How much light for autoflowering cannabis plants?

How much light for autoflowering cannabis plants?
Steven Voser

18/6, 20/4, 24/0 or something else? Find out about the best lighting schedule for autoflowering cannabis plants.

Autoflowering cannabis strains are the fastest and most versatile of all varieties of marijuana. Ruderalis genetics enables autoflowering cannabis to flower independently of the light cycle.

Wild Cannabis ruderalis is believed to have originated in Siberia and has evolved to enter flowering according to age rather than seasonal sunlight. This unique ability is what differentiates autoflowering varieties from typical photoperiod cannabis strains (indica and sativa).

Don’t worry, rapid flowering and toughness are pretty much the only two traits modern autoflowering strains express from their ruderalis background. Contemporary autoflowering strains are actually pedigree hybrids consisting mostly of indica and/or sativa genetics, with just enough ruderalis to induce flowering automatically usually in 30 days or less following germination.

The latest generation of autoflowering strains produces highly potent connoisseur-grade cannabis, that is a far cry from its humble “ditch weed” origins. Moreover, just because autoflowering cannabis can adapt to difficult growing conditions, doesn’t make them invincible, nor does it guarantee the grower a fat stash come harvest.

Autoflowering plants can be successfully cropped in all seasons, bar winter, outdoors and on a range of lighting schedules indoors. The trick is to find the optimal amount of the right light spectrum for autos, in order for them to reach their full potential and deliver a big fat frosty harvest.


This is a question, that is still being hotly debated, as only the 5th generation of autoflowering cannabis strains are being released in 2017. It’s really only in the last decade, that ganja farmers have had the opportunity to crop high-quality ruderalis hybrids. So nobody really definitively knows for 100% certain.

However, the 18-6 light-dark schedule is the most popular amongst indoor growers and the most economic light cycle. For growers planning on running a grow op year-round through the hot summer months, this is probably the best fit. In our opinion, this is always the right amount of light for cropping autoflowering plants indoors from seed to harvest.

Running 24 hours is another option with autoflowering strains; although, it will increase the power bill and grow room temps too. This is really an option to consider during an especially cold winter to maintain optimal temperatures.

Otherwise, this light schedule is unnecessary, as it is not proven to significantly increase yields, nor further quicken flowering. 22-2 and 20-4 are common enough too, but hard data validating their superiority is absent for now.

A viable alternative is to bloom autoflowering plants on a 12-12 light-dark schedule just like or perhaps alongside photoperiod strains. Including an autoflower or two in the grow show is a handy way to crop some herb in a hurry while you await your dank photoperiod strains to fully mature.

Autoflowering cannabis will bloom just fine in 12 hours of light, but yields and overall size will be reduced. It’s not the perfect lighting schedule for autos, but for photoperiod strain growers it’s a means of gaining familiarity with autos and giving them a trial run.


The short answer is sunlight. But for the indoor grower, you can come pretty close with the latest generation of full spectrum LED lighting systems. Full spectrum LEDs are perfect for the entire cannabis life cycle and the most efficient grow lights on the market.

In the long run, an investment in LEDs is worthwhile, as power bill will be reduced and both yield and potency will reach the next level. The initial purchase costs of a high-quality LED kit can be daunting, but this technology is the future of indoor cultivation.

As the LED flatscreen TV ushered in the obsolescing of the clunky box TV set, LED grow lights will soon antique HIDs.

HID grow lamps are still a great short to medium term lighting system to grow autoflowering strains with very low start-up costs, although the running costs will be higher than LEDs. An MH bulb is not really necessary if you want to crop autos using old school grow lamps.

The short vegetative growth stage, rapid transition to the bloom phase, and shorter stature of autoflowering strains, with the exception of some 4th generation monsters, make an exclusively HPS grow the simplest solution.

While the HPS lamp is usually reserved by photoperiod strain growers for the bloom stage due to the high level of orange/red light rather than blue/white, which is better suited for vegging plants the stockier autos won’t stretch to the same degree under an HPS.

Cool white CFL bulbs are great for getting autos started and nursing them through vegetative growth, but limited by the number of lumens they can effectively put down in the bloom spectrum.

The purple dual spectrum CFL bulbs can also fulfil this role. And if switching to more powerful LEDs for the bloom phase, the similar spectrum of light they emit makes for a smooth transition and a most eco-friendly grow.

Autoflowering cannabis strains can survive in as little as 6 hours of sunlight, but if it’s top-shelf buds in 60 days or less, that you desire, it’s best to ensure they thrive and get 18 hours of light - either naturally or artificially.

Steven Voser
Steven Voser

Steven is a long-time veteran of cannabis journalism, having delved into every aspect of the subject. His particular interests lie in cannabis culture, the emerging science of cannabis, and how it is shaping the legal landscape across the globe.