Best Light Schedule For Autoflowering Cannabis

Autoflower varieties are unique, thanks to the exclusive trait allowing these plants to flower under different light cycles. With a handful of autoflower lighting schedules to choose from, curious growers want to know, what is the best light cycle for autoflowers? Light drives growth, but is there such a thing as too much light, do plants need to sleep? What about the light spectrum for autoflowers, does that make a difference? Fortunately, we have the information you need to make an informed decision on your autoflower light cycle.

What’s the best light schedule for autoflowers?

Although there are a number of possible light schedules for your autoflowers, there are four commonly used light cycles for autoflowering strains.

24/0 light schedule for autoflowers

At face value, it is easy to assume this would be the best lighting schedule for your autoflowering strains. Plants need light to perform photosynthesis, which drives growth. However, there is such a thing as too much light. Cannabis plants exhibit this with droopy or tired-looking leaves. The appearance is similar to a thirsty plant. 

If the light intensity is too high and the light is positioned too close to the canopy, this combination can create leaf burn and discoloration. A 24/0 light schedule for autoflowers is not widely used. The largest benefit it offers is to heat the grow room if you are growing in colder climates.

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20/4 light schedule for autoflowers

This is a popular autoflower light schedule for growers. This schedule provides lots of light to fuel the creation of sugars and carbohydrates via photosynthesis, but it also provides the plant with a rest period. During the dark hours, many beneficial phytochemical reactions occur in the plant (2). 

Some autoflower growers will switch from an 18/6 light schedule to a 20/4 lighting schedule for their autoflowers once the plant begins to show signs of flowering. Regular and autoflower varieties benefit from a higher light intensity as they progress to the flowering stage. 

18/6 light schedule for autoflowers

An 18/6 lighting schedule for autoflowers is widely used by autoflower growers. It has the benefit of encouraging healthy growth, which includes a rest and recovery period during the dark cycle. This is also a common light cycle for photoperiod plants in the vegetative stage, meaning your autoflowers can be flowering in the same tent, maximizing a grower’s footprint.

Over the course of the average autoflowers lifecycle (90 days), an 18/6 lighting schedule is best for your autoflowers can save 180 hours of electricity compared to a 20/4 light cycle. This schedule can also reduce the heat needed to be managed by other equipment in the grow room. 

If you want to know the best light schedule for all types of marijuana plants, check out our guide on: What is the best light cycle for weed plants?

12/12 light schedule for autoflowers

A common yield-limiting factor in the indoor garden is the lack of lighting. Lower-powered lights or shorter lighting durations will produce lower yields. For this reason, many autoflower growers stay away from this lighting schedule. If a grower has a very high-intensity light, this can still be a viable option.

An autoflower plant will continue to flower using a 12/12 lighting schedule, which means it can be grown alongside flowering photoperiod plants. This flexibility gives growers with only one flowering area the ability to grow a variety of cannabis types. For growers trying to reduce heat in the grow room, 12 hours of light daily produces less heat to mitigate. Although not a common lighting schedule for autoflowers, it has its place.

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What light spectrum should I use for autoflowers?

Determined by wavelength and energy, there are seven wavelengths of color in the visible spectrum (3). These wavelengths are measured in nm (nanometers). Plants react in predictable patterns when grown under specific wavelengths of light. This knowledge can be used to select the best light spectrum to grow autoflowers, depending upon the growth stage. 

6500k blue light during the vegetative stage

The K at the end of 6500k stands for Kelvin. Kelvin temperature identifies the perceived color emitted by the light source. Compact fluorescent lamp (CFL) bulbs are commonly labeled in Kelvins ranging from 1000k to 8000k. The higher the number, the bluer the light will appear. Kelvins are not the same unit of measurement as nm, which measures the wavelengths of light.

Blue light sits in the wavelength range of 400-500 nm. Blue light regulates the opening of the stomata, which control transpiration and uptake of CO2 (4). This spectrum is also used in the photosynthesis process, which drives growth. Autoflowers grown under this spectrum generally have thicker stems and shorter intermodal spacing. Blue light has been shown to suppress extension growth. Ceramic Metal-Halide light (CMH) bulbs are known for their characteristic blue color output, making them a popular choice for the vegetative stage.

2700k red light during flower

This Kelvin range appears to the eye as a yellow, orange, reddish combination often described as warm lighting. High-pressure sodium (HPS) bulbs are commonly known to be in this range, giving them a distinct color signature. The visible spectrum for red light sits in the 620-700 nm wavelength range. Red light has been shown to increase flower production but can cause stem elongation. The red spectrum is also the most efficient group of wavelengths for powering photosynthesis.

Although these two colors are highly beneficial to the growth of autoflowers, growers will not achieve optimal results with red and blue lighting exclusively. The best light spectrum for autoflowers includes a wider range of wavelengths, often referred to as full spectrum lighting. 

Common questions about light and autoflowers

Do I even need to change the light schedule for autoflowers at all?

No, you do not need to change your lighting schedule to trigger flowering in autoflower varieties. Autoflowers have an internal clock that will initiate these changes. If growers want to switch their lighting schedule while growing, this will not negatively impact the plant.

Do Autoflowers need darkness? And Why?

Autoflowering plants do prefer a period of darkness. It is completely possible to grow an autoflower on a 24/0 light schedule, but the results will be less than optimal. Plants undergo chemical processes during the dark cycle, which contribute to the health, vigor, and pest resistance of the plant. By giving autoflowers at least a few hours of rest each day, growers can increase their yields.

What are the total light hours recommended for autoflower seeds?

This is the most pertinent question when determining the absolutely best lighting schedule for autoflowers. To answer this question most effectively, growers must know their DLI (daily light integral). DLI measures the total amount of Photosynthetically active radiation (PAR) received in a day as calculated by intensity and duration that arrives at the canopy. This calculation is also known as Photosynthetic photon flux density (PPFD). 

Cannabis plants will perform best under varying DLIs during different stages of growth. Seedlings need less light and, therefore, less DLI. As the cannabis plant grows, the DLI requirement will increase. Flowering plants produce best with a 40-50 DLI and roughly 25% lower for the vegetative stage. Whether growing autoflowers or photoperiods, the DLI numbers remain constant. The variable that changes is duration. 

This chart represents the level of PPFD needed to hit optimal growth potential under the different light schedules.

Lighting manufacturers will commonly publish PPFD charts for their lighting online. To find the best lighting schedule for autoflowers under your own light, find the PPFD of your light. The recommended hours will be directly related to the intensity and quality of the light source. Once you know your PPFD, use the chart above to set your autoflower lighting schedule appropriately. Knowing that you can calculate the total light hours for your autoflower seeds.

How far should an autoflower be from the light?

There are many variables to consider, but a constant principle is that the light distance from the canopy will change depending on the age of your autoflower. Lights will be placed furthest away during the seedling stage. Flowering is the stage when growers keep the light closest to the canopy. 

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Lights placed close to the canopy will have a higher PPFD in the center of the canopy, but the PPFD will be far less in the corners. Raising the light will provide a more even distribution of PPFD, but those numbers will be lower. It is important to know that light follows the inverse square law in regard to distance (6).

**Inverse Square Law relates to the intensity of the light on an object at different distances from the light source. 

A light source at a distance of one meter from a floor is lighting one square meter (1×1), which is receiving 100% light. At a distance of two meters from the floor, the light now covers four square meters (2×2), which is receiving 1/4 of the light than it received at one meter away (1 m2 vs 4 m2). At a distance of three meters from the floor, light is now covering nine square meters (3×3), and receiving only a ninth of the light compared to one meter away (1 m2 vs 9 m2). 

As you can see, light intensity rapidly falls off the further from an object the light source is. It doesn’t follow a linear reduction. Instead of decreasing in equal increments, each interval is decreased by a factor of the base.** 

Life stage, intensity, and distance are important things to consider. Many lighting manufacturers will provide recommendations on their websites. The chart below shows common hanging heights for commonly used lighting sources.

What are the best lighting options for growing autoflowering cannabis indoors?

LED

Over the past ten years, lighting companies have poured millions of dollars into the research and development of this lighting sector, primarily fueled by demand from indoor gardeners. As the discoveries were made and the implementation refined, LED now stands as the top lighting choice for growing autoflowers. 

The efficiency per watt has surpassed HID/HPS systems. Color spectrums can be precisely tuned and, in some cases, adjustable by the grower. LED lighting converts more energy into usable light. It also does so with less heat signature aimed at the canopy. The lower cost of entry and savings on energy bills are all reasons why growers are switching to LED.

High-intensity discharge lamps (HID lamps) – MH/HPS

HID systems were the powerhouses of indoor gardening. They have been the go-to lighting solution for gardens growing more than a few plants at a time. The cost of entry is low, which makes them more accessible. Autoflowers will grow under both metal-halide (MH) and HPS exclusively, but it is preferred to switch bulbs depending on the stage of growth. 

MH is preferred for the vegetative growth stage of autoflowers due to the boost in the blue spectrum. Autoflowers prefer a bump of red spectrum lighting during flowering and the HPS bulb suits that need perfectly. Growers in cold regions can take advantage of the high heat output from HID systems to assist in heating the grow space during winter months.

CFL

This type of lighting has the lowest cost of entry, although you will need several to successfully grow a meaningful harvest. CFL bulbs come in various shapes, colors, and power outputs. Growers can find the right CFL bulb to grow autoflower plants based on their needs. Because CFL bulbs have a lower heat signature than HID or LED systems, CFL lighting can be run closer to the canopy. This is an advantage for growers with low ceiling clearance.

Conclusion / Summary

The best lighting schedule for autoflowering plants will be custom to each grower, but the requirements of the plants remain the same. Using the information and charts provided in this article, growers can confidently select the best lighting schedule and hanging height for their autoflowers. 

Lighting, or lack thereof, is one of the largest yield-limiting factors. Use this information to grow the best autoflowers on the block. Be sure to check out more of our insider tips by downloading the ILGM Grow Bible. Please let us know what you are using to illuminate your autoflowers and how it stacks up on the PPFD chart in the comments below.

Additional FAQs

Is It okay to change the lighting cycle during the grow?

Yes, unlike photoperiod plants, the growth cycle of autoflowers is not tied to light hours. The majority of autoflower growers will pick a lighting schedule and stick to it. Some growers will increase the lighting hours during the flowering stage to increase DLI.

How do I know if my autoflower is getting too much light?

The most conclusive way to know if your autoflower is getting too much light or not is to calculate the DLI. There is such a thing as too much light, and this will result in diminishing returns. There is a point where the amount of light can damage the plant.

What are the first signs of stress?

Commonly when autoflowers receive too much light, there is also a heat buildup at the canopy level. Too much heat will result in the leaves folding upwards like a taco. The more severe the taco, the likelier there is a heat issue. Autoflowers also tend to transpire more to deal with extra heat, and this can be manifested by the very edges of the serrated leaf portions curling upwards. 

Some LED lights can put out very intense light without much additional heat. The most obvious sign the light is too intense will be manifested by the bleaching of bud tips and occasionally on the foliage. In most cases, the foliage will show nutrient deficiencies or toxicities first. The extra light and heat create rapid transpiration and root uptake, which can deplete the nutrient reserve in the soil. This reaction can also draw too many nutrients into the tissue as the plant seeks to pull in more water.

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