Autoflowering plants as we have determined earlier grow best when you give them between 18 and 24 hours of light each day but there are many circumstances when a grower need to give his plants only 12 hours of light. And today I will try to explain the what, why and when about autoflowers and 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 those plants will receive less light then their yield will be smaller than if you compare them with plants that are grown under 20/4 or even 24/0 light cycle.

Why use 12 hour light cycle?

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. And 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 the 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 you great buds.
  • Too hot temperatures 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 in the 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 in the nighttime.
  • Cutting electricity costs – If you are on a tight budget then you can reduce the light cycle from 24 to 12 hours and cut your electricity costs in half but still get some decent buds. But be careful because some autoflower strains will take longer to mature when grown under 12 hours so your savings may end up eroding due to longer grow time and less yield.
  • Bad genetics – If your “autoflower” plant does not automatically flower under continuous light cycle after 3 to 6 week 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.

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 drawback.


  • Increased stretching – 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 so 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 night-time so that the plant doesn’t try to stretch to the light that leaks in your grow room trough some openings.
  • Slower growth – 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 and less leaves / branches and fluffier buds.
    Longer growing cycle – Some autoflower strains will take longer to mature if you give them only 12 hours of light. And I have seen plants that need more than 30% longer growth time to get to the harvest stage when grown under 12/12.
  • Less yield – And ultimately when you are giving your plants less light then they will give you less yield and that can be a big drawback to many growers. But sometimes 12/12 is your only choice and even with less yield from a single plant you are still getting at least some buds and you can also grow more plants to compensate this decreased yield.

So the most popular light cycles for autoflowering plants are growing under 12/12 from the start or switching from 24/0 to 12/12 when the autoflower flowering phase starts.

  • 12/12 from the start – If you are growing autoflowers in your flowering room then you can grow them from start to finish under 12/12 light/dark cycle but that will produce the worst results as your plants will get the least amount of light.
  • 24/0 then switching to 12/12 – The best option if you want to use the 12 hour days is to start your plants under 24/0 or 18/6 while they are in their vegetative growth stage and then switch that 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.

And there are also some more exotic light schedules like 14/10, 14/8 13/11 (light/dark) that are used of regular photo-sensitive plants and where autoflower have been grown. But I do not see the advantage in these schedules as they don’t produce more buds or if they do then the difference is usually miniscule.

And now you can make an educated decision if you want to grow your autoflowers under 12/12 or better wait and grow them under 18/6 or 24/0 light cycle.



  1. I have autos in my veg room witch is on 12/1 (12 on 5.5 off 1 on 5.5 off) Iv had some monster plants out of it, I look at it as a free plant,


    Hi everyone,

    I’m a new grower. I’m just starting to growing Easy cheese Stilton . I am running 12/12. My question is should I use other lights than my 150 HPS bulb(16,000 lumens)?. I was thinking purchasing a Full spectrum grow light with Red 630nm and 660nm, Blue:460nm and White light 2700K. Would it help my plant?

    Or just only just have the HPS bulb.

    Thanks, you.

    • midowo

      Full spectrum lights should give your plant some extra boost in the develpoment stages, but if they are expensive and you are on a budget then the HPS will be enaugh for the first couple of grows.

  3. Tj gardens on

    I have a problem where my auto leaves are starting to curl a little is it because of the little white bugs in my soil or is it over watering there almost four weeks old and I’m about to put in my hps I really need help

  4. My grow room is always running at 12/12, so it’s handy to throw in autoflowering and regular photoperiod seeds in at the same time to experiment. Occasionally, I’ve had to have a room full of autos on 12/12, but I compensated for the loss in yield with some extra plants. If you grow 2 regular plants for your own personal consumption normally, you’ll need 3 autoflowering plants running on 12/12 to get a similar yield.

Leave A Reply