Best Light Cycle for Cannabis Plants

You want to grow cannabis indoors, and you’re learning how to get the best lighting setup for your plant. And one of the questions that’s bound to pop up is about the light cycle. What’s the best light cycle for your cannabis plant? 

How many hours of light should your cannabis plants receive? And, what about the dark period? Well, growers use specific light cycle schedules, depending on the strains and the cultivation methods used. It also depends on whether you’re growing photoperiod or autoflowering strains. Most importantly, you must provide light according to its stage of growth. 

Chances are, you may not even know what a light cycle is. Light cycle refers to the number of hours of light and darkness you give to your cannabis plant. Let’s dive into it. 

Relation Between Cannabis Plants And Light Cycle 

THe relationship between cannabis plants and light isn't all that complex. It's just any other plant. However, when we consider light cycles for cannabis plants, it’s photoperiod plants that are the focus here. These plants rely on the light cycle to transition from vegetative to flowering stages — the change in the light cycle triggers the transition. 

But there’s more to it than that. For example, cannabis is a short-day plant. So, it does not need a lot of light hours; instead, it requires more hours of complete, uninterrupted darkness. When it gets this long dark cycle, the flowering stage is triggered. 

Essentially, when you change the light cycle to have longer hours of darkness, the plant sees it as a change in the weather or the coming of the winter. So, it starts blooming. It really does think, “Winter is coming.”

How does that work? Well, the plant isn’t sitting there counting the days. Instead, the change in the photoperiod period makes the plant produce hormones like cytokinins and gibberellins, which helps the plant move to the blooming stage. The same hormones also influence the production of buds along with terpenoid and cannabinoid synthesis. 

How Does the Light Cycle Help You as a Grower? 

On the other hand, maintaining a proper light cycle is important because if the darkness is interrupted by light, the plant can experience stress. This can confuse the plant and push it into re-vegging — it starts vegging again. Similarly, it can also stress out the plant and turn it into a hermie, where the buds start developing seeds. Both have a potential for disaster. 

As a grower, the light cycle comes in handy in three ways. One, with the right light cycle, you kickstart your plant’s life by helping the seeds germinate better. Now seeds don’t need light but a seedling only stretches if there’s light above it. 

Second, it helps you control the growth of your plant. Is the growing season shorter than expected? Switch your plant to bloom. Do you want your plant to grow bigger? Allow your plant to veg for a bit longer. 

Third, it helps you maintain your plant’s flowering stage so it can mature on time and be ready for harvest when you want it. Maintaining the right light cycle prevents issues like hermies and re-vegging, as mentioned earlier. 

What is the Best Light Cycle for Clones and Seedlings? 

Seeds don’t need light to germinate; they can do so in complete darkness, too. But you still need to use light for seedlings and clones. Here, generally, most growers prefer to use a light cycle of 18/6 — 18 hours of light and 6 hours of darkness. 

You can use this light cycle for the initial days of your plant’s growth although some growers prefer a 24/0 cycle with no dark period at all. The 24/0 cycle, as the name suggests, is about providing the plants with 24 hours of light and 0 hours of darkness. 

Typically, growers use the 24/0 cycle when the plants are in their seedling stage to enhance initial development and establish healthy root systems. Growers introduce seedings to light as soon as they germinate. At this stage, they are fragile and highly sensitive to everything around them. Thus, the idea behind subjecting the plants to continuous light exposure is that it may help stimulate root growth, promote vigorous foliage development, and establish a solid foundation when it’s so young.

The consensus varies, but most experienced growers argue that a 24/0 light cycle helps maintain consistent humidity and temperature for the seedlings and clones to grow well. Remember, they are quite fragile. 

Another reason a 24/0 light cycle comes in handy here is that the clones and seedlings don’t have a lot of stored nutrients or energy. So, they won’t have enough energy to power them through the night. 

What is the Best Light Cycle for Vegging Cannabis Plants?

This is where things get interesting and the light cycles start making a difference between a healthy plant and a dead one. This is the time when your cannabis plant puts on most of its mass in the form of leaves, stems, branches, etc. 

Here, the standard light schedule is 18/6 for vegetative plants. It gives enough light for your cannabis plant to thrive while ensuring it gets enough dark hours for other processes, like circadian rhythm and hormonal development. 

But that’s not the only way to do it. Some growers prefer giving their plants 20 hours of light to speed up their growth. On the other hand, some growers give longer nights to make the plant flower sooner although it may not produce large buds. It all depends on what your goals are. 

24/0 Light Cycle 

As mentioned earlier, some cannabis growers also use a 24/0 cycle on their vegging plants. Generally, 16 hours of light is more than enough, but by giving your plant 24 hours of light, you’re essentially forcing explosive growth of your plant. 

The more light your plant gets, the more energy it produces. 

The method is still up for debate, though. Many growers completely reject the idea of this whereas others swear by it. According to the former group, a 24/0 light cycle can make the plant prone to various problems and diseases. 

During the 24/0 cycle cannabis plants experience lighting resembling perpetual daylight. This ongoing light exposure supplies the plants with an energy source for photosynthesis and other important processes. You can use either CFLs or LEDs during the seedling stage. Whatever you choose, don’t use super intense lights. 

In addition, an uninterrupted light source boosts metabolism and nutrient absorption, leading to accelerated growth rates that eventually lead to more buds when the plant shifts to the flowering stage. Most importantly, this may help the plants develop extensive root systems that allow them to effectively absorb water and nutrients from the soil.

Some growers use the 24/0 cycle primarily to prevent the seedlings from stretching too much, but they stick to the 18/6 during the vegetative stage. This is because it has a few disadvantages. 

Firstly, when you provide 24 hours of light, you’re no longer mimicking the conditions outdoors. Plants don’t get 24 hours of sunlight outdoors so they may not perform as you expect even if you’re providing lots of light. Most growers go with an 18/6 light cycle even during the seedlings stage because the plants need to get some rest. 

Also, a 24/0 cycle can raise temperatures pretty quickly in the grow room as there’s no chance to cool down. This can cause heat stress and light burns if you’re not careful. Humidity is another factor you should pay attention to as it can influence the health of the seedling negatively. 

Plus, your cannabis plant can only carry out photosynthesis for a limited time during the day. So, even if you give your plant 24 hours of light, some growers argue that, after a point, the light is a waste. 

Therefore, it’s best to go with an 18/6 cycle to ensure that the plants are happy. Sure, 24 hours of light sounds awesome, but even your grow lights and other equipment cannot function for 24 hours every single day. 

But, still, if you want to go with a 24/0 cycle, you need to strike the perfect balance. Before we get into it, understand that this method may not always produce better results, and sometimes, it can also backfire. 

However, if you still want to use it, you should focus on DLI or daily light integral. It is the amount of light your plant needs in four hours. This is measured in PPFD or photosynthetic photon flux density — the amount of PAR light being absorbed by your plant. 

Ideally, you need to give your cannabis plant 40 moles of light per day. To make sure you reach this, you need to give your plant the following PPFD levels:

• 200 to 300 μmol/m2/s with an 18/6 light cycle for a seedling 
• 300 to 400 μmol/m2/s with an 18/6 light cycle but you can go a notch higher for a vegging plant 
• 1000 μmol/m2/s per day in a 12/12 light cycle for flowering stage 

You can measure the PPFD using a lux meter. There are many lumens to PPFD converters online that you can use. It’s not super accurate, but it works just fine for most growers. 

What is the Best Light Cycle for Cannabis Plants?

Growers provide 18 hours of light and 6 hours of darkness during the vegetative stage, and this light cycle is one of the most common ones used widely as it is very effective in encouraging the plant to grow strong leaves and stems. 

Essentially, an 18/6 cycle mimics the conditions for the plants as if they were growing outdoors. For instance, summer has long days and short nights, so with an 18/6 cycle, the plants are stimulated to develop lush leaves and strong stems. 

As they have 18 hours of light that’s quite ample to conduct photosynthesis, the plants produce all the energy needed to grow and also take care of their cellular processes.

Also, plants focus on the leaves and stems more during the vegetative stage, so you must go with an 18/6 during the growing phase. At this point, the plants will expand their canopy, and strengthen their structure and root system. So, you can use grow lights like HPS, LEDs, Metal Halides, and CFLs to help the plant conduct photosynthesis and grow rapidly. 

Despite the benefits, you need to keep a few things in mind for implementation. Remember that the right amount and type of light are crucial for maximizing the benefits of an 18-hour light cycle. Meaning, you should ensure that you have the right grow light that produces intense light in the right spectrum for the plants to grow. Lights with a blue spectrum are best for plants during the 18/6 or vegetative stage. 

What is the Best Light Cycle for Flowering Cannabis Plants?

Once your cannabis plant has reached the desired height during the vegetative stage, it’s time to move the plant to the flowering stage. You can do so by switching the light cycle to 12/12, which will trigger the plant to bloom. 

Some growers also use a 10/14 light cycle, but it may not work for everyone. This light cycle makes the buds smaller and slower to mature for harvest. 

The 12/12 light cycle plays an important role in growing cannabis as growers implement it when they want their plants to start flowering. As you can already guess by now, it involves giving cannabis plants 12 hours of light followed by 12 hours of darkness without interruption. 

During the 12/12 light cycle, cannabis plants go through uniform periods of light and darkness mimicking the natural changes in daylight that signal the start of autumn. This change in lighting triggers some hormonal changes that stimulate the plant to start flowering. 

Thus, when a grower wants his photoperiod plants to start flowering, he shifts from an 18/6 or 24/0 cycle to 12/12 to help the plant start flowering. This is when the plant's focus shifts from growing leaves to producing buds and flowers. By providing 12 hours of darkness, you’re essentially prompting the plants to channel energy toward flower development rather than more growth. This lighting regimen is crucial for encouraging buds with high levels of cannabinoids and terpenes. Most growers use timers to implement this effectively. 

However, to maximize the benefits of the 12/12 light cycle, you should consider a few things. First, as always, ensure a 12-hour dark period without any interruptions to prevent disruptions in the flowering process. Even brief exposure to light during this phase can impede flowering and bud development. 

In fact, some strains may even start reverting back to their vegetative stage! This is known as re-vegging and the plant may even display abnormal growth patterns. Other strains may also start producing seeds as they become hermaphrodites due to all that stress. Ensure that you are very particular about the light cycle during this time as it dictates how well your plant grows. 

Another factor to keep in mind is the spectrum. Flowering plants will appreciate the red spectrum compared to the blue spectrum during the growing stage. So, you can either get a light that allows you to switch from blue to red or a full-spectrum grow light that allows you to grow the plant from start to finish. 

Finally, it is crucial to maintain temperature, humidity, and proper airflow during the flowering phase to improve bud quality and prevent issues like mold or nutrient deficiencies.

What is the Best Light Cycle for Autoflowering Cannabis Plants?

Autoflowering cannabis strains have transformed cultivation practices as they are super easy to grow. Unlike photoperiod strains, autoflowering varieties do not rely on changes in cycles to initiate flowering. Instead, they follow a predetermined schedule and bloom automatically after several weeks regardless of light conditions. Meaning, they start flowering once they attain a certain age. 

Autoflowering cannabis strains stand out due to their genetic composition consisting of ruderalis genetics. As they possess a trait that allows them to flower based on their age rather than depending on the light cycle, they grow and flower very quickly, so you can typically harvest them faster than photoperiod plants. In short, autos start flowering based on their internal genetic clock, not the light cycle. So, you can essentially use the same light cycle throughout its growth cycle. 

Most autoflowers need only 8 to 12 weeks from seed to harvest. The best part of growing autoflowers is that they grow rapidly and allow you to harvest them multiple times per year. Whether indoors or outdoors, they are perfect to set up a perpetual harvest so you don’t need to worry about being short on your favorite supply. 

These plants are less affected by light cycles, so you can explore various cycles to give them what they want schedules without compromising plant health or yield potential. However, you should always maintain the right environmental conditions such as temperature, humidity, and airflow even when you’re growing autos. 

If you want to be on the safe side, use an 18/6 light cycle for indoor autos — the plant stays healthy and this is the ideal light cycle for novice growers. 

On the other hand, you can use 20/4 or 24/0 light cycles for more explosive growth of your auto cannabis. But be careful with these as you’ll have to tweak other things in your growing room to keep everything stable for your plant. It all depends on your preferences, environmental conditions, and the specific traits of the strain you’re growing. Most growers go with an 18/6 or 20/4 light cycle from the seedling stage until harvest. In essence, stick with whatever light cycle works for you. The only exception is when you are growing autos and photoperiods together. In such cases, stick to 12/12 during bloom. 

Here’s a quick overview of how you grow autos with an 18/6 or 20/4 light cycle:

• Seedling Stage (Week 1 to 2)

Start by germinating the seeds using your preferred method. Once the seeds have sprouted, transfer them to a growing medium like soil or coco coir in pots or seedling trays. Provide lighting for 18 to 20 hours daily while maintaining moderate humidity levels. Ensure ample airflow to support healthy seedling growth.

• Vegetative Stage (Week 3 to 5)

If needed, move the seedlings into containers to accommodate their expanding root systems. Generally, it’s best to grow autoflowers in one container rather than transplanting them as they don’t have too much time to recover from transplant shock. Continue with 18 to 20 hours of light at this stage to make sure the plants grow well in their vegetative phase and produce lots of leaves and stems. 

• Flowering Stage (Week 5 to harvest)

You can maintain an 18/6 even at this stage for autoflowers. Some growers use the 12/12 cycle but an 18/6 is best at any stage for autoflowers. This will help them develop big buds with lots of resin and cannabinoids. 

Summary: Best Light Cycle for Cannabis Plants

To summarize everything, the best light cycle for a cannabis plant is 24/0 for seedlings, 18/6 for vegging plants, and 12/12 for flowering plants. 

The light cycle for your cannabis plant is super important but it is only a part of the picture. Without the right kind of grow lights, correct intensity, and spectrum, even the best light cycle will fail to sustain our cannabis plant. 

So, take this as a stepping stone to getting the right light setup for your plant. Follow us for more advice on getting your light setup right for your cannabis plant, so you can enjoy a hefty yield by the end of the harvest.