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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!

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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.

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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.

Autoflower Light Cycle – Best Light Schedule For Autoflowering Cannabis

Unlike photoperiods that need at least 12hrs of darkness to trigger flowering, autoflowers automatically enter the flowering cycle when they’re mature enough. They start producing buds based on age thus they don’t depend on darkness to start the flowering cycle.

This means that the best light schedule for autoflowers is either 24/0, 20/4, or 18/6 so there are multiple options and the best one will depend on your growing environment and setup.

Tip: Light cycles are used to simulate seasons in nature, more light = summer, more darkness = winter.

1. What is the best light cycle for autoflowers?

Most growers agree that the optimum amount of light to give autoflowering strains is somewhere between 18-24 hours of light a day. As a grower you should adjust the light depending on the cultivar you’re growing.

There are basically four light cycles aka Light schedules: 24/0, 20/4, 18/6, and 12/12 (the first number is hours of light, the second is hours of dark). You can always adjust the cycle as long as they are receiving more than 12 hours of light a day (to achieve the best results). An example would be 19/5, 16/8, 22/2, etc.

Having that in mind, as mentioned above, there are growers experimenting with schedules like 16/8 but if you’re new to growing you should stick to the basic schedules mentioned above.

Light Spectrum

Now, despite cannabis plants relying on the amount of light they get, you should know that depending on the phase your plant is in, it will need a different spectrum and that can also affect your harvest.

As you may know, cannabis plants go through 2 stages, the vegetative and flowering stage, during these stages your autoflowers will obviously need light, but they will also need certain wavelengths of the spectrum.

It’s essential you provide the ideal light spectrum for each stage of plant growth and preferably, a full spectrum.

For example, during the vegetative stage, your plants will need a blue spectrum that ranges from around 400 to 550nm whereas during the flowering stage your plants will need a red spectrum, with wavelengths that range from 550-700nm.

This doesn’t mean that you won’t be able to have a successful harvest if you don’t provide a full spectrum but you will definitely have better results by providing the whole spectrum from seed to harvest or at least the spectrum your plants need during each stage.

Have in mind that this applies to all Light fixtures, including light bulbs, usually good-quality LEDs will emit a full spectrum but you can get mixed spectrum bulbs that will get the job done.

2. Do autoflowers need darkness?

There are growers who believe autoflowering plants need a dark period and won’t be as healthy if they get a 24/0 light schedule. There’s no real evidence of that but there may be exceptions. Also have in mind that a 24/0 will lower humidity , increase temperature , and increase the light bill .

Schedules like 18/6 will save you electricity and if the few hours of darkness indeed help the plant to grow better, it’s a win-win. We recommend starting with an 18/6 light cycle and if you see your autoflower has the potential to grow more, you can always try again in the next cycle.

3. 24/0 Light Cycle

Plants usually grow faster when they get more light. This schedule is a good choice if you live in a cold climate, keeping the lights turned on 24hrs a day will keep your autoflowers warm.

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Obviously, this is the easiest autoflower light cycle as you don’t even need a timer, just turn on lights until harvest.

  • Doesn’t require a timer.
  • May result in the best yields.
  • Uses more energy, can get expensive.

4. 20/4 Light Cycle

The 20/4 has become a popular autoflowering light schedule amongst growers because it’s in the middle between 18/6 and 24/0, providing a good growth without having to spend too much like when growing under 24/0.

Despite obviously leaving the lights on for a bit longer than in 18/6, your plants still have some time to “rest”, allowing you to have slightly better results without having to spend too much like in a 24/0.

  • Allows autoflowers to rest.
  • Better results than 18/6.
  • Uses more energy than 18/6, can get expensive.

5. 18/6 Light Cycle

This is the most common cycle for autoflowers. You’re providing enough light for your plants to develop and a few hours of dark for them to rest, encouraging healthy growth. This can be good in hot climates.

You can leave your lights at night and have them turn off during the 6 hottest hours of the day. By using this schedule you’ll be using around 25% less electricity, which adds up to be a good economy at the end of every harvest.

A lot of growers who have experimented with 24/0 and 18/6 light cycles say they didn’t notice any major difference in final yield. The only minor inconvenience with this schedule is having to buy and adjust a timer. They cost around 10 bucks so it shouldn’t be a problem at all.

  • Allows your plants to rest.
  • Uses less electricity, allowing us to save on electricity.
  • Can help lower temperatures and increasing humidity in hot climates.
  • Requires a timer. This shouldn’t be a problem at all, as this is the first thing you should buy when growing indoors.

5. 12/12 light cycle

The 12/12 schedule is normally used for photoperiods. It can also be used for autoflowers but it’s not that popular. Normally growers give autoflowers this light cycle when they have them growing in the same tent as photoperiods. By giving your autos 12/12 you’re not growing them to their full potential.

Because your plant is getting less light each day, it isn’t able to make as much energy to promote growth. It will underperform compared to the other light cycles, that being said, you can grow them just fine if you don’t mind reduced yields and an overall smaller plant.

  • Uses less electricity, allowing us to save on electricity.
  • Allows you to grow autos and photos in the same grow tent.
  • It’s recommended to grow under 18/6 so 12/12 usually results in aery and underdeveloped buds.
Autoflower grow light schedule pros and cons
Light Schedule Pros Cons
24/0 Faster growth. May be too much for some strains.
20/4 Good growth and allows plants to “rest”. Slightly less growth than 24/0.
18/6 Less heat and cheaper. Slightly less growth than 20/4.
12/12 Cheapest. Underdeveloped buds.

6. In Conclusion

There isn’t really a proven best light cycle. Before going for any light cycle, think about electricity costs and the climate you’ll be growing in. If you are in doubt, start with 18/6. This cycle is the most used and should work for all autoflowers.

When growing outdoors you have to work with what you have as you can’t control the sun (obviously). Outdoor growers should be aware that with the change of seasons the amount of daylight may increase or decrease but autoflowers should grow fine in all seasons.

Even though there are the most common light cycles, make sure you experiment with all of them and find what suits you better because every plant and grow setup is different so there’s a chance that you have better results growing under an uncommon light cycle.