weed light schedule

Cannabis Light Schedule For Seedlings, Vegging And Flowering Photoperiod Weed, And Autoflowers

Getting your light schedule right is an important part of growing cannabis. Actually, the size and the quality of your harvest depends largely on the amount of light your cannabis plants receive and its timing. The seedling, vegetative and flowering stages each require unique lighting conditions. Every grower does things in their own way, and we have tried to capture a wide range of successful strategies for implementing marijuana light schedules. Let’s check them out!

Photoperiod Strains And Their Light Needs

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 most of the summer, after which it “flowers” and grows buds 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.

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Light Schedule For Photoperiod Cannabis Seedlings

Once your seed has germinated (or your clones rooted), your plant is at the seedling stage. This will last for only 2-3 weeks indoors but up to 6 weeks outside. During this stage, your cannabis plant will be exerting a lot of its energy to develop its root system. As a seedling, your plant will transition from growing single blade leaves to growing leaves with multiple blades (also known as “fingers”) – this is an indication that the seedling stage is over.

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Light Schedule Options For The Seedling Stage

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, and 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.

Light Schedule For Photoperiods In 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. The length of the vegetative stage can differ widely among growers – some prefer to take longer before switching the plant to 12 hours of darkness to make it flower. However, they will generally have an indoor cannabis light schedule during the vegetative stage for at least 4 weeks depending on the phenotype and genetics of the plant.

Growing Marijuana – Vegetative Stage

Marijuana Light Schedule Options For The Vegetative Stage

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 16 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 ensure a successful result and 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.

Light Schedule For Photoperiods In 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 12 hours of sunlight or less 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.

Flowering Stage

Light Schedule Options For The Flowering Stage

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 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.

Dialing-In Light Schedule For Autoflowering 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 don’t require the switch to 12/12 to start flowering. Some growers opt for an 18/6 light schedule throughout the lifecycle of 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 productivity of 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.

Cannabis Light Schedule: Short Overview

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 16-18 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.

The right light schedule is an important part of growing cannabis. Here you can learn creating perfect lighting conditions for seedling, vegetative and flowering stages

The Perfect Light Schedules For Autoflowering Cannabis

Growing autoflowering strains? In this article, we explore the ideal light schedule to help your autos produce the best possible harvests.


Autoflowering cannabis strains flower based on age, rather than a change to their light cycle. Nonetheless, providing your autos with the right amount of light remains crucial to ensuring the best possible harvest. Read on to learn all there is to know about lighting for autoflowering cannabis strains.


Cannabis is a C3 plant, meaning it can absorb CO₂ for photosynthesis even during light hours. And since autoflowering plants have short vegetative phases and often grow shorter than photoperiod strains, you’ll typically want to give your autos at least 18 hours of light. This allows for robust growth, without overspending on energy. Your autos love light just as much as traditional photoperiod strains, they just don’t rely on it to begin bloom.


Some growers will even give their autos a full 24 hours of light, arguing that this helps maximise vegetative growth. Growers who stick to 18-hour light cycles, on the other hand, argue that this gives their plants a short “recovery” period that is essential for healthy growth.

There’s no real consensus on whether autos grow better under 18 or 24 hours of light, and we’ve seen growers achieve great results with both methods. To make up your mind, we recommend playing around with both variations and sticking to what works best for you.


Some growers still decide to keep their autos under a 12/12 light cycle during flowering. And that’s fine, as autos grown in these conditions can still produce a fair harvest. Just remember that the buds you harvest under a 12hr light cycle will be noticeably smaller than what you’d get under a 18–24hr cycle. Some reasons you might consider keeping your autos under 12/12 include:

  • Heat concerns: If you live in a very hot climate, you might want to turn your grow lights off during the day to avoid overheating your grow room. In that case, growing your autos under a 12/12 light cycle—with lights on during the night—might be your best bet to avoid causing your plants heat stress.
  • Growing autos alongside photoperiod strains: If you’re growing autos alongside feminized photoperiod strains, you’ll likely have to place your autos in the same room as your flowering feminized plants, meaning your autos will only get 12 hours of light per day.
  • To save money: Running grow lights for 18–24 hours over a few months can get very expensive. If you want to grow autos on a tight budget, you might want to use a 12hr light cycle instead.


SOG, or sea of green, is a cannabis training technique that can produce some great harvests. Rather than getting your plants to grow as large as possible, SOG involves growing several smaller plants in close proximity to form a uniform canopy that maximises light exposure and space.

SOG is a great technique to try with autos because it takes advantage of the naturally smaller stature of these varieties. While every grower will have their own technique for SOG, most will grow between 4–16 plants per m², depending on how big they let each plant grow.

Depending on the size of the particular strain you’re growing, we recommend using 7–10l pots and growing between 4–6 plants per m². This should make the most of your space and lighting while still providing your plants with enough airflow to avoid any mould issues. If you decide to grow more plants per m², remember to use smaller pots to control their size and avoid overcrowding your grow room.


Using the right light spectrum is super important when growing any type of cannabis plant, including autos. Because autoflowering plants have such short life cycles, you really want to maximise the quality of light, nutrients, and soil you give them.

As you might recall from science class, when you shine a light through a prism, it is broken up into different colour spectrums. Plants, which depend on light for photosynthesis, respond differently to these different spectrums. Like with a photoperiod cannabis strain, we recommend sticking to the following light spectrums:

  • 6500K blue light during veg: Blue light spectrums have been shown to encourage vegetative growth, helping cannabis plants grow short and stocky while minimising stretching.
  • 2700K red light during flowering: Red light spectrums are ideal for bloom because they encourage budding and a little bit of stretch, encouraging your plants to produce big, dense flowers.


If you’re new to the world of autoflowers, follow these simple tips for a better harvest:

  • Pick a high-yielding auto: Not all autoflowers are made equal, so be sure to pick a high-yielding strain if harvest size is important to you.
  • Plant in your final container: Up-potting cannabis plants always causes stress. Since autoflowers can go from seed to harvest in as little as 8 weeks, it’s best to avoid this kind of stress and always plant your seeds straight into their final pots.
  • Use an airy growing medium: This helps stimulate root and foliage growth.
  • Use LST: If you want to train your autoflowering cannabis plants, only use low-stress techniques. High-stress techniques like fimming, topping, and supercropping will end up doing more harm than good.

Growing autos? Click here to learn all you need to know about the right light schedule for autoflowering cannabis strains.