Autoflower cannabis plant light schedule

When growing autoflowering cannabis seeds there is one fundamental difference to ‘traditional’ photoperiod feminised seeds. That is the ability of autos to bloom under any light cycle provided by the grower. You can bloom an autoflower indoors under 12, 18 or 24 hours of daily indoor light. Or you can bloom autoflower plants outdoors where they will bloom happily irrespective of how many daily light hours are received.

Autoflower plants don’t have a photoperiod dependency that requires daily light to be 12 hours (or less) in order to bloom. Apart from that, autoflowers grow like normal cannabis, producing the same terpenes and cannabinoids. Quality levels/cannabinoid levels from the best autos are on a par with the best photoperiod strains. You can use the same nutrients and grow systems for your autos as you would use when growing photoperiod feminised strains.

So what is the optimised autoflower light cycle? Does one particular autoflower light schedule produce particularly good results? How do the results from an autoflower 24 hour light cycle compare with an autoflower light cycle of 12/12?

The life cycle of autoflower cannabis plants

When considering the optimised light schedule for autoflowers it should be remembered that in many ways the autoflower plant biology is very similar to that of photoperiod feminised strains. Light is the source of energy that powers the plant biochemistry. Those biochemical reactions produce the same terpenes, cannabinoids, leaf material, roots etc whether they happen in plants grown from autoflower seeds or feminised cannabis seeds.

The only real difference between autoflowering genetics and feminised genetics is that the autoflower genetics have been released from the shackles of photo-dependency thanks to some clever breeding by talented geneticists. This allows the grower new liberties and the ability to grow with the light cycle which most suits them.

For example, you can bloom autos under a 12/12 light cycle (12 hours of daily light followed by 12 hours of darkness) if you wish to grow/bloom your autos in the same room that your feminised strains are blooming in. Or you can grow/bloom autoflower seeds under 18-24 hours of daily light in the same room that you use for your mother plants/vegetative growth.

When contemplating the best light cycle for autoflowers remember that the main benefit of autos is their adaptability and speed of growth. Some growers keep separate mother rooms (18/6 light cycle) as well as a bloom room (12/12 light schedule). Autoflowers will grow and bloom quite happily in either environment. When growing autoflower seeds, you are in charge and can dictate the autoflower light cycle in order to deliver maximum convenience for you.

How long do autoflowers take from seed to harvest?

What is the best light cycle for autoflowers?

Auto Blackberry Kush grown from seed to harvest within 11 weeks under LED grow lights.

The best light cycle for autoflowers is, in many ways, the light cycle which is most convenient for you. Autoflower plants adapt to your situation, allowing you to use a light cycle that suits your needs.

If your grow room gets cold during the ‘lights-out’ period in winter then you may want to run a 24 hour autoflower light cycle, using the heat from your grow lights to keep the grow room temperatures as high as possible. Conversely, if the grow room gets too hot during summer you may prefer to run a 12/12 autoflower light schedule with lights on only during the cooler evening hours.

It’s quite possible that there isn’t a single optimised autoflower light schedule. Instead, you will find that autoflower seeds will grow well under several different light cycles. Autos have earned their reputation for being flexible and adaptable yet still delivering superb results under a broad range of autoflower light cycles in a wide range of grow systems/methods.

Most growers lean towards an autoflower light schedule using 18-20 hours of daily light. This gives great results. With the best modern autoflower seeds you can expect superb quality levels on a par with the best feminised seeds. Auto Skywalker Haze has been independently confirmed to have THC levels over 26%. Many feminised cannabis seeds struggle to deliver such THC levels!

26%+ THC levels with Auto Skywalker Haze

Total light hours for autoflower vs feminised seeds

Autoflower seeds Feminised seeds
Autoflower cannabis seeds
Around 1400 (70 x 20) hours Around 1386 hours
When considering how much light an autoflower does need, it’s worth noting that a typical autoflower takes around 70 days from seed to harvest with 20 hours of daily light. In comparison, a photoperiod cannabis strain grown from seeds might take 5 weeks of 18/6 vegetative growth (630 hours) and 9 weeks of 12/12 bloom (756 hours), so around 1386 hours in total.

Autoflower strains can cram their life cycle into a shorter start-to-finish time period

The 1386 light hours required to grow a ‘typical’ feminised strain is remarkably close to the 1400 hours of light required for autos. However autos fit their growth into 70 days or so, compared to the feminised cannabis seed grow cycle of 98 days (5 weeks veg & 9 weeks bloom). That may be a valuable time saving for some growers.

Of course, it should be recognised that there are some feminised sativa/Haze strains that can take 14+weeks in bloom as there are auto strains that can take 13 weeks or more from autoflower seed to harvest.

For many people, both professional and hobby growers, the faster seed-to-harvest growth cycle of autoflowers makes them much more convenient. But it’s worth adding that some autoflower strains such as Auto Blueberry and Auto Blackberry Kush can be ready to harvest in as little as 8-9 weeks after germination. On the other hand, some autoflower strains such as Auto Desfran and Auto Ultimate can take 13 weeks (or longer) of a 20/4 autoflower light cycle.

Light times for autoflowers can be adjusted to suit the convenience of the grower, that’s one of the reasons that so many indoor growers like to keep a spare pack of autoflower seeds in the fridge. You can grow autos under 12 hours of daily light or 18-24 hours. The choice is yours depending on what works best for you and what else you are growing at the same time.

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12/12 light cycle for autoflowers

A 12/12 light cycle is popular for blooming feminised cannabis strains. The lights are on for 12 hours and then off for 12 hours. Very often, growers may have spare space in their flowering tent and they can put this to good use by planting an autoflower seed or two. However, autoflowers will grow faster with better yields under 18, 20 or 24 hours of daily light than they will on a 12/12 light cycle.

Auto Ultimate grown under 24 hour daily light and 12 hours in the last 3 weeks

18/6 light cycle for autoflowers

How much light for autoflowers? What is the optimum autoflower seeds light cycle? These are two of the most common questions. The 18/6 autoflower light schedule is very common. It means 18 hours of daily light is followed by 6 hours of darkness. Many growers also use this light cycle for vegetative growth of their feminised cannabis seeds. An 18/6 autoflower cannabis light cycle tends to produce harvests around 10-11 weeks after germination.

Auto Daiquiri Lime grown under 18 hours of daily light from seed to harvest

20/4 light cycle for autoflowers

Perhaps the most common autoflower light schedule, and the one which Dutch Passion most often recommends is the 20/4 light schedule. This uses 20 hours of ‘lights on’ followed by 4 hours of darkness in each 24-hour time period. This light schedule delivers great results and seems to give the plants a healthy autoflower light cycle all the way from seed to harvest.

Auto Blackberry Kush grown under 20 hours of daily light from seed to harvest

24hr light cycle for autoflowers

A 24 hour light for autoflowers is often popular during cooler winter months. In winter, grow room temperatures can often decrease faster than the grower would like during periods of darkness when the grow lights are switched off. One way to mitigate cold grow room temperatures is to grow autoflower seeds under 24 hour light. The grow light helps maintain grow room temperatures.

It’s worth noting that not all auto growers recommend the 24 hour light cycle, feeling that an hour or two of darkness has certain benefits for auto plant development. But this view isn’t universally held.

A 24 hour autoflower light cycle is also popular with those that grow their autos in the same room that they use for vegging feminised seedlings or keeping their mother plants. It’s also worth adding that those with particularly powerful lights and high PPFD levels may find that the DLI (daily light integral) is simply too much for some strains when the lights are on a 24 hour autoflower light schedule . Problems can arise when autoflowers are bathed in excessively high light intensities for 24 hours each day. If issues are seen (such as bleached flowers) they can be solved by reducing light intensity and/or giving the plants a few hours of darkness each day.

Auto Think Different grown under 24 hours of daily light from seed to harvest

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LED grow lights and light spectrum for indoor autoflowers

Auto Cinderella Jack grown indoors from seed to harvest under SolarSystem 550 grow lights

Which is the best light for autoflowers? What light for autoflowers will give the best result? Many professional growers would only consider LED grow lights due to their greater efficiency, superior spectrum, reduced levels of heat stress and long life (compared to HPS). Dutch Passion’s best autoflower THC levels (measured by independent test labs) have always been produced with LED grow lights rather than HPS or ceramic metal halide.

What light cycle for autoflower with LED grow lights? Most growers tend to use a 20/4 autoflower grow light schedule, but as previously explained, autoflower seeds will grow well under a range of autoflower light cycles. The autoflower-to-light distance needs to considered. Check the manufacturers recommendations at each stage of growth. Some lights will need to be around 30cm above the canopy, other lights will need much greater distances.

In general the best light for autoflowers is LED. But HPS light delivers great results too for those on a budget. The only remaining benefit for HPS over LED is that the initial HPS purchase cost is much lower. LED light for autoflower has a better spectrum with more usable blue & red light wavelengths and fewer of the less useful orange/green/yellow wavelengths. LED delivers a purpose-designed autoflower light spectrum.

These days the ‘blurple’ (blue/purple) autoflower light spectrum seen in the early LED grow lights has been largely replaced with a more eye-friendly warm-white spectrum which is easy to work in and produces excellent results.

Of course it is impossible to grow an autoflower without light, light is the energy source which drives photosynthesis. Without light, cannabis simply cannot grow. But get the right conditions combined with the best cannabis seeds and 25% THC levels are yours for the taking.

Auto Cinderella Jack with over 25% THC grown under LED light

Autoflowers light schedule FAQ

Many people feel that autoflowers are a little easier and more forgiving to grow than photoperiod feminised cannabis strains. Autoflowering cannabis seeds can be grown and bloomed under a wide variety of autoflower light cycles and spectrums. For the best results and maximised potency, it is recommended to grow autos under good quality modern LED grow lights with around 20 hours of daily light.

Do autoflowers need a dark cycle?

Autoflowers don’t necessarily need a dark cycle. You can grow them under an autoflower 24 hour light from seed to harvest. Many people do just that whereas others feel that they get slightly better results with a 20/4 or perhaps 18/6 autoflower light cycle.

Can you change the light schedule for autoflowers?

Yes, changing the autflower light cycle can be done without stressing the plant. For example, if you are growing indoors with 20/4 autoflower light cycle, you can easily change the light cycle to 12/12 (e.g. to allow blooming to begin on nearby photoperiod feminised plants). This doesn’t cause any stress to the auto. However, it should be noted that reducing the number of daily light hours can increase the flowering time and/or the yield of your autoflower in some circumstances.

Do autoflowers need direct sunglight when grown outside?

To get the best results and the heaviest harvests outdoors, aim to grow autoflowers in the three sunniest months of the year with plenty of direct sunlight. Growing autoflowers without direct sunglight will result in airy, light buds and poor quality taste/potency. Growing autoflowers outdoors without direct sunlight would produce similar results to an under-powered indoor grow room.

Can autoflowers get too much light?

Yes, excessive light will stunt your autoflower plant growth. One of the dangers of using some of the latest very high powered LED grow lights is that they may output a little too much intensity in some situations. However, many lights have adjustable power levels allowing the user to adjust light intensity (or raise LED light height). LED bar lights have different recommended hanging heights compared to LED lights that concentrate their LED chips on a single compact panel. The better LED manufacturers will be able to recommend the right intensity/hanging heights. In many cases, you will be able to see online grow diaries or social media posts illustrating the correct use of various brands of LED grow lights at different states of the plant growth.

Growing autoflower cannabis seeds is fast and easy

The main message about growing autoflowering cannabis seeds is that they are a proven, simple and rapid way to a high quality harvest. Most growers tend to use a 20/4 autoflower light cycle but autoflower seeds are flexible enough to adapt to whatever light schedule you wish to use. These days exceeding 25% THC levels is quite possible with the best autoflower seeds. Choose the highest quality seeds that you can find from a supplier with a reputation you can trust. And enjoy the grow!

5 Comments. Leave new

  • Craig Machen
    27/04/2022 17:18

    Can I change lights to 12/12 from 18/6 during flowering my autos seem av slowed right down and do t seem to be duin much

  • Phillip Jermakian
    09/07/2022 00:05

    I was just watching a actual scientist explain that the plants feed their soil microbe buddies at night so that makes a dark period extra valuable in organic soil.

  • D-vision
    09/02/2024 13:29

    Can you advise me on lighting schedule for “Think fast femised” seeds please. Also how long the grow cycle is expect to be please 🙏

    • MT Dutch Passion
      27/03/2024 13:33

      Think Fast is a relatively fast Sativa (8 weeks of bloom) that likes to be topped several times – which can be a useful technique for experienced indoor growers who sometimes keep an occasional plant growing vegetatively for 2-3 months before switching the light cycle to 12/12.



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