Autoflower Light Cycles: 12/12 vs. 18/6 vs. 20/4 vs. 24/0

Most growers run autoflower seeds on 18 hours of light and 6 of darkness (18/6) from seed to harvest, because autos flower on age rather than day length and that schedule balances growth against power use. 20/4 and even 24/0 work too, while 12/12 is possible but usually costs you yield. Not depending on day length to bloom is the key difference from “traditional” photoperiod feminized seeds.

Autoflowering plants don't need a specific light cycle to start flowering, unlike photoperiod strains, which need 12 hours of light a day or less to kick into bloom. Beyond that difference, though, autos share a lot with regular cannabis, including the ability to produce terpenes and cannabinoids.

So what's the optimal light cycle for autoflowers? Is there a schedule that gets especially good results? Let's break down each option so you can get the most yield and quality out of your autos.

Which light cycles can you run for autoflowers?

Photoperiod growers have set light cycles for each stage, typically 18/6 in veg and 12/12 in flower, but autoflowering strains give you a lot more flexibility. Which schedule you pick comes down largely to personal preference.

The 12/12 light cycle

12/12 is the standard flowering cycle for photoperiod plants, and some growers run their autos on it during flower too. Plenty of growers think it holds autoflowers back, though, since they could benefit from more light.

12/12 has a proven track record in flower, but many growers suggest an 18/6 cycle works better during the vegetative stage.

Technically you can run autos on 12/12 through veg, but the yield probably won't be as good as on 18/6.

Pros of 12/12:

• Tried and tested for the flowering stage.
• Mimics natural light levels in late summer and early fall.
• Lower electricity use.
• Keeps the grow space cooler.
• Lets you grow autos alongside photoperiod strains.

Cons of 12/12:

• A missed chance to grow bigger plants.
• Potentially lower yields.

The 18/6 light cycle for autoflowers

18/6 is the go-to for a lot of growers, through both veg and flower. It was originally meant for photoperiod strains in veg, but it's proven to deliver excellent results with autos as well.

It doesn't exactly match natural outdoor light, but it gives autoflowers an ideal environment to grow and flower, which translates into generous harvests and well-developed buds.

It also hasn't shown any downside at any stage of an auto's life, which makes it easy to run the same schedule from germination to harvest.

Pros of 18/6:

• Plenty of light, which means more energy for growth.
• Generous harvests and well-developed buds.
• Moderate power consumption.
• Lets you grow autos and photoperiod plants in the same space during veg.

Cons of 18/6:

• The grow space can heat up, which may not be ideal in hot climates.

The 20/4 light cycle for autoflowers: the high-yield option

A 20/4 schedule comes closer to summer light in far-northern regions, where the sun barely sets during the season.

You can run autos on 20/4 from day one, and growers report good results with it. It's less common than 18/6, but it's a more light-intensive approach that can push even more vigorous growth and heavy harvests.

Pros of 20/4:

• Bigger harvests thanks to longer light exposure.
• Vigorous plant growth.

Cons of 20/4:

• It's not clear it out-yields 18/6 by any meaningful margin.
• Higher electricity use and costs.
• More heat in the grow space, which may mean extra temperature management.

The 24-hour light cycle for autoflowers: does it work?

Interestingly, autoflowers can grow their entire life without a single dark period. Some growers keep the lights on 24 hours a day from start to finish. But does it actually pay off?

Cannabis plants are known to tolerate round-the-clock light, but it's not clear it offers any real advantage. Some growers argue even autos benefit from a rest period in the dark, while others insist they don't need one.

So far, there's no clear consensus either way.

Pros of 24/0:

• Some growers report big bud harvests.
• Maximizes the light your plants receive, which could boost growth and yield.

Cons of 24/0:

• Uses a lot of power, possibly for nothing.
• Autos may benefit from a daily dark period.
• Can overheat the grow space, depending on how your lights are set up.

So what's the best light schedule for autoflowers?

The optimal light cycle for autos is still up for debate among growers, but we can weigh the options.

Overall, many growers find that an 18/6 indoor schedule strikes the right balance between results and power use. It takes advantage of the fact that autoflowering plants can get more than 12 hours of light a day during flower, without the higher energy bill that comes with 20/4.

It hasn't been fully proven that longer light cycles are better for the plants, so many growers stick with a balance of efficiency and energy savings.

That said, if you want to experiment with a longer schedule, go for it: it won't hurt your autoflowers, and it might even help.