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The Ideal Light Cycle for Cannabis At Different Growth Stages

Written by Taylor Christianson Jan 03 2022

Figuring out the best light cycle for cannabis can take time, in some cases even trial and error, however, this can waste time and money. To avoid any mistakes and waste, follow this guide to get everything right first time.

Understanding light cycles

When it comes to cannabis cultivation, the most crucial aspect is achieving a desirable output that results in smiles and profit when it is ready to harvest. One critical component that determines the success of your cannabis plant is lighting. When cultivating cannabis, whether indoors or outdoors, knowing the ideal light cycle for cannabis is vital. The flowering cannabis light cycle you use directly impacts the quality and quantity of your crop. Indeed, light is required for cannabis cultivation, and here’s all you need to know about the cannabis plant’s light cycle.

How cannabis plants use light

Without sunlight from the sun, there will be no life on this planet. Plants require sunlight to fuel their growth and determine their life cycle. Cannabis is no exception. Photosynthesis, a mechanism that allows cannabis (and most plants) to grow and build healthy root systems and leaves, requires light. And there are a lot of them. The plant’s leaves take light energy, the plant’s roots absorb water and carbon dioxide is absorbed from the air during photosynthesis. Plants fundamentally digest the energy they take to produce water and carbon dioxide, which they subsequently use to grow. Plants cannot photosynthesize without light and will eventually perish. As a result, your plants must get enough sunlight to grow and ultimately produce buds throughout the day.

How much light does a cannabis plant need?

Cannabis is a “photoperiod” plant, which means that the amount of light it receives each day determines when it begins to bloom or form buds. During the vegetative phase, most indoor cultivators provide their cannabis plants with 18-24 hours of daylight.

Ideal light cycle for cannabis

The exact hours required to keep a plant in the vegetative stage vary by strain. However, you may keep most cannabis plants in the vegetative stage for 18 hours or more per day. Your plant requires the right growing circumstances to thrive, from germination to producing flowers and buds, and one of the most important is light. On their way to making resinous buds, cannabis plants go through the following 4 phases:

  • Stage of germination (3–10 days);
  • Seedling stage (two to three weeks);
  • Stage of vegetative growth (3-16 weeks);
  • Stage of flowering (8-11 weeks).

Each stage necessitates unique lighting and nutrient conditions, which cannabis cultivators customize to the strain in question. Whether it is an Indica or a Sativa, an autoflowering or photoperiod plant, each one demands certain growing conditions that define the quality of the buds and the output.

Light schedule for seedlings

The seedling stage lasts 2-3weeks, and it’s during this time, the plants begin to sprout leaves and new fan blades.


The cannabis plant should receive 16/8 hours of light during this stage. Fluorescent CFL blue lights, which don’t emit much heat and have lower power, may benefit cannabis seedlings.


When it comes to seedling light schedules, it’s important to remember that this is the phase of growth that needs the most light. A cannabis plant in the seedling stage can receive up to 18 hours of light. The 18/6 method, which signifies 18 hours of unbroken sunshine and 6 hours of uninterrupted darkness, grows these seedlings.

Best light cycle for veg

The plants develop leaves and stem but no flowers during the vegetative stage of cannabis, which lasts about 3-16 weeks.


Cannabis lighting
Plant sapling growing with LED grow light.

However, if you want your plant to grow as big as possible, you can keep it under 24 hours (24/0) indoor light.


Maintain a minimum of 18 hours of growth light (also known as 18/6) when cannabis plants are in the vegetative stage.

Best light cycle for the flowering stage

Depending on the strain you’re producing, the flowering time might take from around 5-16 weeks. This is when the blossoms appear, followed by the sticky buds that provide us with our favorite cannabis products.


The cannabis plant requires 12/12 hours of light during this stage to encourage flowering.


Sunlight absorption required for sun-grown cannabis to move to the blooming stage is reduced to 12 hours per day. When you grow your cannabis, one of the first signs of flowering stage outdoors is that they develop buds (flowers) as the days get shorter, and they should receive a minimum of 12 hours of complete darkness. It’s critical to understand that you must keep the cannabis plants in total darkness for the remaining 12 hours of the day.

Light cycles for autoflowers

Auto-flowering plants are day-length independent, which implies that the introduction of flowering is not dependent on light or dark intervals. They’re created by crossing cannabis with ruderalis, a day-neutral plant with a short blooming cycle. As a result, longer light cycles can be used during flowering, allowing you to give the plant more overall energy during the flowering period. You can continue to offer 18/6 lighting to auto-flowering plants throughout their life cycle rather than reducing the lights to 12/12. You can enhance your yields, but you’ll also be using more electricity, which will drive up the cost of the growth. As a result, it is not more energy-efficient, but auto-flowering plants may allow you to harvest more within a given time frame, which may be more critical to some producers.

Managing light cycles indoors

Your cannabis plant requires the right growing circumstances to thrive, from germination to producing flowers and buds, and one of the most important is light. If you’re growing cannabis plants inside and don’t have any concerns about room height or space, you can keep it on a 24/0 or 18/6 strict routine during the vegetative stage. This should last for 60 days, the optimal time to increase bud production.

How to maintain light cycles outdoors

If you wish to grow cannabis outdoors, your light cycle for flowering cannabis should likewise be set to 18/6 or 24/0. Still, you’ll want to keep them inside until all unfavorable conditions, such as cold temperatures and frost, have passed. After that, you can safely release them into the open air and allow them to grow and thrive naturally, relying on the sun’s natural light cycle.

Taylor Christianson

Taylor is an indoor expert. Through their writing, they offer a masterclass on growing cannabis indoors, maintaining a productive growing environment, and guiding growers through all the stages of growth both indoors and outdoors. Combining multiple fields of expertise allows for Taylor to give in-depth insights into overall cannabis growing. About this Author

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