules to flower; they flower based on age rather than light exposure:
Constant Light Schedule: Autoflowering cannabis can be kept on an 18/6 or even a 20/4 light schedule throughout their entire life cycle, including the flowering stage. This provides more light for photosynthesis, potentially increasing yields.
24/0 Light Schedule: Some growers keep autoflowering plants under continuous light (24 hours) for maximum growth. However, this can be stressful for some strains, and a slight dark period (4-6 hours) is often recommended.
Consistency is Still Key: While autoflowering plants don’t rely on light schedules to flower, maintaining a consistent light cycle helps optimize their growth and yield, ensuring they produce high-quality buds.
Managing the light schedule during the flowering stage is critical for cannabis growers aiming for maximum yield and potency. By understanding the specific needs of the plants, maintaining strict light and dark periods, and making small adjustments as flowering progresses, growers can optimize their crops and achieve a successful harvest.