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The Importance of PAR for Cannabis

PAR for cannabis

Photosynthetically active radiation (PAR) is the spectrum of light wavelengths that permits plants to photosynthesis. The valid range is 400-700 nanometers (nm). Consider this to be the precise wavelength of light used by cannabis plants (and other plants) to grow. Light is blue in the 400-nanometer range. Light is red in the 700-nanometer range. Plants require blue and red light spectrums, provided in grow lights.

What is the importance of PAR?

Grow lights emit a wide range of light wavelengths at your plants. However, your plants will only absorb a portion of these wavelengths to eat and thrive. The remaining wavelengths will reflect away from the plants, rendering them ineffective for plant growth. Higher PAR, in general, means more growth and healthier plants. As a result, you should select a grow lamp with a high PAR output to ensure that your plants receive as much useable light as possible.

What is the method for calculating PAR?

There are two techniques to calculate PAR:

  • PPF (Photosynthetic Photon Flux) measures the intensity of light by counting the number of photons emitted by the light per second. Micromoles per second (mol/s) is the unit of measurement.
  • PPFD (Photosynthetic Photon Flux Density) – This is an even more essential parameter since it considers both the intensity of the light and the amount of that light that reaches the plant. Micromoles per square foot per second (mol/m2/s) are the units of measurement.

A PAR meter is required to measure your grow light’s PAR output accurately.

 What is an excellent PAR output?

Because flowering plants use more energy and demand more food, your plants require a lower PAR output during the vegetative stage and a higher PAR output during the flowering stage. As a general rule of thumb, the following PAR levels should be considered during each growth stage:

  • 200-400 PPFD – for new seeds or cuts, as well as parent plants.
  • 400-600 PPFD – for plants in vegetative growth.
  • 600-900 PPFD – for plants in the flowering stage.

In general, the larger the wattage of your grow light, the higher the PAR output. But this isn’t always the case.

Consider PAR output before purchasing a grow light

Before purchasing a grow lamp, make sure to verify the PAR for cannabis output. Some manufacturers will supply PPF measurements, while others will provide a single PPFD value taken from the light’s center point. These are tricks employed by low-quality manufacturers to make their lights appear better than they are. Look for detailed graphs that demonstrate the PPFD at various heights or distances to gain a full grasp of your grow light’s PAR. The majority of reputable producers will demonstrate this.

How to boost your PAR

Your grow space configuration might have a significant impact on your PAR output.

1. Investing in a high-quality grow tent with reflective mylar lining will boost your total PAR production by up to 35%!

Reflective cannabis grow tent
Cannabis in grow box/tent with reflective material inside for better PAR production

2. Another method for increasing your tent’s overall PPFD is to daisy-chain many lights in the same tent.

Factors that influence

Determining the optimal PAR levels is a complex process that is influenced by various elements, the most important of which are the plant’s genetics, present development phase, and climate. Photosynthesis and metabolism in plants are highly complex and are influenced by various elements both above and below the soil. The following describes the factors that have the most significant influence on light levels.


Because photosynthesis relies primarily on CO2 and photons (light), CO2 is the most influential factor determining ideal PAR levels. When it comes to genuinely maxing out photosynthetic rates, all plants reach a point when more light does not equal greater growth. Still, more often than not, inadequate CO2 supplementation is the critical limiting issue. Indoor CO2 levels of 400 to 600 ppm immediately limit photosynthetic activity to less than half of what would otherwise be achievable. Increasing CO2 levels to at least 800ppm significantly increases photosynthetic activity and allows for higher PAR values.

Temperature and humidity

In general, a cannabis plant‘s growth rate increases with temperature until an optimal temperature is attained. Plant growth slows or stops when temperatures rise over the ideal level. The tricky part is that the ideal temperature is again determined by the CO2 concentration in the growing environment. Another component is humidity, the ideal rate determined by temperature. Find out the best humidity for growing cannabis.


In terms of climate, you can boost your PAR levels to the upper end of the appropriate PPFD values as long as you keep your plants inside the specified temperature and humidity values.


Cannabis grows wild in a wide range of settings, from tropical to very mountainous – each with particular light intensity. As a highly grown and bred plant, many genetic variabilities would be challenging to discuss in this text. However, most seed companies include some information on the unique strain’s light requirements, which should help you decide whether to place your strain on the low- or high-end of our guideline.

Signs of excessive light 

Light Burn

The most typical symptom of excessive PAR for cannabis levels on a cannabis plant is light burn. It causes yellow leaves with green veins and stems and is also known as leaf burn. Light burn can easily be confused with a nitrogen deficit, which similarly causes the leaves to turn yellow. The strength with which the yellow leaves have adhered to the stem is a simple indicator: Nitrogen-deficient leaves fall off on their own, but light-burned leaves are difficult to remove.

Buds that are loose and airy

Excessive light makes the plant’s metabolism work harder, resulting in nutrient deficits and generally weak, loose, and airy buds that may even feel crisp.

Lightly bleached “White” Buds

Bud bleaching occurs when the buds themselves are exposed to too much light, causing them to lose their color pigments and turn white. This should not be confused with a resinous, trichome-covered bud. Bleached buds are frequently less powerful and may lose almost all of the cannabinoids.

Signs of inadequate lighting

Slowing growth

The most evident factor is slower development than predicted. Because light is one of the fundamental components for plants to photosynthesize, they simply cannot grow as expected when there is insufficient light. To diagnose slow growth, compare the height of your plants to the normal height of the same strain at the same age.


When there is a dearth of light, most plants respond by accelerating stem development in order to come closer to the light. Higher internode spacing and a taller overall appearance of the plant indicate this.

Picture of Ed Rushford

Ed Rushford

Ed Rushford’s impact on cannabis growing is undeniable. Though he tends to focus primarily on 2 areas, plant training techniques and dealing with disease, pests, and other problems, he has offered many insights into how cannabis plants live and grow. That’s not to say that Ed is unfamiliar with the complete life cycle of cannabis, from seed to harvest, but he uses his widespread knowledge to hone in on the minutia and niche areas of growing cannabis. Ed’s goal is to spread knowledge and allow for everyone to become better growers. About this Author

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