5 Lighting Factors To Consider When Growing Cannabis

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Added 16 June 2023

Light plays an essential role in the growth of all plants, and marijuana is no exception. 

Cannabis plants rely on light as a crucial power source for their photosynthesis. 

While plants growing outdoors get ample sunlight, you must supplement light indoors to ensure they grow big and produce good yields. 

If you’re struggling to get decent yields, you should perhaps check your grow light to see if it’s good enough to provide intense light for long durations. 

In addition, you have to keep an eye on the spectrum, distance, PPFD, and a lot more. 

Let’s explore all the lighting factors to consider when growing cannabis in this article. 

The Importance of Light for Cannabis

Light is the crucial component that enables plants to convert CO2 and water into glucose and oxygen via photosynthesis. Thus, without adequate light, cannabis plants can not generate enough energy to perform necessary metabolic processes, leading to stunted development, lower yields, and minimized strength.

Along with supplying energy, light additionally plays an essential role in helping the plants transform from the vegetative to the flowering stage. For instance, photoperiod plants will flower only when there’s a change in the light period. Indoor growers mimic outdoor seasons by using grow lights. They also manipulate the light cycle so the plants can flower and produce buds. 

As you can surmise, grow lights are incredibly important for growing cannabis plants indoors. However, with so many choices, choosing good grow lights can be a tad confusing. Here’s a brief summary of the many types of grow lights you can find today.

Various Types of Grow Lights for Cannabis Cultivation

You’ll find numerous kinds of grow lights in the market, each with advantages and disadvantages. Here are the types of grow lights you can use to grow cannabis plants indoors and get good yields. 

1. High-Intensity Discharge or HID Lights

High-Intensity Discharge or HID Lights

HID lights are popular among cannabis growers due to their high light output and tried-and-tested performance history. HIDs are available as MH and HPS lights.

  • MH Lights

Metal Halides or MH lights send out a bluish-white light spectrum that resembles all-natural daylight, making it excellent for the vegetative stage as it promotes lush green leaves and sturdy stems. In addition, plants develop less internodal space with MH lights. They also stop stretching too much with these lights. They produce high-intensity light that is perfect for cannabis during the vegetative stage. However, MH lights throw significant heat and draw a lot of power that will increase your electricity bill. 

  • High-Pressure Sodium (HPS) Lights

HPS lights produce a yellow-orange light spectrum appropriate for the flowering phase of cannabis. This range triggers hormonal reactions that promote plenty of buds, helping you get maximum returns. HPS lights are famous for their high power efficiency, and growers have been using them for decades. They also perform consistently. However, like MH lights, they produce a lot of heat and can be expensive in the long run (read, electricity bills).

2. Light-Emitting Diode or LED Lights

Light-Emitting Diode or LED Lights

LED lights have become a favorite among cannabis growers because of their convenience and effectiveness. The best part is that they produce light in wavelengths that encourage cannabis cultivation.

What makes LEDs better is that they draw less power compared to other types of grow lights. They also provide bright and intense light that the plants can use from the seedling to the flowering stage. This means that while MH and HPS lights are good for the vegetative and flowering stages, LEDs can be used throughout the plant’s lifecycle, making them very cost-effective. 

In addition, LED lights produce less heat than HID lights, minimizing the risk of heat stress. This also means that you may not have to use additional cooling equipment like air conditioners or coolers. 

Last but not least, LEDs can produce high yields, although the cheap ones may not be as powerful as HIDs. Another disadvantage is that you may have to splurge a bit more on LEDs initially. Still, they are cost-effective in the long run. 

3. Fluorescent Lights

Fluorescent Lights

Fluorescent lights, T5 fluorescent tubes, and CFLs are generally used by growers to tend to seedlings and clones. However, you can use them if you have just a few plants. Or you can purchase more T5 lights to encourage your plants to grow. 

T5 lights are available in 6400k (white-blue spectrum) and 2700k (yellow/white spectrum) that you can use during the vegetative and flowering phases. These lights are cheap and perfect for beginners who want to experiment with just a few plants. Moreover, they produce very little heat and encourage root growth. 

The primary disadvantage is that while fluorescent lights may be good enough during the early vegetative phases, they don’t meet the requirements of your cannabis plant as they grow huge. Plus, they have a short lifespan compared to other grow lights. 

4. Ceramic Metal Halide (CMH) Lights

Ceramic Metal Halide (CMH) Lights

Ceramic Metal Halide (CMH) lights incorporate metal halide and ceramic discharge innovations to use a full-spectrum light that appears like all-natural sunshine. CMH lights provide a well-balanced range that consists of both red, as well as blue wavelengths, making them ideal for the entire lifecycle. This balanced range adds to energetic development, raises resin production, and enhances plant health.

CMH lights are very effective, providing excellent light output while utilizing much less energy. They have a high Photosynthetic Photon Flux (PPF), enabling your plants to absorb all that light. With CMH lights, you’ll also notice that there’s tighter internodal spacing, increased branching, and a boost in overall production. 

Coming to the disadvantages, CMH Lights are expensive and cost double that of MH lights. In addition, they don’t last as long as LED lights. For instance, while most LED lights could go on for about 50,000 hours (many go on for more than 100,000), CMH lights start deteriorating after 20,000 hours. 

You can choose any of these grow lights to cultivate your cannabis plants. Also, consider whether you want to switch lights depending on the growing stage or if you want one light for the entire life cycle. Remember to check the power draw, as it could get very expensive eventually. You could also try experimenting with fluorescents before purchasing expensive ones.  

Understanding the capabilities and limitations of different lights will help you make educated choices and create the best environment for your plants, leading to healthy and bountiful yields of superior quality.

Now, let’s get to the key lighting factors you need to keep in mind when growing cannabis. 

Key Lighting Factors to Consider When Growing Cannabis

1. Light Intensity

Light Intensity

Light Intensity is critical when growing cannabis as it can significantly impact growth and yields. Plants that get sufficient intense light can perform photosynthesis effectively, meaning they can manufacture the sugars and carbs needed for plant development. In addition, you’ll notice that such plants will develop strong stems and dense, vibrant leaves.

The strength of your grow light influences chlorophyll manufacturing which is vital for plants. During the vegetative stage, it enables them to develop thick leaves and absorb light properly. And during the flowering stage, it helps the plants grow thick, resinous buds in good quantities. On the other hand, cheap lights with less intensity will cause the plants to stretch and also develop a lot of internodal spacing. 

One of the most important ways the light intensity helps is by penetrating the canopy. This way, all bud sites get adequate light, and you’ll be left with very few popcorn buds. Maintain light intensity throughout the lifecycle so your plants give you enough returns for your investment. 

Now, how do you know whether your grow lights are producing light with sufficient intensity? Well, you measure that through PPFD or Photosynthetic Photon Flux Density (PPFD), which measures the amount of photosynthetically active photons getting to a certain location. So this tells you how much light your plant is getting in specific areas. 

You can measure your grow light’s PPFD with a PAR (Photosynthetically Active Radiation) meter. Make sure you calibrate it before using it. 

Here are the ideal PPFD values for every growing stage of your cannabis plants:

  • Seedling Stage

Cannabis plants don’t need a lot of intense light during the seedling stage. Anything ranging between modest to strong is okay to promote leaf development. Typically, the preferred range of PPFD throughout this phase is around 100 to 300 µmol/m2.

  • Vegetative Stage

In the vegetative phase, your cannabis plants will develop faster, so they need lights with more intensity. The recommended PPFD ranges between 400 to 600 µmol/m2.

  • Flowering Stage

To develop quite a number of buds with thick resin, your plants will require greater light strength during the blooming stage. The recommended PPFD values should range from 600 to 1000 µmol/m2, with some growers going beyond this range for best outcomes.

Keep in mind that these are standard values, and they differ based on multiple factors, including the strain and your objectives. You can tweak these values according to the way your plants grow. 

2. Distance


Did you know that the distance between your plants and the light, also known as the “hang height,” plays a critical role in growing cannabis? It has an impact on the strength and coverage as well. For example, the farther the lights, the less intense the light is, and vice versa. However, this doesn’t mean that you keep your plants close to the lights as they will experience heat stress and burn. 

Read on to learn how distance affects plant development:

  • Light Intensity and Coverage

The ditance between the plants and the light will impact the light intensity or strength of the light. This means the plants closer to the light will experience faster and stronger growth as they get a lot of energy to perform photosynthesis. If the distance between the light and the plant is beyond the recommended range, the plants will get lanky, develop too much internodal space, and eventually become weak. The leaves may also wilt, and you’ll notice various other problems. 

  • Light Penetration

The distance between your plants and grow lights also influences light penetration. Cannabis plants typically grow in the direction of the light, meaning the leaves or buds at the top get more light than those at the bottom. Therefore, adjust the light according to plant growth so the light penetrates deep into the canopy. 

  • Heat Management

The distance is also closely related to heat. For instance, HID lights throw a lot of intense heat, and the plants that are too close to the light will struggle. To prevent heat stress and burn, make sure you adjust the distance constantly as the plant grows. 

  • Stage of Growth 

The distance between the lights and the plants depends on the stage of growth the plant is in. This means that you can experiment with the range as the plant grows and adjust it. For example, you can keep the lights slightly far away from seedlings. But don’t overdo it, as the seedlings may become weak and topple. The same applies to the early vegetative stage as well. However, the plants need high-intensity light during the flowering stage, so the lights need to be a tad closer compared to other stages of growth. 

As a grower, you need to hit the right balance between light intensity, distance, and distribution and also manage heat to encourage plants to grow stress-free. Make notes of the temperatures to adjust the lights. 

Here’s a table that tells you the recommended distance:

Type of Grow Light

Recommended Distance from Plants

LED Grow Lights

6-24 inches (15-60 cm)

Fluorescent Grow Lights (T5)

6-12 inches (15-30 cm)

CFL Grow Lights

12-24 inches (30-60 cm)

Ceramic Metal Halide (CMH) Lights

12-24 inches (30-60 cm)

High-Pressure Sodium (HPS) Lights

24-30 inches (60-90 cm)

Metal Halide (MH) Lights

18-30 inches (45-75 cm)

Take these values with a pinch of salt as it can depend on the power of your grow light and strain of the plants you're growing. For example, a 400w HPS light can be placed about 22-24 inches away, but a 1000w HPS light should be placed at least 30 inches away from your plants. 

3. Power of Grow Lights

Power of Grow Lights

The power of your grow lights, including the lumens they generate, are essential elements to think about when growing cannabis. Nevertheless, remember that depending only on lumens for evaluating the light output might not accurately depict their viability for growing cannabis. Lumens gauge the overall amount of noticeable light given off by a grow light; however, they do not represent the specific light range needed for photosynthesis.

Therefore, when choosing grow lights, think about the power of the lights in regard to their electrical power or usage rather than only thinking about lumens. Higher-wattage lights usually supply better light strength and coverage, which is critical for optimum plant development. 

Also, consider the light output in terms of the PPFD. We have already covered this so you know exactly how intense and powerful your lights will be. 

Now, let's talk about why the power of the light even matters when growing cannabis plants. The efficiency of grow lights can be measured in several ways depending on its type. While HPS lights are measured in lumens, you can measure the efficiency or brightness of your LED lights by considering the wattage.

For instance, your LED light with 150 Watts provides almost 400 to 700 µmol/m2 of PPFD, whereas an LED with higher wattage of about 800w can produce almost 1000 to 1600!

If you’ve ever grown cannabis, you already know the importance of lights with higher wattages. The higher the power, the better the energy output, and vice versa. This means that plants receiving light from grow lights of higher wattages are more likely to produce greater yields compared to small-wattage lights.

High-wattage lights also provide another important advantage concerning CO2 levels. Now you probably already know that the levels of CO2 in your grow room can have an impact on photosynthesis. If the levels of CO2 are very low, the plants may struggle to grow, and the same applies if the levels exceed 1800 ppm. 

You can use CO2 generators in your grow room and maintain it at 800 to 1200 PPM for greater yields. However, the environment in your grow room should support the addition of CO2, meaning the light intensity should be at least between 1000 to 1500µmol/m2 for it to work effectively. Since low-wattage lights cannot provide enough PPFD, adding CO2 will be meaningless. Thus, as you can see, there's an advantage to using high-wattage lights. Also, remember to adjust temperature levels between 75-80°F (24°C-27°C) to help the plants absorb CO2.

4. Light Spectrum

Light Spectrum

The light spectrum discharged by the grow lights has a role to play in cannabis cultivation as well. Various phases of plant development call for different light ranges for the best growth. 

Here’s the recommended spectrum for every stage of plant growth:

  • Vegetative Stage

For the seedling stage, a light of a greater percentage of blue light is useful. Blue light ranging between 400-500 nanometers (nm) is best to encourage dense foliage and solid stems. It also helps to prevent stretching. You can use either LEDs or T5 fluorescent lights (6500K) for this phase, as they tend to discharge bluish-white light that your seedlings will love.

You can use the same light spectrum for the vegetative as well. Make sure you get full-spectrum LEDs for the vegetative phase. T5 fluorescent lights (5000-6500K) can also help. Those that love MH lights can use them too. 

  • Blooming Stage

This is the phase when the plants will focus on producing buds. You should use lights with a higher percentage of the red spectrum ranging between 620-700 nm to boost the manufacturing of phytochromes which play a crucial function in blooming.

For this stage, you can use LEDs with high levels of the red spectrum. Many lights have options to choose between the vegetative and flowering phases, so you can use them. Or, you can go for HPS lights that are perfect for flowering. 

5. Light Cycle

Light Cycle

The number of hours of light the plants get is another factor that you need to consider when growing cannabis. Without this, your plants will fail to flower or revert to the vegattive stage if they are flowering already. 

  • Vegetative Stage

Throughout the vegetative phase, cannabis plants focus on general plant growth, including leaf and stem development. For this stage, we recommend providing 18 hrs of light and 6 hrs of continuous darkness (18/6). The 18/6 light cycle mimics the lengthy days of the summer when marijuana plants normally experience vegetative development outdoors. Some growers use 24 hours of light; however, like us, the plants also need to rest, and you must give them at least 6 hours. Moreover, letting your grow lights run continuously for 24 hours is not a good idea. 

  • Flowering Stage

The flowering stage is when cannabis plants transition from their vegetative growth to producing flowers or buds. To trigger this transition, you should change the light cycle and switch to a 12-hour light and 12-hour uninterrupted darkness cycle (12/12). This light cycle simulates the natural conditions the plants experience during autumn, when days become shorter.

The 12/12 light cycle induces the plants to shift their energy allocation toward flower production. It signals the onset of the flowering phase and encourages the development of buds. During this stage, it is crucial to maintain a consistent and uninterrupted 12-hour dark period, as any light interruption during the dark cycle can disrupt the flowering process and lead to hermaphroditism or reduced yields. Make sure your grow room is leak-proof, as the plants may revert back to their vegetative phase if there are any leaks. 

If you’re growing autoflowering strains, there’s no need to change the light cycle as they don’t depend on the light to flower. Thus, you can provide an 18/6 throughout their life cycle for the best results. 

Summary: 5 Lighting Factors To Consider When Growing Cannabis

In conclusion, understanding the importance of lighting factors is crucial for successful cannabis cultivation. Light is a vital element that cannabis plants utilize for growth and development through photosynthesis. Different types of grow lights are available for cultivating cannabis, including LED, fluorescent, and HID lights.

Light intensity is critical in cannabis growth, influencing factors such as photosynthesis, plant structure, chlorophyll production, bud development, light penetration, and good yields. Providing the appropriate light intensity ensures that plants receive sufficient energy to grow well. 

Don’t forget that the distance between the grow lights and the plants is also essential. Read the manufacturer’s guidelines to position the grow lights at an appropriate distance from your plants. Although you can place CFLs, fluorescents, and even LEDs a bit close, keep HPS and MH lights a little far away to prevent heat stress.

Regarding the light cycle, vegetative cannabis plants generally require 18 hours of light and 6 hours of uninterrupted darkness (18/6), while a 12/12 light cycle is recommended for the flowering stage. Autoflowering cannabis, on the other hand, can thrive with a consistent 18/6 light cycle throughout its entire life cycle.

Finally, coming to the light spectrum, blue light is beneficial for the seedling stage, promoting compact growth and preventing stretching. The same applies to the vegetative stage, as blue-white light supports leaf and stem development. In the flowering stage, an increased proportion of red light is crucial for promoting bud formation and resin production.

By considering these lighting factors, including light intensity, light cycle, the distance between lights and plants, and light spectrum, you can create optimal growing conditions for your plants at each stage of growth. 


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