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.

Bad genetics

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:

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.

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.

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, 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.

Less yield

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.

Last thoughts

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.



  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.

    • 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.

    • Matty McCormick on

      600w dual spectrum led ideal for any grower amateur or professional don’t blitz electricity

      • What’s the best thing to use for annoying wee flies around my 2 week old veggies (from seed)? First time grower and am chuffed to get this far. Don’t want to fail the little cuties now! Some kind of natural insecticide/food at 4 weeks I was thinking but greatly appreciate any advice offered.

  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.

    • vicki gailey on

      So put hydrogen peroxide in watering water for plant they r disturbing and he gnats

  5. Ho guys, I’m definitely a newbie, as I recently started growing my first two autos baby. (One has sprouted today, the second is still under the soil level). They are inside a 60x60x160 grow box, under a 250w hps, with a nice exhaust fan. I’m now trying to understand which is the best light schedule… but in the process of understanding this, I need to face another problem: I’m freaking scared of fire hazard. I mean, I don’t really feel safety with the idea to leave that light, and that fan, working 24hrs, even when I’m out of home, or when I’m sleeping. So my question is: I’m I crazy? Is the risk of fire real in this situation? I live in a small and new apartment, and I did all the cable work in the best way… but even so I’m scared to leave the light and the fan running when I’m not home. Have you guys ever heard of grow box burning? Is this something that can happen? Thanks, you all

  6. hi
    i seem important reason for best growing autoflower is the brand of seeds last year growing auto of dutch passion company was very well growing and very fast but now growing auto any company is very diffrent and slow growing all the best

  7. I’m new to the indoors can anyone give me some advice I have a 4ft 3 tube T5 set up can i get a decent Yeild using a few or more AUTOS?Any advice would be greatly appreciated!
    Happy Thanksgiving,BMRIVER

  8. My first auto grow. I have LED’s that have veg & bloom switches, should I run the lights as if I’m growing photo’s or is there a specific way I should run them? Any advice is appreciated

  9. Newborn 0420 on

    My autos we’re at 8weeks with no buds. I switched to 12/12 aweek ago starting to get fat tops is this right ???

  10. Buzzsaw28 on

    First time growing autos . So any and all tips are welcome. Thxs Hope everybody grows are doing well and get a great harvest ✌????

  11. So I have a photo plant and its almost ready for flower but I also have a auto in the same tent but the auto is not ready to go in to flower will it be ok to switch to 12/12 with the auto in there or will it push it in to flower aswell

  12. from what i seen in hemp on my farm as i got ripped off by the clone dealer is that in 12/12 auto flowers veg but as soon as the light gets longer then shorter they flower i lost 2 minutes each day morning and night i seen the stretch and over night they shifted i thought it was the duch product for green houses i used,but that not it.these auto grew to un belivable size i was going for it wanting trees by september….thay yelded bout 1 to 5 lbs.i found one or two seeds where they hermed just a little bit due to a neighbors light blinking…poped them as my mother died out and wouldnt shift back to veg..i know i know i didnt see any of these sighns but this is my first grow.poped them in 14 hr veg room and dammmmmmmnnnnnnnnnn it shifted into he ripped me off i know because that seed went from bud to plug in 5 minutes so they will veg in 12 12 its really like they flipped photo periods but weed dose weird things to addapt to question is what to now that cut i cant clone and lyfter is the only thing i got besides that start from new huh…???


    It’s been since December and still no buds it’s my first ever auto flower grow and I’m sure I’ve stressed them out a few times could that make them bus slow? They are finally getting bigger but no bud sights? Should I pull the plug and start over?

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