BackThe cannabis photoperiod: What’s the best indoor lighting schedule?

The control of the photoperiod in cannabis cultivation is one of the most decisive factors for the success of any indoor crop. Just as the sun marks the biological rhythms of plants in nature, artificial lighting in indoor grows dictates the behaviour, development, and final yield of our plants. Mastering this aspect not only helps to optimise growth, but also to manipulate the natural cycles to obtain crops with greater potential at any time of year.

 

In botany, this concept is understood as a unit of alternation between periods of light and darkness, whose repetition controls fundamental biological processes such as germination, growth, flowering, and ripening, enabling plants to use this information to synchronise with the seasons. This means that the full development of plants can be activated or inhibited depending on the light conditions that they’re exposed to. Cannabis is extremely sensitive to light cycles thanks to its sophisticated system of proteins (called phytochromes). These photosensitive receptors detect the presence or absence of light ― and, during the night, phytochromes in their active form naturally degrade, returning to their inactive form. This mechanism allows the plant to "measure" the duration of the periods of darkness. For photo-dependent cannabis plants, this system is crucial: when the nights are longer (exceeding a critical threshold of hours of darkness), hormonal signalling is activated to start the transition to the flowering phase. Therefore, you can keep your indoor plants permanently in vegetative mode with cycles that include short nights; or induce the flowering by providing longer nights. On the other hand, autoflowering genetics derived from Cannabis ruderalis (a strain adapted to regions with long daylight hours during the growing season) automatically flower after a certain period of time, usually between 2 and 4 weeks, without the need for the light cycle to be adjusted. This key difference affects how growers plan their lighting schedules.

 

 

Photo-dependent varieties depend on specific changes in the photoperiod to progress through all their stages of development. These are the recommended lighting times for each stage:

 

During this phase, which usually lasts between 1 and 3 weeks, the plants need constant but gentle light to develop a strong root system and their first pairs of leaves. The most common light cycle is 18/6 (18 hours of light and 6 hours of darkness), which provides sufficient energy for photosynthesis whilst allowing a rest period that can benefit the plant’s metabolic processes that occur during the "night". Some growers opt for 24/0 (24 hours of light) during the first few days to maintain consistent temperature and humidity, which is especially useful in indoor environments where seedlings are vulnerable. However, there’s no conclusive evidence that 24/0 is significantly better than 18/6, but one thing that’s absolutely certain is that it will significantly increase your electricity costs!

 

Top tip: Keep the lights at an appropriate distance (usually 30 - 60 cm for LEDs) to prevent the seedlings from burning or stretching; and use a blue light spectrum to promote compact growth.

 

This stage, which can last between 4 and 8 weeks or even longer (depending on the goals of the grower), is when the plant grows more rapidly, developing a robust structure. The standard lighting cycle is 18/6, which provides an ideal balance between light for photosynthesis and darkness for rest. This cycle mimics the conditions of long summer days, sending a signal to the plant that it should continue growing (given that less than 14 hours of light per day could induce premature flowering). Some growers experiment with 20/4 or 24/0 to accelerate growth, but these schedules may stress the plant or provide benefits that are not proportional to the increase in the energy consumption. A study published in 2022 found that an 16/8 photoperiod was more beneficial than 24/0 in terms of photosynthetic efficiency and reduced damage to chloroplast membranes, suggesting that a period of darkness may be advantageous.

 

Top tip: During this stage, use lamps with a spectrum rich in blue light to encourage compact and leafy growth. Make sure the light intensity (measured in PPFD or Photosynthetic Photon Flux Density) is within the range of 400 - 600 μmol/m2/s to optimise photosynthesis without saturating the plant.

 

To induce the flowering of photo-dependent strains, the light cycle is changed to 12/12 (12 hours of light and 12 hours of darkness). This schedule mimics the natural conditions of autumn, when days become shorter, sending a signal to the plant that it’s time to reproduce. Consistency is critical at this stage: any interruption in the darkness period, even for a few minutes, can cause stress, delay the flowering, or induce hermaphroditism. The 12/12 cycle is standard for most genetics, although the flowering time may vary: indicas typically take 8 - 10 weeks; sativas, between 10 and 12 weeks; and hybrid varieties usually need somewhere in between. If you’ve kept mother plants under a 24/0 or 18/6 cycle for a long time, and want to switch them to the flowering stage, consider making a gradual transition ― decreasing one hour of light every two days until reaching 12/12 can help reduce stress and improve results.

 

Top tip: Shift to a spectrum rich in red light during the flowering to maximise flower production. Adjust the light intensity to 600 - 1000 μmol/m2/s, depending on the variety and the ability of the plant to handle the light.

 

Autoflowering strains don’t require a change in the photoperiod to flower. This genetic programming enables them to change gears automatically without having to wait for external signals, which makes them a favourite amongst rookie growers or those looking for faster growing cycles. The most common light cycle for autoflowerings is 18/6 (18 hours of light and 6 hours of darkness) ― this offers a balance between robust growth and energy efficiency. Many growers also use 20/4 (20 hours of light and 4 hours of darkness) to maximise light exposure and potentially the yielding capacity of their plants. Some others opt for 24/0; but, as with photo-dependent strains, there is no solid evidence that this is significantly better.

 

Top tip: As autoflowerings have a short life cycle, use a full or mixed light spectrum (blue and red) to support both vegetative growth and flowering. Adjust the light intensity gradually, starting at 300 - 400 μmol/m2/s in the seedling phase, and increasing to 600 - 800 μmol/m2/s during the flowering.

 

 

The photoperiod is much more than just "turning the lights on and off". It represents an ancestral language between the plant and its environment, a signalling system that has evolved over millennia and that we can now manipulate to our benefit in indoor grows.

 

The most important thing is consistency: once a cycle has been established, maintaining it rigorously by using programmable timers will ensure that your plants develop their full genetic potential without interruptions or stress. It’s important to remember that, in cannabis cultivation, just as in many other aspects of life, patience and precision are often rewarded, turning your cannabis grow from a simple effort into a resounding success.