Best Light Schedule For Autoflowers Explained

Best Light Schedule For Autoflowers Explained

Autoflowers were created by cannabis breeders who crossed the genetics of Ruderalis with existing Kush, Sativa and Indica genetics. Autoflowering cannabis differs from photoperiod plants which need particular light schedules to encourage the plants to produce buds. Autoflowers begin to flower automatically after growing for 20 to 28 days in the vegetative phase.

However, autoflowers - especially when grown indoors - do need an optimum amount of light to guarantee an excellent harvest. Read this article to find out more about the best light schedule for autoflowers.

Why you should grow an autoflower 

Autoflowering cannabis strains take approximately 8 to 9 weeks from germination to harvest, which is significantly shorter than photoperiod weed varieties which need an average of 9 to 12 weeks. Autoflowering cannabis strains are probably the easiest strains to grow as it requires much less attention and gardening effort compared to photoperiod.

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The best light schedule for autoflower strains

As mentioned earlier, growers of autoflowering cannabis do not need to manually set the lighting duration to 18 hours in the growing phase then to reduce it to 12 hours a day for flowering.

This form of cannabis blooms automatically due to its genetic makeup. Since autoflowering cannabis starts flowering only 20 to 28 days after germination and remains in the flowering phase for the rest of its life cycle, it requires different types of lighting and intensities at different times compared to photoperiod cannabis.

Most growers of autoflowering strains prefer a light cycle of 20 hours of light and 4 hours of darkness per day. The short growth phase makes it necessary to use this time as effectively as possible and to maximise the growth of the plants. Yet, you may prefer to save fuel bills by adopting a lighting schedule of 18 hours of light and 6 hours of darkness. This will also give good results, especially if you live in a location where it is naturally hot and sunny as you will be able to turn off the lights during the hottest part of the day.

In summary, anything from 18 to 22 hours a day of artificial light will produce great results with autoflowers.

Related article: The life cycle of autoflowers

Is 24 hours of light ever a good idea?

Some growers give their plants 24 hours of light with no darkness at all. Some growers argue that the Ruderalis hemp genetics derive from northern areas of the world where the sun hardly sets in summers, and so native plants are exposed to nearly 24 hours of sunlight in the natural world. However, we do not recommend this as it tends to overwhelm the plants and puts them in unnatural stress. Some growers report nutritional problems (such as calcium, magnesium, zinc, and iron deficiency) and other problems that can arise from constant lighting.

Which lights are best for autoflowers?

Like photoperiod cannabis, autoflowering strains prefer the 'blue light' during the vegetative phase. Blue light is produced by metal halide and LED lamps and has a colour temperature which is very close to that of the midday sun in late spring and early summer. Autoflowering plants have little time (usually only up to 28 days) to form structures such as leaves, stems, branches and roots in the vegetative phase. If the grower succeeds in providing the autoflowering plants with as much light as possible for as many hours a day as possible during this growth phase, this will help the plants to form the structures that are required to support large flowers.

Gentle lights for seedlings

Just be careful: tender young plants do not tolerate intense light.

Many autoflowering growers only use fluorescent lamps or use appropriate LEDs in the first one to two weeks after germination. Then they add an increasing dose of blue light from until the plants start to flower in the third or fourth week.

When autoflowering plants begin to bloom, growers use a mix of different lamps that continue to provide blue light but also add red and infrared light. High-pressure sodium lamps (HPS) or high-quality LED lamps with a complete cannabis-specific spectrum are used for the red light.

From the fourth week of life, an autoflowering plant should be able to tolerate the same light intensities as photoperiod cannabis during flowering.

The most successful autoflowering growers have revealed that after the first to the second week of flowering, they reduce the amount of blue light compared to red light. If too much blue light is used during the flowering of autoflowering varieties, the Ruderalis genetics ensure that many leaflets form in the flowers, making them less compact and scattered with resin, although this is not a problem for most growers.

In any case, as long as the plants grow in an appropriate growing environment and are supplied with good water and the right nutrients, it depends on the intensity and the right light spectrum to push autoflowering plants to maximum yields.

There are already suitable LED lamps that have been able to deliver excellent results when used with autoflowering varieties - and that for growers worldwide.

Considering the spectrum is essential, but remember that wattage plays a much more significant role when it comes to autoflowers.

A note for outdoor growers: Autoflowering varieties can also deliver acceptable results outdoors, even if they only receive up to 12 hours of light per day. The yields will not be very high under these conditions, and the plants will remain dwarfs. Nevertheless, the plants will go through their full life cycle, will flower automatically and produce flowers.

Final tips for growing lovely autoflowers

  • Plant seedlings in a good size container. Autoflowers don't take kindly to repotting, finding it stressful. 6 to 9-litre container pots would be ideal.
  • Use a potting mix that is well-aerated.
  • Use low-stress techniques of gently bending or tying stems to train your growing autoflowers.
  • Choose a high-yielding autoflower. Not all autoflowers are the same, so always buy seeds from a trusted purveyor of excellent cannabis genetics, such as You will find a wide selection of feminised autoflowers and many other varieties of weed.

Reading tip: how to grow autoflowers