Autoflowering cannabis plants transition from the vegetative to the flowering stage based on age, not changes in the daily light cycle. This unique characteristic leads to the question of whether they still require darkness to complete their bloom cycle. Unlike traditional strains, darkness is not a requirement for autoflowers to begin or complete flowering.
How Autoflowers Differ from Photoperiod Plants
Traditional cannabis varieties are photoperiod-dependent, meaning they use light and dark periods to regulate their life cycle. These plants rely on the phytochrome system to measure night length. An uninterrupted period of darkness, typically 12 hours, is required to trigger the production of the flowering hormone, florigen.
Autoflowering plants, originating from Cannabis ruderalis, bypass this light-based mechanism entirely. Ruderalis evolved in regions with short growing seasons and long daylight hours, developing a trait to flower automatically based on its internal clock. The transition to flowering is determined by maturity, usually occurring two to four weeks after germination, regardless of light hours.
Standard Light Regimes for Autoflowers
Since flowering is triggered internally, autoflowers do not require reduced light hours to bloom, making their light needs flexible. The most aggressive schedule is 24 hours of continuous light (24/0), which aims to maximize the energy available for growth. Providing light without interruption theoretically maximizes photosynthesis and growth speed, though it offers no downtime for the plant.
A common compromise is the 20 hours of light followed by 4 hours of darkness (20/4) schedule. The most popular schedule is 18 hours of light and 6 hours of darkness (18/6), mirroring the cycle used for vegetative photoperiod plants. All three schedules are effective because the plant’s flowering is genetically pre-programmed.
The main difference between these three schedules is not their ability to initiate flowering, but rather the total energy delivered and the efficiency of the operation. Schedules with a dark period allow the grower to save on electricity costs and manage temperatures more easily. Choosing between 24/0, 20/4, or 18/6 depends on a grower’s specific environment, energy budget, and personal preference for plant management.
Biological and Practical Reasons for a Dark Cycle
While a dark period is not necessary for an autoflower to bloom, implementing one offers specific biological and practical advantages. The primary biological reason for including a dark cycle is to allow for dark respiration, a metabolic process that occurs when photosynthesis stops. During light hours, the plant converts light energy into sugars, but in the darkness, it breaks down those stored sugars to fuel other necessary functions, like cellular repair and growth.
This period of metabolic activity is sometimes referred to as “plant rest,” which proponents believe helps the plant process nutrients and consolidate the day’s growth. Certain enzymes necessary for growth and development are also more active during the dark period. The four to six hours of darkness in an 18/6 or 20/4 schedule is often considered sufficient to facilitate these essential processes.
From a practical perspective, the most straightforward benefit of a dark period is the reduction in operating expenses. High-intensity grow lights consume a significant amount of electricity, so turning them off for four to six hours daily results in tangible energy cost savings. Furthermore, the lack of light output helps to manage the temperature and humidity within the grow space, which prevents the environment from becoming excessively hot or dry.