Is There a Perfect Autoflower Light Schedule?

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Autoflower Light Schedule - Northern Lights DETAILS
Author avatar By Derek LaRose
July 13, 2021
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One of the first decisions growers make before planting is choosing between photoperiod and autoflowering cannabis strains. Autoflowering cannabis plants are easy to cultivate – especially if you follow a good autoflower light schedule – and can be harvested within a shorter period.

By contrast, photoperiod strains produce much higher yields but they require more space, specific skills and light changes to induce flowering. 

In this light schedule for autoflowers guide, you’ll learn about the different light options for growing autoflowers and which is the best one to use for your cultivar.

Purple Cannabis Under HID
Purple Autoflower Cannabis Plant Under Grow Lights

Do autoflowers need darkness?

Compared to photoperiod strains that require at least 12 hours of darkness to trigger the flowering cycle, autoflowers don’t depend on darkness to flower. They automatically produce buds when they mature.

Some growers use autoflower light schedules that deprive autoflower plants from light for a number of hours each day. As long as the dark period does not exceed 6 hours, the crop will still provide a high yield.

If the exposure to brightness drops below 18 hours, you’re likely to get smaller plants and a reduced harvest.

Autoflowering vegetative and flowering stages

Photoperiods and autoflowering cannabis go through a vegetative and flowering stage. A plant only grows leaves and stems during the former. It’s in the latter when it starts growing buds. 

For autoflowers, the vegetative period lasts about 2–4 weeks. Then, the flowering cycle begins.

Typically, autoflower strains take 8–12 weeks to go from seed to harvest. Our autoflower seeds have growing times that are as short as seven weeks. Growing these autoflowering cannabis seeds feminized will give you a yield in a shorter period.

Cannabis plants under grow lights
Vegetative Cannabis Plants Under Grow Lights

Which lights are best for autoflowers?

With photoperiod plants, growers use blue lights for the vegetative period and more reddish-yellow ones during the flowering period. You can apply the same principle to your auto flower light cycle. 

During the vegetative stage, provide your plants a blue spectrum that ranges from around 400 to 550nm. This helps cannabis plants grow short and stocky.

When they’re in the flowering stage, your autoflowering cannabis strains will need a red spectrum, with higher wavelengths from 550 to 700nm, to promote budding and produce large and dense flowers.

Keep in mind that you can still produce a successful harvest even if you don’t provide a full spectrum. A cannabis plant will grow healthily as long as it gets bright light with some red and blue.

There isn’t a best light for autoflowering cannabis. However, you’ll achieve better yields if you give your plants the spectrum they need during each stage.

Most growers agree that when growing autoflowering cannabis indoors, there should be between 18 and 24 hours of light a day

The four basic light schedules growers use include: 

18/6 light schedule

The 18/6 light schedule refers to 18 hours of light and 6 hours of darkness. It’s the most typical light cycle for autoflowering cannabis plants

You’ll save on energy and the fewer hours of darkness allow the plants to rest, promoting robust growth. If you live where there’s lots of sunlight, you can switch off your lights during the six hottest hours of the day.

Pros:

  • Your plants get time to rest.
  • You save money and energy because the process consumes less electricity than the 24/0 cycle.

Cons: 

  • You’ll need a timer. 
  • Your plant doesn’t grow as fast as in a 24/0 schedule.
Blueberry x Jack Herer Autoflower Plant Flowering Period
Blueberry x Jack Herer Autoflower Plant Flowering Period

24/0 autoflower light schedule

This cycle is perfect if you’re growing autoflowering cannabis in a cold climate. Leaving your LED or HPS lights on 24/7 will mean consuming more electricity, but it’ll keep your plants warm, enabling them to develop faster.

Using this autoflower light schedule means you won’t need a timer. Your lights will be on until harvest time.

Pros:

  • High possibility of a higher yield. 
  • You don’t need a timer since you don’t need to switch off your lights.

Cons: 

  • It’s a costly process, as you’ll use more electricity.
  • Your plants don’t get rest.

20/4 light schedule

The 20/4 light cycle is becoming popular among autoflowering cannabis growers because it’s between the 18/6 and 24/0 light schedules. As you don’t leave your lights on the whole day, your plants will get some rest time. 

It’s a perfect balance. You’ll have better yield results than the 18/6 schedule, and you don’t spend as much on electricity as in a 24/0 schedule.

12/12 light schedule

Growers usually use the 12/12 autoflower light schedule when they grow these crops in the same tent as photoperiods because it’s the light cycle for the latter.

It’s also the preferred schedule for those with a tight budget who can’t afford the electricity costs of practicing the longer light cycles. 

Why the 12/12 light cycle isn’t best for autoflowering cannabis strains

You can use the 12/12 light schedule for the above reasons, but it’s not the best for your autoflowers.

Cannabis plants usually produce impressive harvests if they get at least 18 hours of light a day. When you use a 12/12 cycle, you’re not allowing your autoflowering cannabis plants to grow to their full potential.

By limiting the amount of light, you’ll end up with a smaller plant, and your harvest will be lower than what you would produce by applying the 18/6 and 24/0 schedules.

Purple Kush Autoflower growing indoors
Purple Kush Autoflower Growing Indoors

What is the best light schedule for autoflowers?

There isn’t a best autoflower light schedule because of the many factors that come into play. It depends on the amount of yield you want and the electricity costs you can afford

The cannabis seeds you plant are another key element. Do some research and check out our seed bank for ones that are perfect for  growing autoflowering cannabis indoors.

Growers usually adjust the light based on the strain they’re harvesting. If you’re a newbie, you can use any of the common light cycles, but we advise you to start with the 18/6 because it should work for all autoflowers

As you progress, be sure to experiment with the other schedules to find the one that suits you well.

To keep learning about ruderalis plants & auto light schedules, check out this episode of the Homegrown PotCast:


About the author: Derek LaRose

Also known as Kronic from The Cannabis Kronicles, Derek LaRose is a young ambitious cultivator and a staple educator for indoor cultivation.