Cannabis Light Schedule

14 February 2020

cannabis light schedule

Getting your light schedule right is an important part of growing cannabis. The seedling, vegetative and flowering stages each require unique lighting conditions. Every grower does things in his or her own way, and we have tried to capture a wide range of successful strategies for implementing marijuana light schedules. Let’s check them out!

Sinsemilla Science

best light for growing

Like all plants, cannabis uses photosynthesis to create chemical energy out of light. This process involves three components: light, water and carbon dioxide. Cannabis absorbs light energy in its leaves, takes up water by its roots and consumes carbon dioxide from the air. Light energy is used to metabolize water and carbon dioxide which the plant uses to grow. Light is essential to cannabis’ survival as well as to yielding large crops. Without light, cannabis is prohibited from photosynthesizing and will die. However, with proper lighting, cannabis can thrive.

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Optimal Light Schedules For Photoperiod Strains

Cannabis is an “annual” plant, meaning it lives for just a single growing season. The natural growth cycle for cannabis is to begin life in spring as a seedling, then to “veg” all spring and summer until fall, when it “flowers” and grows buds (or seeds) in order to propagate the next generation. Because of this heritage, photoperiod cannabis will “flower” when exposed to the right darkness conditions. Give cannabis 12 hours of uninterrupted darkness a day consistently and it will go into flower (thinking that it’s fall when the nights grow longer). Each stage of a cannabis plant’s lifecycle has its own unique lighting requirements.

Growing Roots: The Seedling Stage

indoor cannabis light schedule

Once your seed has been germinated (or your clones rooted), you have begun the first part of your cannabis journey. Your plant is at the seedling stage. This will last for only 2-3 weeks indoors but up to 6 weeks outside. Your plant will transition from growing single blade leaves to growing leaves with multiple blades (also known as “fingers”). On average, mature cannabis plants have between five and seven blades on each leaf. Cannabis plants are viewed as seedlings until they develop their mature leaves. During this stage, your cannabis plant will be exerting a lot of its energy to develop its root system.

Light Schedule Options For The Seedling Stage

cannabis seedling light schedule

The optimal marijuana light schedule for seedlings is 18/6, meaning lights are on 18 hours per day and off (giving the plant darkness) for 6. However, some growers experiment with cannabis light schedules of 20/4 and even 24/0. Running lights on all day with a 24/0 light schedule will max out the limit of your equipment, especially fans, and will run up a high electricity bill. Opting for a 20/4 schedule gives your plants a small dark period to respire while still maximizing the amount of light your plants get, while also giving your equipment a short breather. It’s okay to experiment and find out what cannabis seedling light schedule works best for you.

Seedlings do not require bright light to grow well. Contrary to what many may think, light that’s overpowering can sometimes harm seedlings. However, if you dim your lights or use a weaker bulb, make sure to check the distance between the bulb and your plant. If the light is too far away from your plant, it will stretch, and if the light is too close, it may burn (especially if it’s an HID). With an LED, for example, you’ll want to keep your light between 60-76cm (24-30 inches) away from your seedlings.

Growing Big: The Vegetative Stage

During the vegetative stage, cannabis undertakes most of its growth. By now, the plant should have a solid root structure on which to grow. You can gauge from the shape of your plant at this stage whether it’s an Indica or a Sativa strain. Spacing between the nodes will show you, as Sativa plants grow lanky and are open in foliage while Indicas are shorter and denser. The length of the vegetative stage can differ widely among growers. However, they will generally have an indoor cannabis light schedule during the vegetative stage for around 4 weeks depending on the phenotype and genetics of the plant.

Marijuana Light Schedule For The Vegetative Stage

best light for vegetative growth

During the vegetative stage, you don’t want to let your plants go into flower. This means you should have your cannabis plants under consistent light for a minimum of 18 hours a day. There are growers who opt to go even further and provide a period of 20 hours of light. Finding the right amount of light for the genetics of your plant can take some tweaking. That being said, a pattern of 18/6 (18 hours of light followed by 6 hours of darkness) is the standard marijuana light cycle for indoor vegetative growth.

Outdoors, growers don’t have the ability to end the vegetative stage by switching the cannabis’ light schedule. Growing outdoors means being at the mercy of mother nature and waiting for dark days in the late summer or early fall to trigger the flip into flower. Planning your outdoor grow ahead of time is a must to maximize your yields. Ideally, plants should be placed in the ground as soon as the nightly frost ends, allowing your plant a few months of vegetative growth before it flowers. 

Growing Buds: The Flowering Stage

The flowering stage is the third and final phase of a cannabis plant’s lifecycle. This stage occurs naturally outside when the plant gets less than 12 hours of sunlight each day when the days shorten during late summer or early fall. Indoors cannabis can be tricked into thinking it’s time to flower by shortening the light cycle to 12 hours of light and 12 hours of darkness.

Perfecting A Light Schedule For The Flowering Stage

marijuana light cycle

Indoors, once you’re ready to begin the flowering stage, you’ll want to set up a light schedule of 12 hours of light followed by 12 hours of darkness (12/12). This stage lasts for 8 to 12 weeks. Your lights should be on a constant cycle of 12/12, as any interruption can cause havoc on your grow. In fact, too much light can cause your plant to revert to the vegetative stage (“re-veg”) or even turn into a hermaphrodite. For best results indoors, use a color spectrum between yellow and red.

Outdoors, much is beyond the grower’s control. You want to plant early enough so that when fall and flowering hits, you have a large enough plant to pack on bud weight. You also want to choose a location with full sun to maximize the amount of light your plant receives during the day.

Properly Lighting Autoflower Strains

Autoflowering cannabis plants have been crossbred with cannabis Ruderalis to create cannabis plants that flower based on time rather than a light/dark schedule. Autoflowers naturally have a short vegetative phase and grow shorter than photoperiod strains. Some growers opt for an 18/6 light schedule for their autos, believing that the plant needs a short recovery period of darkness. Other growers opt for a marijuana light schedule of 24/0, arguing that it maximizes the plant’s short vegetative growth period. There are even growers who opt to provide their autos with a 12/12 schedule during flowering – however, note this will reduce the size of your buds and is not actually necessary, as autoflowering plants flower based on time, not light cycles.


Properly lighting cannabis takes some work but is fully worthwhile. Each phase of a cannabis plant’s growth requires unique lighting needs. Seedlings don’t want too much light, vegetative plants need at least 18 hours of light, and flowering plants need 12 hours of darkness every night. Knowing what stage your plant is in and how to give it the light it needs is crucial to growing healthy cannabis plants. Every plant’s genetics are unique, so don’t be afraid of experimenting with different light schedules to find out what works best for your needs.

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