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Buyer's Guide

Best Light Schedule For Auto-flowering Strains




Autoflowers are an absolute joy to cultivate, especially for beginner cannabis growers. They mature fast, do not take up much space, and don’t need to be induced into flowering. 

Though they require very little prompting, it would not hurt to know the perfect lighting conditions they would thrive under. To this end, we will take a closer look at the best lighting schedule to spur auto-flowering strains into a bumper, and potent harvest.

Let’s dive right in! 

The autoflower Light Cycle

Unlike photoperiod cannabis strains, the start of a flowering phase in autoflowers is not dependent on a change in its light cycle. Typically, they will shift to the flowering stage based on their age, regardless of their light schedule. Their ability to flower automatically is why they were named auto-flower.

Light Schedule for autoflowers

When it comes to picking the best light schedule for growing auto-flowering marijuana strains, understanding what light schedules are, and how effect different ones have on the way a weed plant will grow, is a good idea.

All cannabis strains, both photoperiod, and auto-flowering, have two distinct stages of life: the first stage is the vegetative stage, and the second, is the flowering stage. You could think of them as “children” and “adult” phases in marijuana growth. In the vegetative stage, they will grow only leaves and stems. When the flowering stage kicks in, the plant stops growing its stems and leaves, and starts growing its buds/flowers.

Most strains marijuana strains (photoperiod strains) are reliant on their light schedule to kick off the process of making buds. Generally, a marijuana plant that receives a constant 14 hours of light/day or more will remain in the vegetative stage (it will keep focusing more on the growth of leaves and stems and not buds).

In contrast, auto-flowering strains do not count on light schedules to trigger their flowering phase. They will automatically stop vegetating and start budding in about 2-3 weeks no matter the light schedule they are put under.

“Auto-Flowering” strains of marijuana differ in their light requirements from photoperiod strains

The auto-flowering varieties of weed are able to trigger their own flowering stage. This they do without a change in light cycle, as they don’t rely on a  light schedule to determine when to flower. These strains, often called “Lowryders” or simply by the name of their parent species, “Ruderalis”, will complete the whole life cycle of their lives in just two-three months, regardless of the amount of light they receive/day. 

Autoflower best light cycle

Does that mean that no attention should be given to the light schedule they are placed under? Absolutely not.

Though autoflowering cannabis strains are bound to flower based on age, instead of a change in light cycle, giving your autos the right amount of light is still crucial in ensuring you get the best possible harvest. We will describe the different light cycles and outline which is the best light for autoflower.

18/6 Light Cycle

Being a C3 plant, cannabis absorbs CO2 for photosynthesis, even during light hours. Because autoflowering plants experience short vegetative phases, and are typically shorter than photoperiod strains, giving your plant at least 18 hours of light is considered to be the bare minimum. This does not allow for robust growth with minimal energy. Though they don’t rely on a light schedule to bloom, they love light in the same way traditional photoperiod strains do.

24HR Light Cycle

Though some growers insist that giving their autos a full 24 hours of light maximizes growth in the vegetative stage, others stick to 18-hour light cycles. The latter group argues that the plants need a “recovery” period in order to achieve healthy growth. Though both groups may have a point, autos do well in both 18, and 24 hours of light. We recommend that you try out both and see which will work out best for you. 

Can you grow autoflower seeds on 12 12?

Even though it is not a requirement, some cannabis growers still decide to practice ‘a 12 12 for autoflower’ during the flowering phase. This is actually fine as  autos grown on 12/12 still yield a fair harvest. It is important to note however that buds harvested under a 12 hr light cycle may be noticeably smaller than with 18–24hr light cycles

Some reasons growers consider keeping their autos in autoflower light cycle 12 12 include:

  • Heat concerns: If a grower lives in a very hot climate, turning the grow lights off during the day prevents the grow room from overheating. In this case, growing the autos under a 12/12 light cycle that has lights on only during the night, may be the best bet in avoiding heat stress. 
  • Growing autos alongside photoperiod strains: If for some reason you had to grow your autos alongside feminized photoperiod strains, you will have to give your flowering feminized plants the required light schedule, which will mean that your autos also get only 12 hours of light each day.
  • To save money: Having your grow lights on for 18–24 hours over the period it takes to mature the cannabis plants could get quite expensive. If you have a tight budget while you are growing your autos, a 12 hr light cycle could be more pocket friendly.

In conclusion, can I grow auto-flowering plants on a 12/12 schedule?

As we have discussed, in some situations, it may be easier to plop your auto-flowering plant into a 12/12 schedule. An example we had given before is if you are growing flowering photoperiod plants which have to be on a 12/12 light schedule, could you stick your autos in there?

Yes, but…

Your auto plant thrust into a 12/12 schedule will grow just fine. However, because it is receiving less light every day, it will not be able to make enough energy to power its growth. You will end up with a smaller plant, which will have reduced yields in comparison to autos that receive more light per day.

One of the most impressive attributes of autos is their ability to produce impressive yields in a very short period of time. Giving your plants at least  18 hours of light/day ensures the buds get enough energy as they receive 50% more light than they would under 12/12. This ensures that they grow faster and more robustly. Putting your autos on a 12/12 light cycle does not explore their full advantage. That said, if reduced yields are something you can factor in, they will grow just fine, even under 12/12! Sticking in your auto into a 12/12 and watching it do its thing might be just what you need to keep your growing interesting! 

Do Autoflowering Plants Need a Dark Period?

Though many auto growers put their plants on a 24hr light schedule, others believe that the auto-flowering weed plants need a period of darkness, and will not be as healthy without it.

This however does not seem to be grounded in fact because many growers achieve great results on a 24 hr light schedule. Additionally, the Ruderalis hemp, which is the wild ancestor of all plants that auto-flower grows in Russia’s northern parts, which naturally experience almost 24 hours of light  in the summer. In this vein, expecting autos to do well under 24 hours of light a day is not a crazy expectation. 

Pros and cons of the different light schedules

We will provide you with a few more “pros and cons” to consider while deciding on the best light schedule for your autoflowering strains:

18/6 Pros

  • In hot climates, turning the lights off for at least 6 hours prevents the plants from experiencing heat stress. 
  • You save on 25% of electricity than if you are using 24 hours of light/day. This can add up to a lot if your electricity is expensive. 
  • Plants on this schedule are “harder to stress”, and they recover faster from issues. This extra hardiness comes in handy if you run into problems, or for first grows.

24/0 Pros

  • Cannabis plants will typically grow faster if given a lot of light, and a 24 hours light/day does this effectively, especially in the first month of life.
  • Ideal for cold climates. Keeping the lights on for 24 hours prevents the room from turning cold.
  • Healthy plants that are growing fast thrive well even under a 24 hour light cycle. 

When do autoflowers start to flower?

Autoflowers typically flower within 3-5 weeks. The flowering stage is usually characterized by plenty of white hairs growing all over the autoflower, and by week 6, the buds will begin develop.

When should you switch to bloom light autoflower?

In case you intend to switch the schedule of your plants to an auto flower light schedule, it would be best to switch to bloom light when you notice the first white hairs popping out. This is most likely when the auto flower is beginning to flower.

Growing autoflower with LED lights 

When growing any type of cannabis, autos included, employing the “right light spectrum” is super important. Because your autoflowering plants tend to have short life cycles, maximizing the quality soil, nutrients, and light is vital in ensuring that they thrive within that short time. 

If you think back to science class, shining a light through a prism usually breaks it into different “color spectrums”. Plants that depend on light for the process of photosynthesis have different responses to the different spectrums. Similar to photoperiod cannabis strains, when growing your autos, you should stick to the recommended light spectrums listed below:

  • 6500K blue light during veg: “Blue light spectrums” have been proven to encourage vegetative growth. They help the cannabis plants grow short and strong, while at the same time minimizing stretching.
  • 2700K red light during flowering: Red light spectrums on the other hand are ideal for bloom. This is because they encourage budding, and also spur a tiny bit of stretch, which encourages your plant to produce flowers that are big and dense.

To this end, you can look for full-spectrum LED fixtures which will surely enhance the quality of your harvest. Growing your plants with good quality LED lights ensures that you end up with denser and more resinous buds, than if you used light bulbs. LED’s also emit less light, are more efficient, and have a longer life span.

Extra tips for growing great autos

If you are still a newbie in the world of autoflowers, the following tips will ensure that you end up with a  better harvest:

  • When deciding on the autoflower to grow, pick a high yielding one. Contrary to popular belief, not all autos are made equal. In this vein, be sure to pick a strain that is high-yielding if the size of your harvest is important to you.
  • Plant in your final container: Though cannabis plants are often up-potted, this process always causes stress. Because autoflowers typically go from “seed to harvest” in just about 8 weeks, they may not have enough time to recover from this stress. It is thus best to avoid putting them in these stressful conditions by directly planting your auto seeds straight in their final pots. This ensures that they grow smoothly without interruptions, as they do not have the recovery time photoperiod strains do. 
  • Use an airy growing medium: An airy medium will help stimulate both foliage and root growth. For the same reasons given above, your auto might not have enough time to establish intricate systems. You should therefore help it along by ensuring its growth medium is well aerated for ease in growth and maximization of processes. 
  • Use Low Stress Techniques: If for some reason you need to train your cannabis autoflowering cannabis plants, use low-stress techniques only. High-stress techniques such as topping, fimming, and supercropping will cause more damage than good to your autoflowers. 


The best autoflower grow light schedule might differ from grower to grower. However, what is most apparent is that they are not as sensitive to the amount of light as photoperiod strains. That said, light periods of between 18 to 24 hours appear to be ideal. Tweaking the light with the information we have provided will ensure you make a good situation even better. So go on and make the best of your autos!

Sebastian is a passionate advocate of CBD's therapeutic potential, dedicating himself to exploring its diverse benefits. As a seasoned writer, he eloquently shares his insights and personal experiences with CBD, aiming to educate readers about its transformative power. His life's mission is promoting holistic wellness, with CBD at the heart of his advocacy.