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what kind of light do autoflowering cannabis seeds need

Best light schedule for auto-flowering strains?

Unlike photoperiod plants where you have to provide special light schedules to get the plant to start making buds, auto-flowering marijuana strains automatically start making buds and are ready to harvest in about 3 months from seed, regardless of what light schedule received.

The next question, then, is what’s the best light schedule to give auto-flowering marijuana plants each day if you are growing indoors?

Short Answer: 18/6 Schedule
18 Hours Light, 6 Hours Dark (perfect for beginners)

Most growers seem to agree that the optimum amount of light to give auto-flowering strains of marijuana is somewhere between 18-24 hours of light a day. When I’ve grown auto-flower plants, I gave them 18 hours of light a day, and 6 hours dark, at least partially because it allowed me to turn the grow lights off during the 6 hottest hours of the day (I live in Southern California where it’s hot most of the year!).

Recommended: 18-24 hours of a light a day

I’ve also seen many great growers give their autos 20, 22 or even 24 hours of light a day.

Do Autoflowering Plants Need a Dark Period?

There are growers who believe auto-flowering plants need a dark period and won’t be as healthy if they get 24 hours of light a day.

I personally haven’t seen evidence of that, and at least in my experience autoflowering cannabis plants seem to do great getting 24 hours of a light a day.

Additionally, the wild ancestor of auto-flowering plants (“Ruderalis” hemp) grows in the northern parts of Russia and may naturally experience nearly 24 hours of light a day in the summer. So it’s not crazy to think that auto-flowering strains will also do well on 24 hours light/day.

However, giving a plant fewer hours of light a day will save you on electricity, and if it does help the plant grow better then that’s a bonus! Combined with the fact that it gets hot here during the day and we have extremely expensive electricity, I always put my autos on 18/6.

I’m very happy with how my plants grow on an 18/6 schedule, but it’s definitely not necessarily the best light schedule to grow autos depending on your situation. Here are a few more pros and cons to consider:

18/6 Pros

  • For hot climates, the ability to turn off lights during 6 hottest hours of the day
  • Uses 25% less electricity than 24 hours of light a day (which adds up if you have expensive electricity)
  • Plants are harder to stress and recover faster from problems. Often the grow light can be kept closer. This extra hardiness can make things a little easier for you if you run into problems, or if it’s your first grow.

24/0 Pros

  • Cannabis plants typically grow faster when they get 24 hours light/day, especially during the first month of life.
  • A good choice if it gets cold and you don’t want lights to turn off! Keeping the lights on 24 hours a day can prevent the grow space from getting as cold at night
  • If plants are very healthy and growing fast, it doesn’t matter if they’re more resistant to problems

Can I grow auto-flowering plants on a 12/12 schedule?

There are situations where it may be easier to grow your auto-flowering plant on a 12/12 schedule. For example, if you’re growing flowering photoperiod plants, you need to keep them on a 12/12 schedule to keep budding. Is it okay to stick an auto in there too?

Yes, but…

Yes, it’s okay and your auto will grow just fine on a 12/12 schedule… except for one thing. Because your auto is getting less light each day, it isn’t able to make as much energy to power growth and you’ll end up with a smaller plant and reduced yields compared to if you’d given more light a day.

One of the really great things about autos is how they’re able to give you pretty impressive yields in such a short amount of time. Part of the way they accomplish this is you’re able to give the plant 18 hours of light a day to power buds, so buds get 50% more light and grow faster than they would under 12/12. By giving your autos 12/12 you’re not using them to their full advantage. That being said, if you don’t mind reduced yields they’ll grow just fine! And it can be fun to stick an extra plant in the tent and let it do its thing!

Auto-Flowering Vegetative & Flowering Stage

When it comes to choosing the best light schedule for growing auto-flowering marijuana strains, it’s a good idea to understand what light schedules are, and how different light schedules affect the way a cannabis plant grows.

All cannabis strains, whether they’re photoperiod or auto-flowering, have two main stages of life: the vegetative (first) stage, and the flowering (second) stage. Think of them as the “children” and “adult” stages of marijuana growth. During the vegetative stage, a cannabis plant grows only leaves and stems. In the flowering stage, the plant stops growing leaves and stems and starts growing buds/flowers.

Most strains of marijuana (photoperiod strains) rely on the amount of light, or their light schedule, to determine when to start making buds. Generally, if a marijuana plant receives 14 hours of light a day or more, it will stay in the vegetative stage (it will focus more on growing leaves and stems without any buds).

Auto-flowering strains don’t rely on light schedules to determine when to flower. Auto-flowering strains will automatically leave the vegetative stage and start budding after about 2-3 weeks regardless of the light schedule they’re given.

“Auto-Flowering” strains of cannabis have different light requirements than photoperiod strains

Auto-flowering varieties of marijuana don’t rely on a change in light schedule to determine when to flower. These strains (sometimes called “Lowryders” or by their species name “Ruderalis”) will finish their whole life cycle in two to three months, regardless of how much light they receive a day. See a grow journal featuring auto-flowering strains.

Light Color Spectrum

Another consideration when growing auto-flowering plants is what light spectrum to use.

Normally with photoperiod plants, you would optimally use more blue lights for the vegetative period and more red lights for your plant during the flowering period. With auto-flowering strains, you can follow the same principle by using the blue lights before the plant has shown signs of its gender, and then switch to more red lights once you see the first signs of gender.

Blue Light for Vegetative – bluish light (like from Metal Halide bulbs or 6500k (blue) fluorescents/CFLs) is a great choice for the vegetative stage, which for auto-flowers is just the first few weeks of life

Yellow Light for Flowering – yellow/orange/red light (like from HPS bulbs or “soft white” colored CFLs/fluorescents) tend to produce the best growth and densest buds in the flowering stage. Their color mimics the reddish light from the autumn sun. It is completely okay to give your plant yellow light for its whole life. In fact, that’s what I prefer to do as yellowish light encourages stretchy growth, and I want an auto-flowering plant that grows fast!

Remember: Spectrum is much less important to yields than the total wattage of your lights! (More Light = More Yields)

HPS grow lights are the most popular grow light for the flowering stage, with good reason. They are one of the most efficient grow lights available today (as far as electricity used for density/yields produced – even more efficient than most LED grow lights), they’re cheap to start with, they’re just the right color spectrum for optimal flowering, and they’re more standardized and intuitive to use than many other types of grow lights. Learn more about finding the right size HPS for your space

However, marijuana plants are weeds, and will usually grow at least okay under either spectrum of light during either stage.

Light in any color spectrum will produce good growth and bud as long as you’re using cannabis grow lights and there’s enough brightness.

In fact, many growers use Metal Halide or other more “blue” colored lights for the entire life of the plant and still produce good results. Some growers even prefer these lights for budding.

But the best yields and density in flowering is usually achieved with reddish-yellow light. For the hobbyist grower who may be low on cash and can’t change up the lights, you may want to consider getting all flowering bulbs for autos since they spend practically their whole life in the flowering stage.

Optimum light cycles for an Autoflowering Marijuana Strain

One of the best things about growing an autoflowering marijuana strain (or several) is that they are not dependent on light cycles for growth. Plants like Grand Daddy Purp Auto or Blue Dream Auto will start to flower when they reach a certain age or maturity. So as a grower, you don’t need to worry about always turning the lights on or off at a certain time. But still, a cannabis plant is still a plant, and so it’s going to need some type of light in order to thrive. So what light cycles will these plants need to get the best results?

Best light cycles for an autoflowering marijuana strain

It doesn’t really matter what type of strain you have. Whether it’s a Grand Daddy Purp Auto, a Blue Dream Auto, or some other type of auto strain, it’s going to need light and there is an optimum light cycle.

Some growers swear by a 24-hour light cycle for their autoflowering plants. This is simply because cannabis is a C3 plant, it can absorb CO2 during periods of light for photosynthesis. Theoretically autoflowering plants do not actually require a period of darkness and they are also indigenous to northern areas of the world, that do sometimes see periods of 24 hours of light. But that might not be best for autoflowering plants.

A marijuana plant, just like people, need a period of rest and recovery. And when they don’t get it, we can see a negative effect on the yield that they provide and sometimes even pro long or stop them from flowering. Some growers drop the light cycle to 18 hours a day. This for many, including us is considered the optimum light cycle because the plants still get the rest they need while also being given enough light to produce large yields. Other cannabis growers even put their autoflowering marijuana strain in a 20-hour light cycle. These growers believe that this is giving the plant the best of both worlds.

In most cases, no grower wants to give their plants less than 18 hours of light a day. That light is vital to the plant’s growth and without it, the plant and the yield will suffer. But there may be times when a lower light cycle is needed.

When to use the 12-hour light cycle

The Internet is full of forums. And on many cannabis or autoflowering forums, those forums are filled with threads from people asking for advice on why their autoflowering plant isn’t producing the blooms they would like. Sometimes this happens at the time the plant should be flowering, while other times people are simply wondering if they can speed up the already-fast process of getting buds from an autoflowering plant.

Firstly, never rush an autoflowering plant. Nature has already done all the hard work by eliminating the need for several different light cycles and waiting months for the plants to grow enough so they are ready to be harvested. If waiting the short timeframe for an autoflowering plant is too much, it may be best advised that a local dispensary provides the bud that is needed sooner.

But, if you’re being patient and your autos still aren’t producing, there may be something you can do. And that’s switch your plants to a 12-hour light cycle. Even with the experienced growers in these forums, no one really seems to understand why switching the plants to a 12-hour light cycle seems to work. The most logical explanation is that this re-enforces it’s already trained genetics to believe summer is coming to an end, as the plant starts to receive less light. Therefore forcing it to want to grow faster in time to harvest before the frost sets in.

Growers should know that this is a light cycle last resort for when plant appears to not be flowering or is very slow at flowering. It is never advised to lower the light cycle to any more than 12 hours of light per day. That amount will not provide the plant with everything it needs to grow and thrive and is more than likely going to hurt your plant and yield.

An Autoflowering marijuana strain are one of the easiest types of marijuana plants that a person could grow. They’re convenient, they run on their own schedule, and will 99% of the time, flower early with very little muss or fuss. But there are a few things to keep in mind when growing them and these optimum light cycles can help you get yield you hope for.

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Autoflower 12/12 Light Cycle

Autoflowering plants as we have determined earlier grow best when you give them between 18 and 24 hours of light each day but there are many circumstances when a grower need to give his plants only 12 hours of light. And today I will try to explain the what, why and when about autoflowers and 12/12 (day/night) light cycle.

First of all I need to say that you can absolutely grow autoflowers on a 12/12 light schedule and they will produce great buds but because those plants will receive less light then their yield will be smaller than if you compare them with plants that are grown under 20/4 or even 24/0 light cycle.

Why use 12 hour light cycle?

There are many reasons why a grower would grow autoflowers under 12 hours even if autoflower plants do best under 18 to 24 hours of light. And here are the most common reasons:

  • Autoflowers in the “flower room” – If you are growing regular (photo-sensitive) plants but still want to get some early autoflower buds then you can put autoflower seeds in the flowering chamber and get decent buds well before your photo-sensitive strains have matured. You can even grow your autos together with photos from the start giving them 24 hours of light and then switching the light cycle to 12/12 when you want your photos to start flowering. Autoflowers will not mind the light cycle change and contrary to popular belief this change won’t increase the hermaphrodite rates.
  • Growing autos outdoors – If you are growing autoflower plants outdoors in the spring or in the autumn when the day is about 12 hours long then that won’t impair autoflower plant growth and they will still flower and produce you great buds.
  • Too hot temperatures during the daytime – If you are living in a climate where the summers get really hot then you might want to only turn on your grow lights in the night when the sun is down and the ambient air gets cooler. In this situation you can still get some indoor autoflower buds even with 12 hours of light that turns on only in the nighttime.
  • Cutting electricity costs – If you are on a tight budget then you can reduce the light cycle from 24 to 12 hours and cut your electricity costs in half but still get some decent buds. But be careful because some autoflower strains will take longer to mature when grown under 12 hours so your savings may end up eroding due to longer grow time and less yield.
  • Bad genetics – If your “autoflower” plant does not automatically flower under continuous light cycle after 3 to 6 week then you might have a bad batch of seeds and you will need to manually adjust the light cycle to 12/12 and kickstart the flowering phase.

As you can see then there are quite a few reasons when you might need to grow autos under 12 hours of light but this practice also has some drawback.


  • Increased stretching – If you give your autos only 12 hours of light each day and 12 hours of darkness then that can promote some unwanted and excessive stretching so you need to get your autoflower as close to the light source as you possibly can when those lights are on. And you should also give them complete darkness in the night-time so that the plant doesn’t try to stretch to the light that leaks in your grow room trough some openings.
  • Slower growth – If you give your plants less light they will grow slower and won’t produce as many new branches and leaves so you will end up with a smaller plant and less leaves / branches and fluffier buds.
    Longer growing cycle – Some autoflower strains will take longer to mature if you give them only 12 hours of light. And I have seen plants that need more than 30% longer growth time to get to the harvest stage when grown under 12/12.
  • Less yield – And ultimately when you are giving your plants less light then they will give you less yield and that can be a big drawback to many growers. But sometimes 12/12 is your only choice and even with less yield from a single plant you are still getting at least some buds and you can also grow more plants to compensate this decreased yield.

So the most popular light cycles for autoflowering plants are growing under 12/12 from the start or switching from 24/0 to 12/12 when the autoflower flowering phase starts.

  • 12/12 from the start – If you are growing autoflowers in your flowering room then you can grow them from start to finish under 12/12 light/dark cycle but that will produce the worst results as your plants will get the least amount of light.
  • 24/0 then switching to 12/12 – The best option if you want to use the 12 hour days is to start your plants under 24/0 or 18/6 while they are in their vegetative growth stage and then switch that light cycle to 12/12 when the first pre-flowers appear. This will give your autos the most amount of growth and the yield will not suffer that much.

And there are also some more exotic light schedules like 14/10, 14/8 13/11 (light/dark) that are used of regular photo-sensitive plants and where autoflower have been grown. But I do not see the advantage in these schedules as they don’t produce more buds or if they do then the difference is usually miniscule.

And now you can make an educated decision if you want to grow your autoflowers under 12/12 or better wait and grow them under 18/6 or 24/0 light cycle.