Continuing our series on Cannabis Light Cycles, we now discuss the 18/6 light cycle used during the vegetative stage of marijuana growth.

There are two light cycles for growing marijuana indoors

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

The 18/6 light cycle and the 12/12 light cycle

The first stage of marijuana growth requires eighteen hours of light and six hours of darkness. This 18/6 light cycle is referred to as the Vegetative Stage.

The second stage of marijuana growth requires much less light and more darkness. The optimal light cycle used is 12 hours on, 12 hours off. This 12/12 light cycle is referred to as the Flowering Stage.

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

When does the Vegetative Period Begin?

In order to grow marijuana indoors, you must first start with either clones, or plant seeds. If you are starting with seeds, you will want to use one of these methods to sprout your seeds.

If you are starting with clones, you will also need to “root the clone.” This can be done many ways. We use an EZ Cloner hydroponic set up to develop roots in our clones.

During the process of rooting your clones and handling your sprouted seedling, your plants at that stage need only indirect light. Direct light is fine, but you do not want to subject these babies to any heavy pounding of lumens.

Once you have sprouted your seed, or rooted your clone, you are ready for the 18/6 light cycle.

Using the 18/6 light cycle for your marijuana plants

During the vegetative period, you will take your plants from small seedlings or clones into sizable plants ready to be flowered. The purpose of the vegetative period is growth, not maturity.

Softer light bulbs is preferred during the 18/6 light cycle

Therefore, soft lighting works best. You do not need the same amount of power as will be needed during the flowering period. You should use fluorescent lighting, such as T5 bulbs or lower watt LED lights.

The main objective is to subject your plants to a lot of light, but not necessarily powerful light. Save those lumens and watts for the next stage.

Stronger lights are necessary during the 12/12 light cycle

Initially, the extra darkness of the 12/12 light cycle will be enough to send your plants into the post-veg stretch and trigger flowering. So you can start the flowering stage with low watt lighting, such as 600 watts.

After the third week or so, you will need 1,000-watt fixtures to get maximal development and density of your buds. But you don’t need that much intensity for all of flower. Just in the middle. The end can be tapered down as well.

Other Vegetative Light Cycles, such as 20/4 and 24/0, can be used

There are some growers who prefer to use a different light cycle than 18 on, 6 off. Some growers like to keep the lights on all the time. The theory is that this will cause faster growth, and speed up the vegetative cycle.

A shorter veg cycle would mean you could start the flowering stage sooner. This would equate to getting more flips done in a year, and therefore more harvests.

But we have not found that to be completely true. Our head of cultivation, Dan Phillips, has tried all methods – 18/6, 24/0, and 20/4. Dan has found that it is a waste of power to run the lights on all the time. It does not actually speed up growth.

If you think about it, 18 hours of light is pushing the plant as hard as you can push it. In nature, there is never that much light unless you are in the far north or south regions close to the poles. If you’re in an Alaskan summer or a Siberian summer, you have light all the time. This is why autoflowers were invented. But otherwise, cannabis in nature has never had that much daylight during its vegetative season.

The six hours of darkness in the 18/6 light cycle helps the plant to recover, and that recovery causes as much growth as the additional light in 24/0 light cycle.

How Long should you keep your marijuana plants under 18/6 Light Cycle?

This is a common question that is asked, and the answer is “it depends.” The length of your veg light cycle is determined by your overall goals and objectives. Primarily, how big do you want your plant to be before switching to 12/12? And to answer that, work backwards from how big you want it to be at the time of harvest.

The more you do this, the better you will get at these calculations. In general, we have written before on this blog that most people will do well with a vegetative period of four to eight weeks, before switching to a flowering period that will last eight to nine weeks, depending on the strain.


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