How It Works Start My Diary Login Sign Up

Best Light Schedule For Growing Cannabis

Created by
JoshuaHolt JoshuaHolt
Added 20 October 2020

Cannabis Light Schedules

It goes without saying that most living things could not live without light. The sun provides the Earth with the right energy for optimal health, but the question is, how much is the right amount? Light is a fundamental resource for cannabis plants, but a lack of light causes deficiencies and too much can burn. If your plants do not receive the correct balance they may underperform.

If you are a beginner, understanding light cycles should be the first thing on your list. Fortunately, setting up the correct light schedule is easy once you understand how cannabis plants grow outdoors. This article has all the information you need.

What is a Light Schedule?

When we talk about light schedules, we are essentially referring to the outdoor cycle of the sun. As the Earth orbits the sun throughout the year, the amount of daylight changes with each day. The sun is highest in the sky during summer, providing us with long days. During winter, the sun 'dips' to its lowest point and the days become shorter.

Outdoor Seasonal Light

The light schedule will be different depending on where you live and the particular season you are in. A 'schedule' indicates there is a set arrival and departure time. We know how this works outside because we have studied our own environment for many years. The ability to grow cannabis indoors has come from understanding how light changes outdoors throughout the seasons. This means we now we have full control over the light schedule, allowing us to harvest multiple times per year.

Indoor Vs Outdoor

Cannabis can be grown using a variety of light schedules, but there are some basic rules that need to be followed. Most cannabis strains are photoperiodic, which means they have the ability to respond to changes in light. They recognise when they need to grow and flower based on the length of the days. The Summer Solstice signals the moment days begin to shorten. For the Northern Hemisphere this takes place in June, and for the Southern Hemisphere in December.

An outdoor cannabis plant will grow during the spring as summer approaches. The days are long and plants will focus on developing lots of leaves and branches as they mature into fully grown ladies (if they are female). The flowers bloom when the days start to become shorter as they try to mate in time before winter. Harvesting usually takes place before light and temperatures drop too much.

Outdoor Vs Indoor Light

So, from this we can understand that a cannabis plant in vegetation needs more light than one in flowering. In most regions where cannabis grows well, plants get plenty of sunlight during their vegetation period, and enough to sustain their flowering period. This is not the case for autoflowering cannabis, which we will discuss later on.

Outdoors, there is not really a 'best' light schedule because nature has already determined that for us. We can provide some assistance by placing plants in more direct sunlight or adding shade, but the daylight hours will be the same nonetheless. It can also be a bit inconvenient to manage. You can measure how many hours of sun exposure your growing space receives and plan accordingly.

Indoor growing requires us to set up the lighting ourselves. Controlling the light schedule allows us to shorten the grow cycle by reducing the time spent in vegetation. This is why outdoor cannabis plants tend to be bigger than indoor varieties.

The flowering time depends on genetic makeup of the strain, as well as the environment. Some strains may take longer if you live closer to the equator because of the extended summers. Strains that originate from further North or South of the equator are usually faster flowering and have shorter cycles.

Some growers like to experiment with different light cycles, which can work well if used correctly. For example, a light schedule of 24 hours in the very early stages of growth can help to keep a stable environment while plants are still fragile.

Best Light Schedules For Cannabis

We can take what we understand about the outdoors and apply it to our indoor grow setting. Over the years, growers have managed to perfect their lighting technique meaning they can now have harvests all year round in all kinds of climates.

Each stage of growth benefits from a particular light schedule. We are basically mimicking the ideal outdoor light cycle with a few adjustments. Flowering is gradual when growing outdoors as the daylight reduces slightly each day, but this process is not particularly necessary.

Outdoor Light Schedule Is Determined By The Seasons

Cannabis plants can be forced to flower by suddenly reducing the amount of the light they receive. The majority of growers stick with what works best in most cases. The standard schedule is to use 18 hours of light for vegetation and then flip it to 12 hours so plants know it is time to flower.

The best thing about having control over the light is that plants can be grown to the desired size before flipping them into flower. You can use this to your advantage whether you are growing indoors or outdoors. Many growers start their plants indoors to be later planted outside, which helps to manage their size.

Let's take a look at how each stage of growth benefits from different light schedules:

Clones (24/0)

Cannabis Clones

Having the lights on constantly for clones can be a good idea as a way to maintain consistent conditions. Lights help to provide a stable temperature and maintain humidity levels. A clone does not have a proper root structure to absorb energy so it must take it from the exterior environment. Any extreme changes in the environment could decrease the chance of a clone rooting.

If you are planning to keep clones, we recommend keeping them on a 24/0 light schedule unless you have the temperature and humidity absolutely spot on at all times, which can be tricky without the right equipment. Once clones have rooted, the amount of light can be reduced.

Seedlings (20/4)

Cannabis Seedling

A seedling is still quite delicate and will be working hard to root itself securely into its grow medium. Water absorption is relatively slow at this stage, so a 20/4 light schedule might encourage the plant to drink a bit faster by providing warmer conditions. Seedlings love warmth and temperatures are less likely to drop for too long if the dark period is shorter.

When a cannabis plant emerges from its seed, two small leaves (cotyledons) appear and photosynthesis begins to take place. Light is a primary source of energy for the plant, and is especially helpful while roots are young. A longer daytime can also help prevent seedlings from stretching too much in search for light, meaning the plant can focus its energy on developing a strong root structure.

Vegetative (18/6)

Cannabis Plant In Vegetative Stage

The vegetative phase is the main period of growth in a cannabis plant's life. Most of the foliage appears during this stage, and there is a noticeable change in growth speed. It is possible to use longer light schedules during vegetation, but most growers reduce it to 18 hours per day with a dark period of 6 hours. This helps to save on lighting costs, which can add up significantly.

Using a longer light schedule, for example a 20/4 schedule, may help to control the size and shorten the vegging period, but it probably won't affect yield or potency of the final harvest. Plants also need a resting period with enough dark hours in order to function correctly. Being able to recognise when it is day and night facilitates their growth process and helps to keep them at optimal health.

Flowering (12/12)

Cannabis In Flowering Stage

Once our cannabis plants are big enough, we can switch the lighting schedule to 12/12. This means they have an even amount of light to dark over a 24 hour period. Sometimes growers give even less as a way to further save on bills, but 12 hours is perfect. Anything under 12 hours may mean the plant underperforms from lack of light, as well as potentially causing problems with mold.

After lights are flipped to 12/12 there is an explosive growth period, often referred to as the pre-flowering stage. The sudden change in light hours is a catalyst for hormones to start developing, and plants can double or triple in size in a matter of 2-3 weeks.

Outdoor plants can be forced into flowering but it can be difficult to manage afterwards. You might be interested in doing this if you live very far North or South, where winters arrive earlier and last longer.

Harvest (0/24)

Cannabis Ready For Harvest

There is a technique growers use at the end of the flowering period which is designed to increase potency and flavour of buds. It has been stated that switching the light schedule to 0/24 for 48-72 hours before harvest can do wonders for your bud, however, there is no concrete evidence for this so it should be experimented with care.

Leaving your plants for too long in the dark at the end of flowering may increase the chance of mold, so if you try this method, make sure you place oscillating fans around your grow room to prevent mold spores settling, but not pointing directly at your buds.

Autoflowering Cannabis

Autoflowering Cannabis

Autoflowering cannabis isn't dependant on light like photoperiodic plants are. Due to their Ruderalis heritage, autoflowers mature based on their age. It is in their genetic makeup to start flowering just a few weeks after germinating. That means they can be grown under different light schedules and flower regardless.

In order for autoflowers to grow properly they still need a minimum amount of light. Some growers go all out and use a 24/0 schedule for the whole grow. The theory is that the more light plants receive, the faster they will develop. This method can work, but it is expensive as your lights will be on for the entire duration of the grow.

Although cannabis plants love light, they also benefit from a dark cycle. Just a few hours of dark can be enough to encourage growth and promote overall health. A 18/6 schedule may be the best option for your autoflowers because this way the plants get plenty of light but also get enough resting time.

Tangerine Dream (Barney's Farm) by SativaSteve88 from GrowDiaries.

Providing your autoflowers with at least 12 hours a day is recommended otherwise they may underperform and produce small yields. If you are trying to save on electricity costs, it is possible to grow autoflowers under a 12/12 schedule for the whole grow. However, this schedule is only really used if autoflowers are grown in a flowering room. Otherwise, autoflowers can be left in the vegging room under an 18/6 schedule.

Light Schedule Grow Tips

We couldn't arrive at the end of this article without a few light schedule tips to get you started. Setting up grow lights is easy but an efficient setup requires strict maintenance. We do have to rely on some equipment which requires some investment, but putting in the effort here will definitely pay off.

Setting up a consistent environment makes the difference between large or average harvests. Any mistakes with lighting could be a problem for your plants if left unattended. Timers can save you a lot of hassle so you won't have to worry about turning your lights on and off each day.

Temperature also plays a big role in cannabis' ability to thrive so you may want to adjust your light schedule depending the kind of climate you live in. Growers that live in hot climates often choose to have their lights come on at night and set the dark period during the hottest hours of the day.


Vegetative Light Cycle

Setting up timers connected to your lights makes perfect sense. Turning your lights on and off manually can be a real inconvenience, and with just a small investment the whole system can be easily automated.

Timers are cheap, easy to use and setting up your light schedule shouldn't take you more than 5-10 minutes. A grow light timer still requires your attention, and you should check it is working correctly every so often. If the timer fails you could confuse your plants by overextending the daytime, or worse, leave them in the dark for too long, which might force them to flower.

Grow Lights

Cannabis Light Schedules

You may find that changing your light during the grow can have positive results. It is common for growers to use different lights to encourage growth at different stages as it can help to provide the optimal conditions.

  • CFL - Compact fluorescent lights are cool and do not use much electricity so they are ideal for the early stages when a longer light schedule is required. They provide the right spectrum and can be left on for a long time without producing too much heat.
  • LED - LEDs can be used throughout the grow but work particularly well during the vegetative stage. Usually, they emit blue and red spectrum wavelengths with some white light to create a 'full spectrum'. LEDs emit low heat so they can be placed close to plants to help prevent stretching.
  • HID - High intensity discharge lamps are well suited to the flowering stage. These lamps are powerful, produce a lot of heat, and require more maintenance, but give great results. HIDs emit plenty of red/orange wavelengths which are fundamental for healthy flowers. Think about how the natural light changes at the end of summer/beginning of autumn.

Cannabis Can Be Grown Under CFLs

Dimmable Lights and Ballasts

Using a dimmable light or ballast allows you to control the intensity of your grow lights. When a cannabis plant is young, it is still sensitive and a strong light can be harmful. Turning up the intensity as plants mature can help to ease them into their environment without shocking them by accident.

You can even turn up the intensity for a few hours each day as plants get bigger. Dimmable lights give you much more flexibility and allow you to increase or decrease intensity without having to change the bulb or lighting fixture. 

Having dimmable lights isn't entirely necessary but can improve your grow significantly if they are used correctly. If the light is powerful enough and emits the correct spectrum, the same light can be used for the duration of the grow.


One of the most important things when it comes to growing cannabis is that your plants receive the correct amount of light. Now you've learnt about the ins and outs of a proper light schedule, planning your grow cycle should be much easier.


Have you experimented with different light schedules? What were your successes or failures? We'd love to hear your thoughts, so if you like, leave a comment down below!

External References

Seasons and Photoperiodism. - Binkley, Sue. (2020).

The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L.. Medical Cannabis and Cannabinoids. - Magagnini, Gianmaria & Grassi, Gianpaolo & Kotiranta, Stiina. (2018).

This article was updated October 2020



Hi. I´ve been doing a combined light procedure.
Veg.- During spring I apply Led Cree light from 11pm to 8am untill the sun is up, then I put the plants to direct sunlight untill sunset.
Forced Flowering.- Direct sunlight for a 12 hour period and 12 hours in a dark room with fan.
I get up to three havests per summer. I´m trying to get four harvests this year.
In my country we are allowed to have up to 6 plants in flowering and unlimited in vegetative but you have to be registered in a goverment agency that regulates growers.