Understanding Cannabis Light Schedule for Seedlings

A proper understanding of cannabis light schedules and setups is one of the most important topics any aspiring home grower can learn about. Cannabis is, after all, a plant. So, if you decide to grow your at-home garden indoors, it will require specific amounts of light and dark that mimic the natural conditions outside. 

Each stage of your plant’s life cycle requires specific lighting. In fact, this process begins when your plants are mere seedlings. However, with a little research and practice, proper lighting becomes quite intuitive for most growers. Below, we cover how light cycles for growing weed work, and how to best nourish your plants at every step of the growing journey.

How Light Affects Cannabis Plants

Before we talk about the ideal seedling light schedule, vegetative light schedule, and beyond, let’s first address why lighting schedules are important in the first place.

At the most basic level, light is essential for photosynthesis. Photosynthesis is the process a plant uses to convert light into energy. In a nutshell, it allows plants to synthesize carbohydrates from carbon dioxide and water. This energy is then used to fuel the plant’s growth and development, resulting in the leaves and flowers we know and love.

Photoperiod Lighting

It’s also worth noting that some cannabis seeds are photoperiods, meaning they require a change in the amount of light they are exposed to each day to flower. Photoperiod strains such as Birthday Cake feminized seeds, Purple Kush feminized seeds, and many more, including some of the best hybrids are available at Growers Choice Seeds.

These photoperiod plants require proper weed light schedules to produce potent buds. In general, extended periods of light promote vegetative growth, while shorter periods of light promote flowering. Managing the duration of light exposure is essential for optimizing the growth phase and ensuring that plants flower at the appropriate time.

While light is important, so too is darkness. Strategically scheduled dark periods prompt a process referred to as “respiration.” Respiration allows carbohydrate growth in your garden by allowing your plants to break down stored carbohydrates. In turn, this provides the necessary energy for metabolic processes and enhances the overall growth and development of your cannabis plants. Darkness also allows your plants to rest. 

Light Schedule for Cannabis Seedlings and Clones

The light schedule for seedlings and clones can vary from 18 hours per day to 24 hours of light per day. For a short period of time (typically a few days), some growers choose to expose their seedlings or clones/cuttings to 24 hours of consistent lighting. This helps provide their sprouts ample energy. Since plants can photosynthesize around the clock, 24-hour lighting cycles tend to maximize growth.

After a period of constant lighting, most cultivators choose to do 18 hours of light followed by 6 hours of darkness. This schedule provides a long period of light each day to support photosynthesis and growth, while scheduled periods of darkness offer your crop a much-needed rest. This balance helps promote healthy development in young plants.

Adjusting light intensity and type to suit the delicate nature of seedlings is essential for preventing stress and promoting healthy early growth. As seedlings mature, they can gradually be acclimated to more intense light in preparation for the vegetative period.

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Light Schedule for Vegetative Stage (18/6)

The vegetative stage is when cannabis plants grow in size and develop a robust structure to support future buds. Throughout this phase, your plants will reach their natural size and shape. The vegetative stage also prepares your garden for flowering.

We recommend ensuring your plants are healthy, well-nourished, and properly pruned. This can significantly impact the overall yield and quality of your cannabis. Make sure to keep an eye on temperature, humidity, and ventilation, adjusting as needed to maintain the best-growing conditions for your special green sprouts.

Lighting Tips for the Vegetative Stage

To give your cannabis plants the light they need during this stage, you will need to expose them to 18 hours of light and six hours of uninterrupted darkness. This light cycle emulates the long days of summer that your plants would naturally be exposed to if grown outdoors. As your plants move into the vegetative stage, they can also handle more intense light. We recommend gradually increasing the intensity of your weed grow lights or lowering them closer to the plants over several days to avoid shocking them.

It is worth mentioning that some growers use a 24-hour lighting cycle instead of an 18:6 lighting cycle during the vegetative stage. Both options have unique advantages and disadvantages. Continuous light produces a constant state of photosynthesis; accordingly, plants may grow more quickly.

However, this type of lighting results in higher energy consumption and therefore, higher electricity costs. Additionally, your plants are not receiving any rest with 24 hours of light. This can potentially stress your garden. For this reason, many home growers opt for an 18:6 schedule. While this does allow your plants to rest, it may also result in slower growth.  

Light Schedule for Flowering Cannabis (18/6)

After a few weeks, your plants will have grown enough foliage to enter the flowering stage: the phase of growth where they start growing the potent buds the plant is known for. If you’re growing photoperiod seeds such as OG Kush, you’ll need to adjust light schedules to trigger flowering. But, if you go with any of our autoflowering options, your seeds will start to flower automatically—regardless of the number of daylight hours they’re exposed to.

Lighting Tips for the Flowering Stage

The flowering stage is going to require fewer daylight hours than other phases. Most cultivators tend to go with a 12/12 schedule (12 hours of daylight, 12 hours of darkness), as the extended exposure to darkness triggers the plants’ natural response to the changing seasons, indicating that it’s time to reproduce. Thus, your flowers are going to start to bud.

When you first switch to the 12/12 cannabis light schedule, the plants may take a week or two to show signs of flowering. This period is often called the “stretch phase.” During this period, plants may rapidly grow in height. You can support your plants through this transition by adjusting nutrient formulations and environmental conditions as needed.

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Cannabis Lighting Recommendations

The seedling stage can be tricky; thankfully, there are always things you can do to make your cultivation journey easier. Whether you opt for lower-maintenance autoflowering strains like Tangerine Dream or Northern Lights or prefer (non-autoflowering) feminized seeds Growers Choice Seeds is here to help both novice and experienced growers alike choose their optimal lighting setup.

Here are some of our favorite tips and tricks:

  • Save yourself time and energy by investing in autoflower strains. Strains like White Widow are known for being easy to grow, and you don’t have to worry about changing weed light cycles to trigger flowering.
  • Choose the right light type for the size and space of your grow set-up. Not all lights operate the same. LED cannabis grow lights, for instance, optimize cannabis production by improving light distribution and heat generation. 
  • Make sure to adjust your light distance to ensure it doesn’t burn your plants. While fluorescents can be placed close, LEDs need to be placed at a further distance.
  • Keep an eye out for stretching! If you notice your plants are growing in unusual ways, or stretching toward the light, this means they aren’t receiving enough light, and are compensating by trying to reach the light source themselves. This can be easily remedied by adjusting your light source. 
A cannabis plant with bright purple lighting
Learn to identify the signs of too much light 
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What is the optimal light schedule for cannabis seedlings? 

Ideally, cannabis seedlings should have between 18-24 hours of light exposure during their early stages of growth. Some growers opt for 24 hours of light, while others prefer to provide their seedlings time to rest with an 18:6 cycle.

The lighting should also be gentle: Some home cannabis growers prefer fluorescent lights, while others opt for LED lights.  Be sure to position the light source close enough to your seedlings to trigger photosynthesis—but not so close your seedlings overheat. 

Can seedlings and clones be grown under a 24-hour lighting schedule?

Seedlings and clones can be grown under 24-hour lighting. However, many growers prefer to allow their seedlings 6 hours of rest per day. 

Why is the 18/6 lighting schedule recommended for the vegetative stage? 

The 18:6 lighting schedule is recommended for the vegetative period because it mimics the light and dark patterns that occur during long, summer days. Were you to grow your cannabis plants outdoors, they would be exposed to similar ratios of light and dark. An 18:6 schedule allows plenty of time for photosynthesis, yet eliminates the potential stress caused by constant lighting. 

What is the significance of the 12/12 lighting schedule for flowering cannabis? 

Just as the 18:6 lighting schedule mirrors the natural light that would occur outside during long, summer days, the 12:12 lighting schedule signals to your plants that summer is ending. The days are becoming shorter, and therefore, it is time to enter the flowering stage! As mentioned, photoperiod strains will require a shift in lighting to flower—but auto-flowering strains will flower without this shift in lighting. 

How can LED lighting technology improve cannabis production?

LED stands for “light emitting diode.” LED lighting conditions can be helpful for cannabis plants as they are both energy-efficient but also create more usable light while simultaneously producing less heat. In this sense, LED lights are both cost-effective and gentler than traditional sources—as reduced heat helps avoid any potential stress on your marijuana plants. 

LEDs also offer growers the option of adjusting wavelength to cater to different growing stages. For example, blue wavelengths often promote the robust, dense growth that occurs throughout the vegetative phase while red wavelengths are ideal for bud and resin production throughout the flowering phase.

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