Autoflowering plants grow best when you give them between 18 and 24 hours of light each day. But there are many circumstances when a grower needs to give their plants only 12 hours of light.
And today I will try to explain the what, why, and when about autoflowers and the 12/12 (day/night) light cycle.
First of all, I need to say that you can absolutely grow autoflowers on a 12/12 light schedule and they will produce great buds. But because the plants will receive less light their yield will be smaller than if you compare them with plants that are grown under a 20/4 or even 24/0 light cycle.
Why use a 12/12 hour light cycle for your autoflowers?
There are many reasons why a grower would grow autoflowers under 12 hours even if autoflower plants do best under 18 to 24 hours of light.
Here are the most common reasons:
Autoflowers in the “flower room”
If you are growing regular (photo-sensitive) plants but still want to get some early autoflower buds then you can put autoflower seeds in a flowering chamber and get decent buds well before your photo-sensitive strains have matured.
You can even grow your autos together with photos from the start giving them 24 hours of light and then switching the light cycle to 12/12 when you want your photos to start flowering.
Autoflowers will not mind the light cycle change and contrary to popular belief this change won’t increase the hermaphrodite rates.
Growing autos outdoors
If you are growing autoflower plants outdoors in the spring or in the autumn when the day is about 12 hours long then that won’t impair autoflower plant growth and they will still flower and produce great buds.
For more info on growing autoflowers outside check out our guide to outdoor autoflower growing.
Cutting electricity costs
If you are on a tight budget you can reduce the light cycle from 24 to 12 hours and cut your electricity costs in half but still get some decent buds.
However, be careful with this because some autoflower strains will take longer to mature when grown using a 12/12 autoflower light cycle so your savings may end up eroding due to longer grow time and less yield.
The temperatures are too hot during the daytime
If you are living in a climate where the summers get really hot then you might want to only turn on your grow lights at night when the sun is down and the ambient air gets cooler.
In this situation, you can still get some indoor autoflower buds even with 12 hours of light that turns on only during nighttime.
You can learn more about controlling the temperature in your grow room in our autoflower temperature guide.
If your “autoflower” plant does not automatically flower under a continuous light cycle after 3 to 6 weeks then you might have a bad batch of seeds and you will need to manually adjust the light cycle to 12/12 and kickstart the flowering phase.
Disadvantages of 12/12 autoflower light cycle
As you can see then there are quite a few reasons when you might need to grow autos under 12 hours of light. But this practice also has some drawbacks:
If you give your autos only 12 hours of light each day and 12 hours of darkness then that can promote some unwanted and excessive stretching.
To fix this, you need to get your autoflower as close to the light source as you possibly can when those lights are on. And you should also give them complete darkness in the nighttime so that the plant doesn’t try to stretch to the light that leaks into your grow room through the openings.
If you give your plants less light they will grow slower and won’t produce as many new branches and leaves so you will end up with a smaller plant, fewer leaves and branches and fluffier buds.
Longer growing cycle
Some autoflower strains will take longer to mature if you give them only 12 hours of light.
I have seen plants that take more than 30% longer to get to the harvest stage when grown under a 12/12 light cycle.
And ultimately when you are giving your plants less light then they will give you less yield which can be a big drawback to many growers.
But sometimes 12/12 is your only choice, so even with less yield from a single plant you will still be getting at least some buds and you can also grow more plants to compensate for this decrease in yield.
Can you grow autoflowers on a 12/12 light cycle from seed to bud?
Yes, you can grow auto autoflowering plants under a 12/12 light schedule from start to finish. But that will produce the worst results as your plants will get the least amount of light.
Can you change the autoflower light cycle to 12/12?
Yes, you can switch to a 12/12 light-dark cycle even if you started your autos under a different light cycle.
When to switch to a 12/12 light cycle?
The best option if you want to use the 12-hour days is to start your plants under 24/0 or 18/6 light schedule while they are in their vegetative growth stage and then switch the light cycle to 12/12 when the first pre-flowers appear.
This will give your autos the most amount of growth and the yield will not suffer that much.
There also are some more exotic light schedules like 14/10, 14/8, and 13/11 (light/dark) that are more commonly used for regular photo-sensitive plants but some use these to grow autoflowers as well.
I personally do not see the advantage in these schedules as they don’t produce more buds or if they do then the difference is usually minuscule.
Anyways, now that you know more about the 12/12 light cycle for autoflowers, you can make an educated decision if you want to grow your autos under a 12/12 cycle or better wait and grow them under an 18/6 or 24/0 light cycle.
And if you want to learn more about which is the best light schedule for autoflowers, check out our article on autoflowering cannabis plant light cycles.