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Should You Change Light Cycle For Autoflowers?

It’s a question that comes up regularly among beginners to cannabis cultivation – but is also a topic of dispute among veterans. Should you change light cycle for autoflowers? The short answer is – you don’t have to, but you might want to.

It’s a fact that autoflowering cannabis doesn’t need the same light manipulation as photoperiod cannabis to switch to flowering. The clue’s right there in the name, after all. Still, the debate rumbles on about the best light cycle for autos in an indoor grow.

What is Autoflowering Cannabis?

Autoflowering cannabis plants differ from photoperiod plants. Photoperiod cannabis is given a long light schedule during the vegetative phase, and then the amount of light, or hours of light, are cut dramatically to provide the plants with a dark cycle. At this point, photoperiod strains switch into the flowering mode, where they begin to produce buds.

With autoflowering weed plants, the flowering phase is guaranteed across the autoflower light cycle, regardless of hours of light or dark. Autos flip to flowering as a result of age – think of it as if the plant starts to produce buds due to its biological clock. Whether you turn the grow lights off or not, you’ll still find your autos produce buds.

It’s a trick of genetics, bred into the plants via hybridisation. There are different types of cannabis – cannabis sativa and cannabis indica, and then there’s cannabis ruderalis.

Ruderalis strains were discovered growing in the wild climates of Siberia, and they evolved to produce flowers even in those harsh Siberian climates. It’s like a unique survival mechanism. By crossing ruderalis strains with indica or sativa photoperiod strains, breeders found the best of both worlds suddenly available to them.

Autoflowering cannabis seeds deliver the strain of your choice and all its desirable traits, but with a guarantee of flowering without the need for light manipulation.

How Do You Grow Autoflowering Cannabis?

This is where the light cycle question becomes a debate. Since flowering is guaranteed, there are contrasting theories on the best light schedule when growing autoflowers. Let’s delve deeper into the different light cycles for your grow space.

24-0 Light Cycle

You could – and many people do – give your autos 24 hours of light for the duration of their growth. Indoor growers who use this light schedule argue that it provides the plants with maximum vegetative growth and encourages larger cannabis plants.

Naturally, there are pros and cons to this. In theory, yes, 24 hours of light can produce larger plants and maximise harvests simultaneously. There’s no need to worry about buying a timer, setting a timer, or rushing back to your plants to switch the lights off manually. But 24 hours of lighting for your cannabis plants is pretty expensive. The more plants you have, the more lights you need. The more lights you need, the more energy you use, and your electricity bill will be higher. Right now, the last thing many of us want is a huge electricity bill.

light autoflowers

In addition to the increased cost of growing using a 24-hour light schedule, lack of any dark period means your plants get no recovery period, meaning they’re using energy all the time. This elevates the stress levels put on the plant, and while they contain those super-resistant ruderalis genes, they don’t necessarily have 100% ruderalis qualities.

18-6 Light Cycle

An 18-6 light cycle means 18 hours of light with 6 hours of darkness. Many experienced cultivators contend that giving your autos at least 18 hours of light is essential. This is because autoflowering strains typically produce smaller plants than their photoperiod counterparts, and to maximise plant growth, 18 hours of light is optimum to facilitate maximum development and stretch.

Remember that autoflowers love light as much as photoperiod strains; they just don’t need it to flower. Giving autos a 6-hour window of darkness means they won’t overspend their energy – we all need a rest after a hard day, right?

With an 18-6 light cycle, your autos will produce a healthy harvest and yield nicely. You’ll save a bit on energy since the lights are off 6 hours of the day, every day, but you’ll need a timer. Either that or set your watch, so you don’t forget to kill the lights manually.

12-12 Light Cycle

It’s certainly not unheard of for cultivators to keep their autoflowering plants to a 12-12 light cycle during the flowering stage. The advantage of this method is that you’ll save even more on energy use and still produce a decent harvest of weed.

This is a valuable tactic for cultivators growing autoflowering cannabis in hotter climates. Suppose you’re in a climate zone such as the Mediterranean or South Africa, for example. In that case, keeping your lights off during the day is worthwhile to avoid heat stress to your plants, then fire them up at night when the overall temperature has dropped a bit. You reduce the risk of overheating in your grow room, and that’s only wise.

A 12-12 light cycle also means you can put your autos in with any photoperiod strains you may be growing, and the two can happily grow side by side in the same environment.

One thing to keep in mind – while yields will still be respectable under the 12-12 lighting regimen, your autos will produce smaller buds. Something to think about, for sure.

Is Light Spectrum More Important than Light Cycle?

It’s essential to consider the impact of the light spectrum when growing autoflowering cannabis plants. If you want to get the most out of your grow, this attention to detail should not be overlooked. Consider these notes at the different stages of your autos’ life cycle.

light cycle autoflowers

Blue Light

During the vegetative stage, it’s best to ensure your autos receive light from the blue spectrum. Blue light encourages the growth of short, stocky cannabis plants, which is ideal for autos to help them support larger flower formations. It’s also suggested that light from the blue or UV spectrum can increase cannabinoid concentrations, including that highly-prized THC[i]. Ramp up the blue during the veg stage.

light cycle autoflowers

Red Light

When it comes to flowering, switch over to a red light spectrum. Doing this can encourage your plants to stretch at this point while maximising bud growth. It’s thought that the red light spectrum mimics the sun’s angle through the late stages of summer and into autumn. As such, you’re imitating nature and getting similar results in your grow room.

How to Light Autoflowers: In Conclusion

Autoflowering cannabis is a great way to grow faster-finishing cannabis with less hassle. Yes, you’ll produce smaller plants, and yes, possibly lower yields than some photoperiod strains are capable of.

However, as with all cannabis cultivation, starting off with the right genetics makes a huge difference to your final product. Seedsman specialises in autoflowering cannabis strains, so hop over to before you start your auto project and check out some of the best autoflower seeds we offer.

If yield is a big concern, choose one of our high-yielding autos for your next grow. Then it’s up to you to choose how to set your lights. Like many aspects of cannabis cultivation, lighting is a trial and error process, and what works best for one grower won’t necessarily suit another. You have plenty of options, so consider the above and do what works best for you. Ultimately, you don’t have to change the light cycle when growing autoflowering weed – but there are reasons why you might want to.


Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.

This post is also available in: French

Duncan Mathers