When to Switch to 12/12 Light Cycle for Flowering Marijuana (2024)

This article continues our series on Cannabis Light Cycles for growing indoors.

Early in the process of growing marijuana, you utilize a vegetative light cycle. However, your plants will never produce bud unless you switch to the 12/12 flowering light cycle. Since the entire reason you are growing marijuana is to have luscious, flavorful buds to smoke, you can’t stay in veg forever. You have to eventually switch to the flowering light cycle.

This article covers when to make the switch from the 18/6 veg cycle to the 12/12 flowering light cycle.

How to Determine if you are ready to switch to 12/12 Light Cycle for your marijuana plants

The first thing you must do before switching from the 18/6 veg light cycle to the 12/12 flowering light cycle is to determine if your plants are ready. In order to determine if your plants are ready, you must first set goals and objectives for what you are trying to accomplish with this flowering cycle.

In order to get you thinking about these concepts, ponder some of these questions:

• Are you wanting to flower a lot of plants, or only a few?
• Are you seeking to maximize yield per plant, or yield per batch?
• Are you wanting to maximize how many cycles you can run in a year? Or are you just flowering a crop less frequently, say a couple times a year?

All of these considerations will play into the larger picture of when to switch to the 12/12 light cycle. The reason is that, as you know, there are two photoperiods (which means two light cycles). The Vegetative Period is accomplished indoors with an 18 on, 6 off light cycle. The flowering period is accomplished indoors with a 12 on, 12 off, light cycle.

What we are trying to determine for you now is how long the first cycle – 18 on, 6 off – should last.

Are your plants ready to be flowered under a 12/12 light cycle?

The longer you run the vegetative light cycle, the larger your plants will grow. A bigger plant will equate to a bigger yield. But it will also occupy more space, grow taller, and depending on your grow space limitations, this may or may not be feasible.

Once a plant is placed into a flowering light cycle, that plant will double or triple in size over the next 60 days.

So you definitely need to grow it to the proper height during the vegetative cycle in order to ensure everything goes smoothly during the flowering cycle. Too small, and the yield won’t be there. Too large, and you may not have enough space and will run into problems controlling the plant, controlling the atmosphere, and keeping the lights from burning the plant (if using HPS).

How long should plants be grown under the vegetative light cycle?

It all starts in Veg. What you do in veg will largely affect your successes and struggles during the flowering period, so let’s discuss those issues now.

During the vegetative period, you are trying to accomplish a few things before switching to the 12/12 light cycle:

• Grow the plants to suitable height
• Train the plants for optimal growth during flower
• Top the plants to optimize center cola development
• Prune the plants to stay ahead of growth that occurs during flower

If you do all of these things, you will have a much smoother ride during the flowering cycle, and you will optimize your results at harvest time.

But this article is about answering the question, “When to make the switch to the 12/12 light cycle?” In order to answer that question, we must determine how long the plants should remain in the vegetative period light cycle.

The question now becomes, how long will it take to accomplish all of the above-listed tasks during the vegetative cycle? That will tell us whether it is time to switch to the 12/12 flowering cycle.

Getting everything done during the 18/6 light cycle.

So we have a list of things we need to accomplish during the 18/6 vegetative light cycle. Let’s go over that list and determine how long these plants will stay in the vegetative cycle.

After you pot your clone, and the clone takes root, you will need about 2 weeks to get that clone to a suitable size to do anything else. Once that plant is at least a foot tall, it can be topped. It can actually be topped sooner, but we do not recommend that.

We actually recommend waiting until your plant is more like 18 inches tall. You should allow your plant to grow for a few weeks. Then top the center cola. This will retard the growth of the center cola, but it will allow the side colas to catch up to the center. This will help shape the plant so that it can have a more even canopy.

If you top too soon, you will slow growth and lose time. You are also over stressing a tiny plant that is just getting started in its growth pattern.

Also during the time spent in the vegetative light cycle, you will want to remove the tiny little branches and bud sites at the bottom of the plant. As the plant growth begins to accelerate, your plant will become bushy with leaves. Take this time to prune the plants.

All of this work means you let your plant grow for about three weeks before topping. Then you will prune the overgrowth of leaves. And you will let it grow a couple more weeks.

So to answer the question of when to switch from 18/6 light cycle to the 12/12 light cycle, it will be after you have accomplished your goals and tasks for the vegetative period.

For most growers, this takes about 4 to 8 weeks. You can get it all done in four weeks, or take as long as eight weeks. The longer your plants stay in veg, the bigger they will get. The less time spent in veg, the small they will be, and the less time you have to train and top them.

Decreasing time spent in veg, and switching to the 12/12 light cycle as soon as possible

Yes, you can utilize a very short veg cycle, such as three weeks, or maybe less. Your plants will be very small. If utilizing a short veg cycle, you may skip topping your plants altogether.

One reason for short veg cycles is if you plan to flower a lot of plants at once in an area. Just running a single cola branch in the middle, and then surrounding it with more plants, is one strategy.

There are no right or wrong answers you. You have lots of options depending on your goals, and your environmental limitations/conditions.

Bigger plants for bigger rooms. Smaller plants for low ceilings. Or small batches of large plants; large batches of really small plants. The choice is yours.