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Ideal Light Cycles for Weed’s Growth Stages

November 16, 2021

Cultivating cannabis can be a challenging adventure, especially for beginners. However, you can produce a gratifying yield if you understand and master one critical element in the growing process: the light cycle for weed.

Marijuana plants need light, lots of it, to grow big and strong. When you cultivate indoors, you must mimic the amount of light that plants get in nature.

Timing is also essential. Ensure your plant gets the optimal amount of light at the right time during its growth stages.

The weed light cycle you implement will determine the health of your cannabis plant and the yield it delivers.

In this straightforward guide, you’ll learn how light affects your plant’s growth, how much light to give it, and the best time to do it. 

Cannabis flower under grow lights
Cannabis flower under grow lights

The science behind the light cycle for weed plants

To understand the science behind the light cycle for weed, it’s essential to know that while cannabis plants need light for growth, it’s the amount of darkness that influences their development.

Unlike autoflowers that automatically transition from the vegetative stage to the flowering phase when they reach maturity, photoperiod weed requires a certain number of hours of uninterrupted darkness to do the same.

When you change the weed light cycle of a photoperiod marijuana strain from 18 hours of light and 6 hours of darkness (18/6) and give the plant 12 hours of the latter (12/12), it begins to flower.

You can adjust the schedule of your grow lights for indoor plants to mimic what happens in nature when the seasons change and the number of daylight hours get shorter.

Why marijuana lighting cycles help you get bigger yields

Your cannabis plant needs light to thrive, and using optimal marijuana lighting cycles will maximize the quality and quantity of your harvest. Knowing when to apply the schedules also matters.

There are two weed plant stages where light is critical, namely:

The first phase is when your plant focuses on growing bigger and taller, and the amount of light it gets directly corresponds with the size it grows to. More light means larger plants

By manipulating the cannabis light cycle and giving your crop more light during this period, you’re helping it grow better and potentially increasing its future yield.

The minimum amount of light to encourage healthy growth is 18 hours. That’s how long you should keep your grow light switched on every day. 

If you want larger plants, you can switch to a 24/0 marijuana light schedule, giving continuous light throughout the day. However, this schedule may cause your crop to develop too fast.

Weed light cycle for each growth stage

What makes photoperiod marijuana plants unique is that they need different light levels during each growth stage.

Here’s a brief explanation of the light cycle for weed at each growth stage:

Light schedule for seedlings

If your climate is suitable for seedlings, you can cultivate them outdoors. Alternatively, you can grow them indoors and wait till it’s warmer before you move them outside.


At the seedling stage, you can use inexpensive grow lights such as LED or fluorescent bulbs. What matters is that you provide your seeds with lots of light.

The optimal light schedule for seedlings is 18/6. Therefore, leave your grow lights on for 18 hours and switch them off for the next six. Having a timer will save you the trouble of doing this manually for the duration of the stage.


Nature should do its job when your seedlings are outside. If they don’t get enough sunlight for 18 hours, use artificial lighting for the rest of the period.

Marijuana seedling outdoors
Marijuana seedling outdoors

Cannabis vegetative lighting

During the vegetative stage, the cannabis light cycle you use should focus on giving your plant plenty of light to help it grow big and healthy.


When cultivating indoors, make sure you use at least the 18/6 light schedule for weed. Keep your grow lights on for a minimum of 18 hours and off for six. 

You can give your plant up to 24 hours of light if you want a larger crop. However, we discourage using the 24/0 schedule because your plant can develop too quickly with that much light.  

Cannabis in veg stage under LED lights
Cannabis in veg stage under LED lights


The marijuana light schedule for outdoor plants is based on the available number of hours of daylight. So, be sure that your plant will get at least 18 hours of sunlight if you cultivate outside.

Alternatively, you can keep it indoors under grow lights until the climate is suitable before moving it outside. 

Flowering light cycle

The flowering light cycle refers to the amount of darkness that stimulates marijuana plants to transition from vegetating to producing buds.

In nature, this phenomenon occurs when seasons change. As summer moves into fall, the days get shorter, and there are more dark hours, signaling cannabis plants to start flowering.


When growing indoors, you can nudge your plant to begin budding by changing the weed light cycle, mimicking the change in seasons.

Set the timer for your grow lights to switch off and on every 12 hours. Using this 12/12 cycle, you can expect your plant to start bud production for 8–12 weeks. Some strains have a shorter flowering period.

Cannabis flowering indoors
Cannabis flowering indoors


There’s very little to do except to let nature take over. The flowering light cycle begins automatically when fall approaches.

In late June, the shorter days and longer dark hours will cause your cannabis plant to flower, and within weeks, you’ll be able to harvest beautiful nugs.

Autoflower light cycle

Marijuana cultivars that are autoflowers will begin producing buds when they attain a certain maturity point. They don’t depend on any light cycle for weed to signal them when to bloom. 

So, it’s not critical to have an autoflower light schedule if you cultivate these strains. 


You should be aware that autos are typically smaller than photoperiods because of their shorter vegetative stage. As such, if you want a robust plant and better yield, give it lots of light.

The 18/6 flowering light cycle is the optimum schedule to implement. Growers have also seen excellent results when giving their crop 24 hours of light.

Some cultivators stick to the 12/12 cycle, which triggers flowering in photoperiods to save energy costs, but this schedule produces smaller buds than the 18/6 and 24/0.

There’s no hard and fast rule. We suggest you experiment with the various weed light cycles and choose the one that meets your requirements.


It’s best to leave it to nature if you cultivate outdoors. Alternatively, you can decide on a light cycle that you think will give you the results you desire.

If there’s not enough sunlight for your chosen marijuana light schedule, you may use grow LED lights for cannabis to make up for the lost hours.

Cannabis in sunlight
Cannabis in sunlight

If you have specific questions about marijuana lighting cycles, the following section may have the answers you want.

Alternatively, you may get in touch with us if you have additional inquiries.

When should I change my light cycle to 12-12?

As cannabis plants switch from vegetative mode to flowering when there are more dark hours than light, you should set your weed light cycle to 12/12 when you want your plant to start producing buds.

Can a weed plant get too much light?

Although marijuana plants love light and need it for their growth and development, too much light is not recommended in some cases. 

For example, while an 18/6 marijuana lighting schedule during the vegetative stage can boost your plant’s growth, giving it 24 hours of light may cause it to develop too quickly.

Too much light can also hurt your cannabis plant if you place your grow lights too close to it. The intense heat can burn its leaves and potentially destroy your crop.

How long can seedlings go without light?

Seedlings need at least an 18/6 marijuana light schedule to develop into a healthy plant. As such, it’s advisable not to let them go more than 6 hours without light at this stage. 

Weed plants under LED lights
Weed plants under LED lights

What’s the best weed light cycle?

There’s no best light cycle for weed per se. It depends on your cultivation goal and when you implement it during your plant’s growth stages.

However, if you apply the fundamental principle that marijuana plants need lots of light to grow big and strong and at least 12 hours of darkness to start flowering, you’ll know which cannabis light cycle to use and when.  

Check out our blog for more cultivation tips from expert growers.

About the author: Derek LaRose

Also known as Kronic from The Cannabis Kronicles, Derek LaRose is a young ambitious cultivator and a staple educator for indoor cultivation.

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