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PAR Level Meaning: The Key to Optimal Cannabis Growth Lighting

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The intricacies of plant lighting, especially for cannabis cultivation, begin with a grasp of the Daily Light Integral (DLI) and its pivotal role. DLI, a crucial measure in plant lighting, quantifies the total amount of photosynthetically active radiation (PAR) that a plant receives over a 24-hour period, highlighting the critical nature of PAR in the photosynthesis process. This understanding sets the stage for exploring PAR level meaning, which refers to the spectrum of light wavelengths that plants utilize for photosynthesis, measured in Photosynthetic Photon Flux Density (PPFD). Identifying the optimal PAR levels becomes a key factor in maximizing cannabis growth and efficiency.

The journey to optimal cannabis growth involves not just understanding what PAR levels are but also, comprehending how different light sources compare and how one can maximize PAR efficiency for healthier plants. As we delve deeper, topics such as the significance of the light spectrum and how to monitor and adjust PAR levels to meet the specific needs of cannabis plants will be thoroughly examined. With terms like ‘par level calculation,’ ‘par value formula,’ and ‘par level meaning’ central to mastering indoor horticulture, this article aims to provide a comprehensive roadmap.

What is Photosynthetically Active Radiation? PAR level Meaning

Photosynthetically Active Radiation (PAR) refers to the range of light wavelengths that plants use for photosynthesis, converting light into chemical energy to fuel growth. This spectrum spans 400-700 nanometers (nm), covering blue, green, and red light. Just like humans need a balanced diet, plants require the right mix of light within the PAR range to flourish.

Key Metrics in PAR Measurement

  1. Photosynthetically Active Radiation (PAR): This refers to the spectrum of light, ranging from 400 to 700 nanometers, which is crucial for photosynthesis 68910.
  2. Photosynthetic Photon Flux (PPF): Measures the total amount of light produced by a light source per second, expressed in micromoles per second (µmol/s) 46.
  3. Photosynthetic Photon Flux Density (PPFD): Indicates how many photons from the PAR spectrum hit a specific area per second, measured in micromoles per square meter per second (µmol/m²/s) 610.
  4. PAR Watts: Quantifies the energy available from light for photosynthesis, emphasizing the efficiency of light sources 5.
  5. Daily Light Integral (DLI): Represents the total amount of PAR received each day per square meter, crucial for understanding light exposure over time 10.

Why is PAR important for plant growth?

PAR is critical for plant growth as it directly influences the efficiency of photosynthesis. Different wavelengths within the PAR range have distinct effects on plant growth and development. For example, blue light (400-500 nm) promotes vegetative growth, while red light (600-700 nm) encourages flowering and fruiting. A well-balanced PAR spectrum ensures that plants receive the right amount of energy at each growth stage.

Importance of Specific Wavelengths

  • Chlorophyll Absorption: Chlorophyll-a and Chlorophyll-b absorb light most effectively at 430nm, 662nm, and 453nm, 642nm respectively, critical for the light-dependent reactions of photosynthesis 5.
  • Carotenoids Role: These pigments absorb light at 449nm and 475nm, playing a protective role and assisting in light absorption 5.

Practical Application in Cannabis Cultivation

  • Measuring Tools: Utilizing PAR meters or apps is essential for accurate PPFD measurements, ensuring optimal light levels for different growth stages of cannabis 1.
  • Lighting Systems: LED grow lights provide a spectrum that includes crucial PAR wavelengths, mimicking natural sunlight to optimize plant health and yield 36.
  • Evaluation of Grow Lights: When selecting lighting systems, considering the PPFD data provided by manufacturers, including distribution and intensity at various distances, is vital 6.

Understanding these aspects of PAR not only helps in choosing the right lighting systems but also in fine-tuning the environment to maximize the photosynthetic potential and overall health of cannabis plants.

Measuring PAR

PPFD and DLI To determine the light intensity within the PAR range, we use Photosynthetic Photon Flux Density (PPFD), measured in micromoles per square meter per second (µmol/m²/s). PPFD indicates the number of light photons that reach a specific area every second. For a comprehensive understanding of light exposure over time, we use Daily Light Integral (DLI), measured in moles of photons per square meter per day (mol/m²/day). DLI helps cultivators optimize light exposure for various plants and growth stages.

Optimal PAR Levels for Cannabis

General Recommendations for PAR Levels

  1. Seedling/Clone Stage: 100 – 300 µmol/m²/s 14
  2. Vegetative Stage: 250 – 600 µmol/m²/s 14
  3. Bloom/Flowering Stage: 500 – 1050 µmol/m²/s 14

Influence of Environmental Factors

  • CO2 Levels: Higher CO2 levels can allow for higher PAR levels, enhancing photosynthesis. Recommended CO2 levels are:
    • Seedling/Clone: 400 ppm 1
    • Vegetative: 400 – 800 ppm 1
    • Bloom/Flowering: 800 – 1400 ppm 1
  • Temperature and Humidity: Optimal levels vary based on growth phase and CO2 concentration, influencing PAR efficiency 1.

Signs of Inappropriate PAR Levels

  • Too Much Light:
    • Light burn: Yellow leaves with green veins and stems 1
    • Loose and airy buds: Result from the plant’s metabolism working harder 1
    • Light-bleached “white” buds: Caused by excessive light, reducing potency 1
  • Too Little Light:
    • Slow growth: Slower than expected growth rate 1
    • Stretching: Higher internode spacing and taller plant structure 1

Measuring and Adjusting PAR Levels

  • Tools: Use a PAR light meter or a PAR meter app for accurate PPFD readings 1.
  • Adjustments: Ensure no area of the canopy receives less than 500 µmol/m²/s and no more than 1000 µmol/m²/s to prevent damage 12.

Optimal PPFD for Various Growth Stages

  • Clones: 150-200 µmol/m²/s 11
  • Seedlings: 200-400 µmol/m²/s 11
  • Vegetative: 400-600 µmol/m²/s 11
  • Flowering: 800-1000 µmol/m²/s 11

Understanding and managing PAR levels according to these guidelines ensures optimal growth and health of cannabis plants, adjusting for specific strain needs and environmental factors 81.

Comparing Light Sources

Energy Efficiency and Spectrum Range

  1. Energy Efficiency: LED grow lights are notably more energy-efficient compared to High-Intensity Discharge (HID) lights, converting at least 50% more electricity into usable light 3. This efficiency is measured in umol/j, indicating the number of photons generated per unit of energy 3.
  2. Quantum Efficiency: Among LED options, 660nm monochromatic red diodes stand out with a high quantum efficiency of approximately 5.5 umol/j, making them one of the most efficient in converting electricity to photons 3.
  3. Total Efficiency: The overall efficiency of an LED is calculated by multiplying its radiant efficiency with its quantum efficiency, providing a comprehensive measure of its performance 3.

Impact of Spectrum on Plant Growth

  • Full Spectrum LEDs: A true full spectrum LED, covering the entire 400-700nm PAR range, generally has a lower umol/j efficiency compared to LEDs focused heavily on the red spectrum. This is due to the inclusion of less efficient violet and UV wavelengths 3.
  • UV and Far Red LEDs: UV light plays a crucial role in enhancing cannabinoid and terpene production, while Far Red light can accelerate flowering and increase biomass. However, LEDs emitting Far Red light tend to have lower efficiency 3.

Advanced LED Features and Yield Improvements

  • Photon Flux Density (PFD): Advanced LEDs with more than 10% of their light outside the traditional PAR range demonstrate a higher PFD, which can significantly impact plant growth and yield 3.
  • Yield Enhancement: By 2024, it is projected that advanced LED grow lights could increase cannabis yield by 30% or more compared to older LED technologies 14.

Choosing the Right LED Grow Light

  • Light Quality: When selecting LED grow lights for cannabis, it is crucial to consider the Photosynthetic Photon Flux (PPF) and Photosynthetic Photon Flux Density (PPFD) alongside the electrical power usage to ensure optimal light quality and energy efficiency 4.
  • Chip Quality: High-quality LED chips, such as those from Cree or Bridgelux, are recommended for their superior performance and reliability in cannabis cultivation 15.
  • Durability and Heat Output: LEDs not only last longer but also produce less heat compared to traditional High-Pressure Sodium (HPS) bulbs, making them more energy-efficient and cost-effective over time 15.
  • Sizing Guidelines: The size of the LED light required depends on the number of plants and the size of the grow space, with a general guideline of providing 32 to 50 watts of LED light per square foot 16.

Maximizing PAR Efficiency

Implementing PAR Level Systems

To maximize PAR efficiency in cannabis cultivation, implementing a PAR level system proves essential. This system focuses on optimizing inventory levels, reducing waste, improving cash flow, and enhancing customer satisfaction 17 [59] [60]. By determining PAR levels based on historical sales data, demand forecasts, perishability, and lead time, cultivators can automate the ordering process, ensuring that inventory is managed more efficiently 18. This approach not only reduces the risk of stockouts and overstocking but also minimizes costs and enhances the customer experience 18.

Environmental Optimization for Growth

In addition to managing PAR levels, providing optimal environmental conditions is crucial for maximizing the growth rate potential of cannabis. The ideal temperature range for a grow room is set between 77-86 degrees Fahrenheit (25-30 degrees Celsius), with relative humidity levels tailored to different growth stages—80% during the seedling or cloning process, 60-70% during the vegetative phase, and 50% during the flowering stage 9. These conditions, alongside appropriate CO2 levels and light intensity, ensure faster and healthier plant growth 9.

Advanced Monitoring with Blakthumb’s System

Blakthumb’s innovative PAR level system includes features such as automated ordering and regular monitoring of inventory levels. This advanced system not only improves the bottom line by optimizing resource allocation but also significantly enhances the overall customer experience by maintaining consistent product quality and availability 18. This integration of technology into PAR management allows cultivators to maintain precise control over their cultivation environment, adapting quickly to changes in plant growth needs or environmental conditions.

The Role of Light Spectrum

Impact of Light Spectrum on Cannabis Growth

The light spectrum plays a pivotal role in cannabis cultivation, influencing plant morphology, growth rates, and cannabinoid content. Here’s how different light wavelengths impact cannabis plants:

  1. Blue Light (400-500nm):
    • Promotes strong stem development and compact growth, essential for robust energy production 20.
    • Keeps plants shorter and squatter, beneficial during the vegetative stage for dense foliage 15.
  2. Red Light (600-700nm):
    • Encourages energy channeling into developing large, resinous buds during the flowering stage 20.
    • Influences plant height and branchiness, promoting expansive growth 15.
  3. Green Light (500-600nm):
    • Though contributing minimally, it regulates plant architecture and aids in photosynthesis 20.
    • Involved in various growth stages including seedling development and initiation of flowering 21.
  4. Far-Red Light and UV Wavelengths:
    • Far-red light stimulates stem elongation in early vegetative stages and affects flowering responses 20.
    • UV light enhances terpene production, boosting aroma, flavor, and potentially the plant’s potency 20.
  5. Optimal Spectrum Mix:
    • For vegetative growth, a focus on blue light is ideal 20.
    • During flowering, red light becomes crucial for bud development 20.
    • An optimal mix for overall growth is approximately 80% red (660nm) and 20% blue (440nm) light 15.

Understanding the specific light needs of different cannabis strains is also essential, as most seed manufacturers provide tailored information about each strain’s light requirements 1. This knowledge allows cultivators to adjust lighting systems effectively to meet the unique needs of each strain, optimizing growth conditions and enhancing yield.

Monitoring and Adjusting PAR Levels

Tools for Monitoring PAR and PPFD

To effectively monitor and adjust PAR levels for optimal cannabis growth, a variety of tools are available. PAR meters, spectrometers, and light meters are essential for gauging the Photosynthetic Photon Flux Density (PPFD) that influences plant health and productivity 7. Additionally, mobile apps and specialty sensors provide convenient, on-the-go solutions for regular checks 7.

Adjusting Light Intensity and Position

Adjusting grow lights is crucial in maintaining the desired PAR or PPFD levels. This involves measuring the current light levels using a PAR meter and then modifying the distance or intensity of the light source to achieve optimal conditions 7. Regular adjustments ensure that cannabis plants receive the right amount of light without the risk of damage from excessive PAR or PPFD, which can lead to plant cell damage and excessive heat buildup 7.

Daily Light Integral (DLI) Management

The Daily Light Integral (DLI) is a critical measure that represents the total amount of PAR received per day. Optimal DLI levels are essential for different growth stages of cannabis:

  • Seedling/Clone Stage: 10 – 15 DLI 22
  • Vegetative Stage: 30 – 45 DLI 22
  • Bloom/Flowering Stage: 30 – 40 DLI 22

Adjusting DLI involves modifying both the PAR/PPFD levels and the duration of lighting to ensure plants receive the appropriate amount of light for their specific growth phase 22.

Practical Steps for Measurement and Adjustment

  1. Measure Current PPFD: Utilize a PAR meter to obtain accurate readings at the recommended height and various distances to ensure even light coverage 10.
  2. Calculate Average PPFD: To get a more accurate representation of light intensity, calculate the average PPFD across different points of the grow area 10.
  3. Adjust Lighting Setup: Based on the measurements, adjust the height or intensity of the light sources to maintain optimal PPFD levels, ensuring no area of the canopy receives less than the minimum required PPFD and no more than the maximum to prevent light stress 10.

Utilizing Advanced Monitoring Tools

For more sophisticated monitoring, the Photone PAR/PPFD and DLI meter app offers a cost-effective solution, available on both iOS and Android platforms. This app allows growers to measure and adjust light levels conveniently, ensuring that autoflowering cannabis plants, which thrive under extensive light exposure, receive adequate lighting without exceeding PPFD levels of 700 µmol/m²/s during peak bloom phases 23.


Understanding Photosynthetically Active Radiation (PAR) is fundamental for optimizing plant growth and development. By measuring and adjusting PAR values, you can create a thriving environment for your plants, whether you’re growing cannabis, tomatoes, or orchids. So, harness the power of PAR and let your plants bask in the light they need to flourish!


What is the recommended PAR level for a grow light?

The recommended PAR (Photosynthetic Active Radiation) level for a grow light can vary depending on the plant species and growth stage. However, there is no specific number given as it can differ widely. Growers should aim to provide sufficient PAR that matches the needs of their particular plants.

Can you explain what PAR level means in terms of lighting?

PAR stands for Photosynthetic Active Radiation, which refers to the wavelengths of light within the visible range of 400 to 700 nanometers (nm) that are crucial for photosynthesis. Although widely used in horticultural lighting, PAR is not a unit of measurement like feet, inches, or kilograms; it’s a term that describes the type of light needed for plant growth.

What type of lighting is best for growing cannabis?

For the flowering stage of cannabis growth, the optimal light spectrum falls within the range of 620 to 740 nm, with a peak at 660 nm. This range promotes strong flowering without causing the plants to grow too tall and thin, which can reduce yield.

How does par value differ from lux in lighting?

PAR value represents the total amount of light energy within the 400-700nm wavelengths, which is crucial for plant photosynthesis. In contrast, lux measures the total perceived brightness of a light source as seen by the human eye. Essentially, PAR and lux are different interpretations of light photons, with PAR being relevant to plant growth and lux to human vision.

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