Illuminating Auto-Flowering Cannabis: A Guide to Optimal Light Conditions

Many growers ignore the importance of light when growing autoflowers simply because it doesn’t depend on it to start flowering. However, light is equally important for autoflowers too. 

If you’re a beginner, know that autoflowers are cannabis strains that are produced by crossing ruderalis strains with regular sativa or indica varieties. Unlike photoperiods that depend on the light cycle to start flowering, autoflowers have no such demands. They flower automatically, according to their internal clock — hence the name auto. 

When you’re growing auto-flowering cannabis, getting the light just right is a balancing act. Light plays a role in shaping the growth, development, and ultimate yield of these plants. In this guide, we will delve into the importance of light in the world of autoflowering cannabis cultivation. 

However, it will take hours if not days to discuss everything about light. Thus, we will specifically explore three factors: choosing the right grow lights that act as a source of light for indoor cultivation, Daily Light Integral (DLI) which is an often underestimated metric that quantifies total light exposure critical for plant well-being, and lastly Photosynthetic Photon Flux Density (PPFD) which measures light intensity reaching plants and holds the key to achieving optimal growth.

Let’s get to it!

The Significance of Light for Growing Autoflowers

Understanding how light influences plant physiology will help you grow cannabis better, including autoflowers. Although autoflowers don’t depend on light to flower, they need all the light they can get to produce good yields. 

Now, if you’re growing outdoors, you don’t have anything to worry about; however, if you’re growing indoors, choosing good grow lights is the first step that determines whether you’ll have bad or good yields. 

Let’s take a brief look at the types of grow lights available to cultivate autoflowers indoors. 

Types of Grow Lights To Grow Autoflowers

• LED Grow Lights

LED grow lights, especially modern ones, have become very popular for cultivating auto-flowering cannabis simply because they deliver good results. These lights are energy efficient, emit less heat, and offer a customizable light spectrum that mimics sunlight. 

Moreover, LED lights last for a long time, often lasting more than 100,000 hours, thereby reducing the need for replacements and minimizing environmental impact. However, you should know that high-quality LEDs are expensive. Sure, there are thousands of cheap ones, but they often don’t produce the results advertised. Additionally, understand the light spectrum requirements for each growth stage and purchase appropriate lights. 

• Fluorescent Grow Lights

Fluorescent grow lights, T5, and CFL (Compact Fluorescent Lamp) bulbs are suitable for most setups. They are also user-friendly, which is why beginners prefer them. If you want to try growing just a few plants before switching to expensive lights, CFLs are perfect. 

Not only are they energy efficient, but they also provide the right spectrum that accommodates both the vegetative and flowering stages. While the cool daylight bulbs are good for the vegetative stage, go for warm yellow when you switch the plants to the flowering phase. 

However, one disadvantage is that they don’t produce yields like LEDs or HPS lights. So, once you decide to go for more powerful lights, you can use these lights as side lighting to provide more light to your autoflowers. You can also use fluorescent lights, T5, and CFLs for clones and seedlings as the light they produce isn’t very intense. 

• HID (High Intensity Discharge) Grow Lights

Growers have used HID grow lights, including Metal Halide (MH) and High Pressure Sodium (HPS) lamps for decades to grow cannabis. HID lights produce intense, powerful light that’s simply perfect for growing cannabis. While you can use MH Lights for the vegetative stage due to their blue spectrum, HPS Lights with their red spectrum are perfect for the flowering phase. They are unbeatable in terms of yield; however, modern LEDs are almost as good as HID Lights now. 

One major disadvantage of using HID lights is that they generate ample heat and consume too much energy. Although autoflowers grow and flower quickly in a matter of weeks, the heat can be unbearable for them if you don’t take care of the ventilation and temperature. 

• CMH (Ceramic Metal Halide) Grow Lights

Ceramic Metal Halide (CMH) grow lights combine the advantages of many modern grow lights, including HIDs, making them great for cultivating auto-flowers. They provide the right spectrum, including UV and IR wavelengths, which contribute to the plant’s well-being. 

Not only are they more energy efficient compared to HID lights, but they also generate very little heat, which is going to be easier on your wallet. Remember, the more power a light consumes, the more the bills! CMH lights produce light that closely resembles sunlight, making them suitable throughout the life cycle of auto-flowering cannabis plants. Their efficiency, spectrum coverage, and reduced heat emission make them an appealing option for growers who aim to strike a balance between performance and energy conservation. In addition, many growers state that the yields produced by a CMH light are as good as HPS lights. 

You can choose any of the grow lights to grow autoflowers indoors. Make sure you consider many factors including your budget, space, and the effort needed to set up everything. 

How Much Light Do Autoflowers Need?

Autoflowers need as much light as photoperiods. They will grow healthy and produce ample buds that are dense and big if you provide sufficient light. Typically, a grower starts with 18 hours of light during the vegetative phase. For photoperiods, he shifts to 12 hours once he’s satisfied with the growth. However, you don’t need to do this for autoflowers. 

This means that you can continue to provide 18 hours of light even during the flowering stage. Some growers like starting with 20 hours of light during the vegetative stage and continuing with the same even for the flowering stage. The remaining 4 hours allow the plants to get some rest. However, this is optional as the plants will do well even with just 18 hours. 

A small section of growers take it up a notch and go for 24 hours of light and 0 hours of darkness. But, this is not recommended as the plants will not be able to grow well if they don’t have some time to recoup. 

Photosynthetically Active Radiation (PAR)

PAR is known as the visible light or light perceived by plants. In other words, it’s nothing but light wavelengths within the 400-700nm visible light that helps with the process of photosynthesis in a plant.

When above 700, it’s known as IR or infrared. Conversely, when it’s below 400, it’s UV or ultraviolet. Growers always look for ways to increase PAR while growing cannabis. Most grow light manufacturers state that a PAR intensity of 800µmol/m²/s hits the sweet spot for great yields. Sure, plants can even use about 1500µmol/m²/s; however, you’ll need to use CO2. 

For autoflowers, the requirement is slightly different. Since you’re not going to be reducing the light cycle from 18 hours to 12 hours to let them flower, the PAR intensity can be lower. Autos typically need around 20 hours to grow to their optimum potential and they still have 4 hours to rejuvenate.

Since they have 20 hours unlike the 12 hours photoperiod plants get, a PAR intensity of just 550 µmol/m²/s is good enough for them to flourish. This will also hit the recommended 40 DLI (explained below) for them. 

Photosynthetic Photon Flux Density (PPFD)

PPFD (Photosynthetic Photon Flux Density) simply measures the intensity of PAR at a specific point. Or, the light that’s arriving at the plant at one point. Since different parts of the plant receive different levels of light at the same time, PPFD can be handy in identifying whether the levels are optimal for photosynthesis. Also, the light is strongest at the center and depletes as you move away. You can observe variations in the intensity of light throughout the plant's canopy and understand how much energy each part of the plant is receiving.

Unlike photoperiod plants, autos don’t need high levels of PPFD as they get about 20 hours of daylight, or artificial light, in this case. If you think that your plants will love more light and install extremely powerful lights, the DLI gets too high, and the plants will not like it. Thus, aim for about 700 µmol/m²/s when the plants are in their flowering phase. This level is good enough to stimulate the plant’s natural pigments called phytochromes. If the plant is getting only 18 hours of light you can aim for a range between 500 to 700 µmol/m²/s. 

In the seedling stage, keep the PPFD at about 100-200µmol/m²/s, and 200-500µmol/m²/s will be good for the vegetative stage. That said, every cannabis strain is unique and while one strain may not appreciate extreme light intensity, another strain might love it. So, if your plants are doing well even with high-intensity light, you can experiment a bit by increasing the PPFD. However, note that you will have to add CO2 to ensure that the plants use all that light. 

Daily Light Integral (DLI) 

Daily Light Integral (DLI) measures the photosynthetically active radiation (PAR) that falls on a specific region within 24 hours. In short, DLI is an indicator that tells you how much light the plant is exposed to in its daily cycle. Think of it as the light the plants can use per day. 

As plants receive the light and use it for photosynthesis, it promotes the growth of vibrant foliage, creating a foundation for the entire plant. It also becomes better equipped to defend itself against diseases, pests, and environmental stressors. 

The role of Daylight Integral (DLI) becomes more important as autoflowering cannabis enters its flowering stage. This is when the plant channels its energy into producing flowers and ultimately buds. The quantity and quality of light measured by DLI directly influence the size, density, and resin content of these buds. Insufficient DLI during this stage can result in less bud development and even affect the entire harvest.

Insufficient DLI can have negative effects on autoflowering cannabis plants. For instance, your plant can experience delayed flowering and the size of the buds could be small too. This is one of the reasons why you see so many airy thin buds. 

Therefore, DLI is very important as it regulates the growth and flowering processes of your autoflowering cannabis. At its core lies photosynthesis, which is responsible for powering the plant's metabolic functions. It’s the fuel for energy production that influences the plant’s health and yields. So, providing your autoflowers with appropriate DLI should be your primary goal. 

Typically, you will vary DLI based on the stage of growth. Cannabis plants need a lot of sunlight to grow well, and this applies even indoors, so they will appreciate good, intense light. Of course, you must also consider various factors such as the distance from the grow light, light output, and the number of hours the light is on to provide the perfect environment. 

In addition, seedlings will not require too much light. For example, the plants will grow well with just 12-16 DLI at this stage. Even in the early stages of the vegetative phase, too much intense light will be detrimental to its progress. This means that you will gradually increase the DLI from 16 to 55 (peak stage) according to the stage of growth. 

It’s good to maintain an average of about 45 DLI. The plants will grow well even if you go above this and reach 55, but you may see signs of some light burn and if you don’t have proper ventilation, there will be issues. 

For photoperiods, the DLI reduces as you need to reduce the number of light hours. However, for autos, you can continue with the same DLI from the vegetative stage until you harvest it. Meaning, a 45 DLI will work best. 

By keeping an eye on these levels, you can easily ensure that your plants get all the light they need. 

Now, how do you measure PAR, PPFD, and DLI? Well, you could get a PAR meter. But, they are expensive, ranging from $200 to even $700 for sophisticated ones. Since you’re a home grower, you don’t need all that. Instead, get a cheap LUX meter to figure out the best distance to position the lights, or use apps that calculate PAR, PPFD, and DLI. A quick online search will provide many apps that help you get readings in minutes. 

Summary: Illuminating Auto-Flowering Cannabis: A Guide to Optimal Light Conditions

Do not underestimate the importance of lighting just because you’re growing autoflowers. They too need as much light as photoperiod plants. The only difference is that autos get more light as daylight even during the flowering stage. 

When it comes to lighting, you should consider the PAR, PPFD, and DLI to ensure the plants get as much light as possible. The combination of these factors is crucial to cultivate cannabis. Grow lights provide the light for photosynthesis and growth, whereas DLI measures the exposure required to maintain plant health. Then, you have PPFD which evaluates the intensity of light reaching the plants at a specific point. 

With these three factors in mind, it’s pretty easy to provide the right amount of light to your autoflowers, which ensures that they produce top-quality buds.