When growing marijuana indoors, you will have to utilize two different light cycles for your plants.

  • The first is the vegetative light cycle, which is 18 hours of light, six hours of darkness.
  • The second light cycle is for the flowering period, and consists of 12 hours of light, 12 hours of darkness.

There are also variations of these two light cycles, which we will discuss later in this article.

The purpose of the 18/6 vegetative light cycle

The purpose of the 18/6 light cycle is to cause growth of the marijuana plants. It is during this stage – the vegetative stage – that your plants are growing bigger and taller. This is a good time to train your plants, and prepare them for the second photoperiod – the flowering stage.

While subjected to 18 hours of light and only six hours of darkness, your plants will not have sufficient recovery time to mature into budding flowers. Because these plants cannot get into a mature state, they remain immature – and therefore young and pliable, capable of being bent and trained. Your plants can also be topped. This technique will produce more cola yielding branches later during the flowering stage.

The purpose of the 12/12 light cycle

The flowering light cycle is for the purpose of causing bud development. While the plants will still grow with the flowering light cycle, the growth eventually stops and all energy is focused on bud development.

When you are ready to flower your plants – meaning they are of the right size, and you have topped, pruned, and trained them – you can begin the flowering phase.

To cause your plants to stop vegetative growth and to start flowering, you must reduce the amount of light.

How to Use the Two Light Cycles for growing Weed

You will start off by sprouting your seeds. You don’t even need a grow light for this. Simply follow one of the methods here, such as the paper towel method or the cup of water trick.

After your seeds have sprouted, you will want to plant them in your soil. This can be a red solo cup, or a small pot. From here, you can start the 18/6 light cycle and begin vegging your plants.

For a good schedule to follow, we suggest our Week-by-Week Guide to the Vegetative Stage. In that guide explain when to transplant, when to top, how to prune, and how long to keep your plants in the vegetative state.

After you have followed that guide, you will want to start flowering your plants. Now it is time to switch to the 12/12 light cycle.

Depending on the strain you are growing, this phase will last about eight to nine weeks. Most strains flower in 56 to 63 days, but some may go as long as 70 days or more.

If you are looking for a detailed guide to get you through the flowering stage, check out our Week-By-Week Guide to the Flowering Stage. This series of articles will inform you on how to care for your plants to maximize yield, quality, and plant health. And you will be able to follow along with the articles in real time.

Other Light Cycles for growing weed

In addition to the light cycles described above, you may also use variation of these two light cycles, so long as you follow the same principles.

For example, instead of using 18 hours of light, and six hours of darkness for your vegetative stage, you could use 24 hours of light, with no dark periods. However, we do not believe this accelerates your growth in any meaningful way, and therefore we recommend 18/6 over 24/0.

Alternatively, you could also use sixteen hours of light and eight hours of darkness. This could require you to extend your veg period to achieve the same amount of growth as 18/6 in a given time frame.

When it comes to flowering, we prefer 12 hours of light and 12 hours of dark, although we may turn the lights off after 11 hours toward the very end of the flowering stage. Some growers utilize the 11/13 schedule for all of flower. As you become a more advanced grower, you can experiment with these little tweaks.

Understanding the Cannabis Light Cycles

Understanding the two light cycles for growing weed indoors is fundamental if you want to be a grower. Being a grower has its rewards, such as being capable of growing endless amounts of smokable herb for your own medicine or pleasure.

But the light cycles of cannabis is just the beginning of your education in cultivation. Once you grasp propagation, vegetation, and flowering, you will need to really master and refine your harvest techniques, particularly drying and curing.

We have prepared these guides for you to use:


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