Light Cycles for Growing Cannabis

A cannabis plant will grow differently, according to how much light it gets (daytime) compared to how much darkness it gets (night-time). There are many different light cycles for growing cannabis indoors, each have their own pros and cons. In this guide, we will discuss different light cycles for growing cannabis, when to use them, and why. This will help you understand how to grow cannabis a little more.

If you have any questions about cannabis plant light cycles, that aren’t covered here, feel free to ask for help in our cannabis growers forum

Watch the Video of this Guide Here!

What are Light Cycles

Light cycles are the amount of light, vs the amount of darkness a plant receives. A cannabis plant will start to grow in the Spring, flower throughout Summer, and be harvested in the Autumn. Throughout the year, it will receive a different amount of light, depending on what season it is. When you use light cycles indoors, you’re trying to imitate the length of the day for spring time, or summer time. This will keep a plant in “veg” (simulating Springtime growth) or in “flower” (simulating Summer growth).

To understand light cycles for growing cannabis, you should understand how a plant grows in its natural habitat, outdoors.

Cannabis Light Cycles Outdoors

As the year goes from Spring, into Summer, and then to Autumn, the days get longer, and then shorter (unless growing near the Equator). Plants can detect the changes in the amount of light they receive, and this tells them what season they are in.

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Spring Equinox

Sometime around March 20th, in the Northern Hemisphere, or September 23rd in the Southern Hemisphere, the days begin to get longer, and we head into spring. This is when a cannabis seed will germinate, and begin its life cycle outdoors. As the days get longer, the plant will grow more and more, and continue into its vegetative state, until the days start to become shorter. This happens shortly after the summer solstice.

Summer Solstice

The Summer Solstice is when the days reach their longest. Up until now, the plant has grown in its vegetative state. This is midsummer, and the plants will soon prepare themselves for flowering. It will depend on what strain you’re growing, but the flowers will start to grow when the plants get around 14 hours of daylight, and ten hours of darkness. When the flowering stage begins, it will continue for around 8 to 16 weeks, until it is harvested in late Autumn

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Autumn/ Fall Equinox

Autumn Equinox will arrive in mid September. The days are 12 hours long, and the night is also 12 hours long. This is when a cannabis plant will be in the flowering stage. By now, the plant should already be showing small flowers. When a plant starts to grow its flowers is strain dependant, and so is its harvest time. Sativa dominant strains will flower for longer than Indica dominant strains, and may need to be harvested later.

A cannabis plant will flower for anything from 8 to 16 weeks. But when growing outdoors, you must harvest the plant before frost sets in, and the colder weather of Winter starts.

Winter Solstice

At this point, your plant should already have been harvested. This is the height of Winter, when the nights are longer than the days. In its natural habitat, a cannabis plant can not survive these conditions, and would have died already. Any seeds made by the female plant, will be laying under the surface of the ground, waiting for conditions to improve. When Springtime comes again, and the climate starts to get warmer. The seeds will germinate around the Spring Equinox, and the whole cycle begins again.

This is how a cannabis plant uses light cycles naturally in the wild. When you grow cannabis indoors, you have to simulate the length of the day and night, to induce certain stages of the cannabis plants life cycle.

Growing around the Equator

The days and nights around the Equator are always around the same length, 12 hours day, and 12 hours night. A cannabis plant can grow fine under 12/12, but it will not stay in its vegetative stage for long. About 4 weeks after germination, a cannabis plant will reach maturity, and it will be ready to grow flowers. Before this, the plant is not ready to grow buds. Even if it is on 12/12, it will only veg. You can grow cannabis using 12 hours of light, and 12 hours of darkness from seed, just like a plant grown at the Equator. But this will not speed the grow up! You still have to wait about 4 or 5 weeks after germination before the flowering cycle begins.

Light Cycles For Growing Indoors

The idea of using light cycles to grow cannabis indoors, is pretty much trying to simulate the sun, at certain times of the year. This is not only done with the length of day and night, but also what colour (spectrum) light the plants get. In Springtime, the light will be slightly more into the blue spectrum, and during Summer, it will be a lot more red. Different bulbs can be used at different stages of growth to imitate this.

You have more control over lighting when growing cannabis indoors, and can control the length of day and night by using a timer.

Timers for Cannabis Grow Lights

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Grow lights are heavy duty, and they use a lot of power. Because of this, you need to be sure you use the correct timer for your lighting rig. You have to be safe with your equipment, if you use a timer that isn’t designed for a heavy load, it may short circuit and set fire to something. At the very least, the timer could break, and the light cycle your plants are used to will be suddenly changed, and this will cause them stress.

If your plant is in flower, and there is an irregular light cycle, it can cause it to “hermie”. You must use reliable, safe equipment. You can’t put a price on piece of mind.

24/0: Lights on all day

A cannabis plant can be grown under 24 hours of light with no dark period. This light cycle tends to be used on seedlings or cuttings in the early stages of growth. Some growers will grow auto flowering cannabis plants under 24 hours of light. But of course when growing indoors, more hours of light means more energy bills. It is not clear whether the extra energy turns into extra yield.

20/4: Lights on for 20 hours

A light cycle of 20 hours light, and 4 hours darkness, tends to be used for seedlings and cuttings, or auto flowering plants. This is preferred over 24 hours of light. Ss it gives the plant some time to rest, and it reduces the energy bills a little. When you are growing indoors, you are trying to imitate a natural environment. All plants will receive some hours of darkness. So 20/4 is usually the most amount of light a grower will give to a plant.

18/6: Lights on for 18 hours

18/6 is the most common light cycle for growing cannabis. It used to vegetate photoperiod cannabis plants, or grow auto flowering plants for their whole life cycle. By keeping a photoperiod cannabis plant under 18 hours of light and 6 hours of darkness, it will stay in the vegetative stage. A plant can be kept in this state for years, and this is how mother plants are kept. A cannabis plant will not start to flower until it receives at least 10 hours of darkness, and 14 hours of light. But this is strain dependant.

14/10: Lights on for 14 Hours

Some strains of cannabis may begin to flower with 14 hours of light, and ten hours of darkness. This is risky though. If the cannabis plant gets confused about its light cycle during the flowering period, it can “reveg” or “hermie”. To ensure a plant goes into flower, and has no side effects, most cannabis plants will be flowered under 12/12.

12/12: An Equal Amount of Light and Dark

12/12 is the light cycle used to induce flowering in a cannabis plant. When the days and nights are the same length of time, a cannabis plant will release a hormone that triggers the flowering process. This hormone takes a few days to a week to build up, and then you will see flowers starting to form.

The night cycle must be undisturbed, with no light interrupting it at all! Make sure that any equipment with lights on, or any light leaks coming into the grow room are sealed. Disturbing the night cycle can force the plant to re veg or hermaphrodite and grow seeds.

10/14: Lights on for 10 Hours a Day

On some rare occasions, growers may opt for 10 hours of light, and 14 hours of darkness. Light degrades THC. So reducing the amount of light a plant receives during the flowering stage, will reduce the amount of degradation of the THC.

10/14 can give the plant extra time to ripen towards the end of the grow, without pushing the trichomes too far. It may improve flavour and potency, but less light means less yield. So it’s a cost you have to weigh up for yourself. The most popular light cycles for growing cannabis are 18/6 for veg, and 12/12 for flower. Just sticking with these two light cycles will be fine. But you can experiment if you want to, it’s your grow.

Photoperiod Cannabis Plants

Photoperiod cannabis plants are plants that react to light cycles. For example, plants that will vegetate under 18/6 but not flower until they are under 12/12. A photoperiod cannabis plant will not reach maturity until it is around 4-5 weeks from seed. Even if a plant is grown under 12/12 from seed, flowers will not start to show until it is mature. It is always best to veg a photoperiod cannabis plant under 18/6 for at least four weeks before flipping to flower.

You can veg a photoperiod cannabis plant for as long as you need to. By growing one plant, and extending the veg period by a couple of weeks, you can achieve the same yield as ten small plants.

Night Cycle During Flower

When growing photoperiod cannabis plants, it is vital that the hours of darkness are undisturbed. Even a small LED light on a piece of equipment is enough to stop your plant from flowering.

If you have light leaks, you risk three things:

1: Your plants will not flower when you want them to
2: Your plants may revert back to the vegetative stage
3: Your plants will hermaphrodite, and produce seeds

Before you change your light cycle to 12/12, you should make sure your tent or grow room is fully light proof, and free from any lights on the equipment. You can do this by getting into your grow room with the lights off, and all doors and vents closed. Wait for a few minutes for your eyes to adjust to the darkness, and you will see any lights coming in.

Seal up light leaks with tape, or direct intake vents to draw air from a dark place. If you have any equipment in the grow room, that does give off light, remove it, or cover the light with tape, to stop it leaking out.

Autoflowering Cannabis Plants

Auto flowering cannabis plants are a little different. They will flower under any light cycle, 12/12, or 24 hours of light a day. This is a great advantage, as you can grow auto flowering cannabis plants along side vegetating plants, or have many autos in the same room, at different stages of growth.

Light leaks can still cause problems with autos. You should still ensure any dark period you give you plants, is fully dark, with no light leaks, or irregular exposure to light. Most growers will grow auto flowering cannabis plants under 18/6, for their whole life cycle. Some may even push it to 20/4. But it is rare that any one grows autos under 24 hours of light once they are out of the seedling stage.

Final Notes on Light cycles for Growing Cannabis

Keep things simple, choose which light cycle will work best for you, and decide if you are going to grow autos, or photos. When you choose you light cycle, you should set the night time, to be over the hottest part of the day. For example, if you are growing in summer, the hottest part of the day will be around midday.

Because grow lights produce a lot of heat, it is advised to run them at night, and turn them off during the day. This will make it easier to keep temperatures under control. Thanks for reading Percy Growers. If you need any more help with light cycles for growing cannabis, you can post in the forum anytime and start a discussion. Good luck with your grow, and stay safe!

28 thoughts on “Light Cycles for Growing Cannabis”

  1. @monkeydo Ahh right, I was thinkin it would be somewhere around 8 weeks but thought I better make sure just incase it creeped up on me and took me by surprise. im checkin on them every day and think I more or less know what to watch out for when they are nearly ready. ok thanks for that mate, cheers 😉  

  2. yeah i see whats up here with my girls, you said somewhere up there that some strains may flower under 14/10 veg and start to reveg in flower or hermie? hope that doesnt happen to me that is a mess. first time in about 15 yrs so i wasnt expecting miracles anyway.

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