One of the best things about growing an autoflowering marijuana strain (or several) is that they are not dependent on light cycles for growth. Plants like Grand Daddy Purp Auto or Blue Dream Auto will start to flower when they reach a certain age or maturity. So as a grower, you don’t need to worry about always turning the lights on or off at a certain time. But still, a cannabis plant is still a plant, and so it’s going to need some type of light in order to thrive. So what light cycles will these plants need to get the best results?

Best light cycles for an autoflowering marijuana strain

It doesn’t really matter what type of strain you have. Whether it’s a Grand Daddy Purp Auto, a Blue Dream Auto, or some other type of auto strain, it’s going to need light and there is an optimum light cycle.


Some growers swear by a 24-hour light cycle for their autoflowering plants. This is simply because cannabis is a C3 plant, it can absorb CO2 during periods of light for photosynthesis. Theoretically autoflowering plants do not actually require a period of darkness and they are also indigenous to northern areas of the world, that do sometimes see periods of 24 hours of light. But that might not be best for autoflowering plants.

A marijuana plant, just like people, need a period of rest and recovery. And when they don’t get it, we can see a negative effect on the yield that they provide and sometimes even pro long or stop them from flowering. Some growers drop the light cycle to 18 hours a day.

Some growers drop the light cycle to 18 hours a day. This for many, including us is considered the optimum light cycle because the plants still get the rest they need while also being given enough light to produce large yields. Other cannabis growers even put their autoflowering marijuana strain in a 20-hour light cycle. These growers believe that this is giving the plant the best of both worlds.

In most cases, no grower wants to give their plants less than 18 hours of light a day. That light is vital to the plant’s growth and without it, the plant and the yield will suffer. But there may be times when a lower light cycle is needed.

When to use the 12-hour light cycle

The Internet is full of forums. And on many cannabis or autoflowering forums, those forums are filled with threads from people asking for advice on why their autoflowering plant isn’t producing the blooms they would like. Sometimes this happens at the time the plant should be flowering, while other times people are simply wondering if they can speed up the already-fast process of getting buds from an autoflowering plant.

Firstly, never rush an autoflowering plant. Nature has already done all the hard work by eliminating the need for several different light cycles and waiting months for the plants to grow enough so they are ready to be harvested. If waiting the short timeframe for an autoflowering plant is too much, it may be best advised that a local dispensary provides the bud that is needed sooner. But, if you’re being patient and your autos still aren’t producing, there may be something you can do. And that’s switch your plants to a 12-hour light cycle. Even with the experienced growers in these forums, no one really seems to understand why switching the plants to a 12-hour light cycle seems to work.

The most logical explanation is that this re-enforces it’s already trained genetics to believe summer is coming to an end, as the plant starts to receive less light. Therefore forcing it to want to grow faster in time to harvest before the frost sets in.

Growers should know that this is a light cycle last resort for when plant appears to not be flowering or is very slow at flowering. It is never advised to lower the light cycle to any more than 12 hours of light per day. That amount will not provide the plant with everything it needs to grow and thrive and is more than likely going to hurt your plant and yield.

Autoflowering marijuana is one of the easiest types of marijuana plant that you can grow. They’re convenient, they run on their own schedule, and will 99% of the time, flower early with very little muss or fuss. But there are a few things to keep in mind when growing them and these optimum light cycles can help you get yield you hope for.