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In ideal conditions, cannabis can absorb 1500 - 2000 μmol/s. For context, in California, the sun can emit up to 2000 μmol/s around midday. This means that the light source should ideally be powerful enough to deliver this intensity over the area the canopy will occupy. Equipped with a powerful and efficient grow light, you may be tempted to use as much power as you can afford, to increase yield - but this is often not the best strategy; leaves can be damaged by overexposure to light (and heat) which will decrease yield. To avoid this, slowly increase the intensity of your light over several days, by increasing power draw by 5-10 watts per day, observing the leaves regularly. Any yellowing leaves can be a sign of light stress. This may be accompanied by curling leaves which are a sign of heat stress, often caused by lights being too intense. Note also that seedlings require less light than flowering plants.
 +
In ideal conditions, cannabis can absorb 1500 - 2000 μmol/s. For context, in California, the sun can emit up to 2000 μmol/s around midday. This means that the light source should ideally be powerful enough to deliver this intensity over the area the canopy will occupy. Equipped with a powerful and efficient grow light, you may be tempted to use as much power as you can afford, to increase yield - but this is often not the best strategy; leaves can be damaged by overexposure to light (and heat) which will decrease yield. To avoid this, slowly increase the intensity of your light over several days, by increasing power draw by 5-10 watts per day, and observing the leaves regularly. Any yellowing leaves can be a sign of light stress. This may be accompanied by curling leaves which are a sign of heat stress, often caused by lights being too intense. Note also that seedlings require and are less able to make use of light than flowering plants.
   
 
== Horticultural light metrics ==
  
== Horticultural light metrics ==
−
There are many different metrics that can be used to measure the intensity and quality of light for photosynthesis in the cultivar.
 +
There are many different metrics that can be used to measure the intensity and quality of light for photosynthesis in the cultivar. These metrics are important for assessing if a source is sufficient but especially notable when comparing two different lights.
⚫
=== μmol/s ===
  
−
The measure of photons per second produced by a light. Photons are measured in micro (10^-6) [https://en.wikipedia.org/wiki/Mole_(unit) moles] there are a huge amount of photons coming from any light source. One µmol is actually 602 quadrillion photons. Simply put, this metric is the intensity per second from a light.
  
⚫
=== PPFD ===
  
−
Photosynthetic Photon Flux Density: its unit is μmol/m^2/s. It is a measure of [[Light#PAR|PAR]] intensity (light that can be used for photosynthesis) over an area per second. This is important to note when buying a grow light. For example, light A may provide an intensity of 800 μmol/s in a 1m^2 area, while light B provides the same intensity in a 4m^2 area. Light B provides the same intensity but over four times the area, so it can support a larger plant. In this case, light B has a greater PPFD.
  
⚫
=== μmol/J ===
  
⚫
The light intensity a horticultural light produces per Joule of power it uses. This is a measure of its efficiency. At the time of writing, the most efficient LED package available is the [https://www.samsung.com/led/lighting/mid-power-leds/3030-leds/lm301h/ LM301H] by Samsung. It achieves 3.10 μmol/J @ 65 mA, 25°C. Note the efficiency will change depending on the temperature and current supplied.
  
⚫
=== DLI (Daily light Intergral) ===
  
⚫
The amount of light energy (photons) a plant is receiving every 24 hours. For example, if you used a light with a PPFD of 500μmol/m^2/s on a 20/4 [[Light#Light_schedules|lighting schedule]] that would result in a DLI of 43.20.
 
−
DLI is useful as a metric as it takes light schedule into account
  
   
  ⚫
=== Intensity ===
⚫
Full light intensity from the sun in summer reaches around 60 DLI<ref>https://www.youtube.com/watch?v=ID9rE5JewVg&t=1170s&ab_channel=ApogeeInstrumentsInc. - Bruce Bugee Grow lighting myths</ref>
  
  ⚫
unit: μmol/s
   
  +
Light intensity is quantified by the number of photons produced per second. Because the number of photons is so large they are measured in micro-moles (µmol). One µmol is actually 602 quadrillion photons. See: [[Light#Measuring light intensity|Measuring light intensity]]
⚫
[https://www.waveformlighting.com/horticulture/daily-light-integral-dli-calculator DLI calculator]
  
  +
  +
=== Photosynthetic Photon Flux Density (PPFD) ===
  +
unit: μmol/m<sup>2</sup>/s.
  +
  +
A measure of light intensity within the [[Light#PAR|PAR range]] over an area. The maximum PPFD a grow light can produce is important to know as well as the size of the area it can deliver that PPFD.
  +
  ⚫
=== Efficiency ===
  ⚫
unit: μmol/J
  +
  ⚫
The light intensity a horticultural light produces per Joule of power it uses. At the time of writing, the most efficient LED package available is the [https://www.samsung.com/led/lighting/mid-power-leds/3030-leds/lm301h/ LM301H] by Samsung. It achieves 3.10 μmol/J @ 65 mA, 25°C. The efficiency of all LEDs will change slightly depending on the temperature and current supplied.
  ⚫
=== DLI (Daily light Integral) ===
  +
unit: mols/m<sup>2</sup>
  +
  ⚫
The amount of light energy in the PAR range a plant is receiving every 24 hours. For example, if you used a light with a PPFD of 500μmol/m<sup>2</sup>/s on a 20/4 [[Light#Light_schedules|lighting schedule]] that would result in a DLI of 43.20 mols. DLI is generally the most useful metric of light intensity the plant is exposed to as it takes the light schedule into account.
  +
  ⚫
Full light intensity from the sun in summer reaches around 60 DLI<ref>https://www.youtube.com/watch?v=ID9rE5JewVg&t=1170s&ab_channel=ApogeeInstrumentsInc. - Bruce Bugee Grow lighting myths</ref>
  +
[[File:DLI equation.png|center|thumb|349x349px]]
  ⚫
[https://www.waveformlighting.com/horticulture/daily-light-integral-dli-calculator DLI calculator]
   
 
== Horticulural light terms ==
  
== Horticulural light terms ==
 
=== PAR ===
  
=== PAR ===
  +
[[File:PAR range.png|thumb]]
 
PAR stands for Photosynthetic Active Radiation: light within the range of 400 to 700 nanometers that drives photosynthesis. For reference, the human eye can see light between 380 to 750 nm.
  
PAR stands for Photosynthetic Active Radiation: light within the range of 400 to 700 nanometers that drives photosynthesis. For reference, the human eye can see light between 380 to 750 nm.
   
 
==Types of lighting ==
  
==Types of lighting ==
−
There are several light technologies that have differing efficiencies, spectrums and form factors
 +
There are several light technologies that have differing efficiencies, spectrums, and form factors.
 
===CFL - Compact fluorescent lamp===
  
===CFL - Compact fluorescent lamp===
 
===HID - High-density discharge===
  
===HID - High-density discharge===
Line 32: Line 43:
   
 
== Light schedules ==
  
== Light schedules ==
  +
This refers to how many hours of light a plant is exposed to in a 24-hour period.
−
Common light schedules include 24/0 (24 hours of light a day), 20/4 (20 hours light, 4 hours darkness), 18/6 (18 hours light, 6 hours darkness), and 12/12 (12 hours of light, 12 hours of darkness).
  
  +
Common light schedules include 24/0 (24 hours of light a day), 20/4 (20 hours light, 4 hours darkness), 18/6 (18 hours light, 6 hours darkness), and 12/12 (12 hours of light, 12 hours of darkness). Typically 12/12 is used when growing photo-flowers as this is very likely to initiate flowering in all strains, however, the light schedule has other effects on the cultivar. One 2023 paper has found that longer light periods can increase yield but decrease THC content.<ref>Peterswald, T.J.; Mieog, J.C.; Azman Halimi, R.; Magner, N.J.; Trebilco, A.; Kretzschmar, T.; Purdy, S.J. Moving Away from 12:12; the Effect of Different Photoperiods on Biomass Yield and Cannabinoids in Medicinal Cannabis. ''Plants'' 2023, ''12'', 1061. https://doi.org/10.3390/plants12051061 </ref>
   
−
Auto flowers can be grown with lights on 24 hours a day (24/0), but constant light can leave the plant with no time to recover from deficiencies, or stress in general. The absence of a dark period can also stave the plant's rhizosphere, negatively affecting growth<ref>Karuna Chourey PhD - Shango Los 2019
 +
Auto flowers can be grown with lights on 24 hours a day (24/0), but constant light can leave the plant with no time to recover from deficiencies or stress in general. The absence of a dark period can also stave the plant's rhizosphere, negatively affecting growth<ref>Karuna Chourey Ph.D. - Shango Los 2019
   
 
https://www.youtube.com/watch?v=u8XNN4V7qsU</ref>. It is a good idea to match the light schedule with the cultivar's [[Plant_processes#Circadian_rhythm|circadian rhythm]]; Many growers recommend 20/4 or 22/2.
  
https://www.youtube.com/watch?v=u8XNN4V7qsU</ref>. It is a good idea to match the light schedule with the cultivar's [[Plant_processes#Circadian_rhythm|circadian rhythm]]; Many growers recommend 20/4 or 22/2.
   
 
== Ideal light conditions ==
  
== Ideal light conditions ==
  +
Cannabis can tolerate an extreme light intensity. Many growers limit light levels early in the plants life but this is not nessesary<ref>Best Grow Lighting for Cannabis with Dr Bruce Bugbee | Far red ePAR - https://youtu.be/VvBTsl_gScw?si=4kS8z9cSu9hprAMu&t=2771</ref>.
−
In the initial stages of growth when the plant is a seedling, it should be exposed to much less light than when in flowering. This is for photoacclimation[citation needed].
 
   
  +
Ideal intensity is around 1000 - 2000 μmol/s. 40 DLI is a good target although it is possible to go higher.
−
{| class="wikitable"
  
−
|+Light over time
  
−
|Life stage
  
−
|style="text-align:center;" | PPFD
 
−
|-
  
−
|Seedling
  
⚫
|100 - 400 μmol/s
  
−
|-
  
−
|Flowering
  
−
|600 - 2000 μmol/s
  
−
|}
  
   
−
Once a plant moves into the vegetative stage the maximum light it can tolerate will be limited by other conditions such as CO2 level or nutrient deficiency. Note that light intensity is proportional the rate of [[Plant processes#Transpiration|transpiration]], meaning higher light intensity can raise the [[Temperature and Humidity|humidity]] of the environment as more water exits the plant. This creates another potential limitation to the maximum light that can be applied in the grower's ability to control the environment.
 +
Generally the maximum light intensity a plant can tolerate becomes limited by other conditions such as CO2 level or nutrient deficiency. Note that light intensity is proportional to the rate of [[Plant processes#Transpiration|transpiration]], meaning higher light intensity can raise the [[Temperature and Humidity|humidity]] of the environment as more water exits the plant. This creates another potential limitation to the maximum light that can be applied; the grower's ability to control the temperature and humidity.
   
 
=== Optimum light intensity===
  
=== Optimum light intensity===
 
In this instance, "optimum light intensity" refers to the amount of light that provides the maximum yield from the cultivar.
  
In this instance, "optimum light intensity" refers to the amount of light that provides the maximum yield from the cultivar.
   
−
The threshold for optimal growth and photosynthesis, from the vegetative stage, is a DLI of 65 moles<ref> Daily Light Integral DLI Relation To Cannabis Yield (Meta-analysis, Matthew Debacco, 2021) https://www.youtube.com/watch?v=au7G-oVDeKg</ref>
 +
The threshold for optimal growth and photosynthesis, from the vegetative stage, is a DLI of 65 moles.<ref> Daily Light Integral DLI Relation To Cannabis Yield (Meta-analysis, Matthew Debacco, 2021) https://www.youtube.com/watch?v=au7G-oVDeKg</ref>
   
−
Yield appears to be linear with ppfd to around 1500 - 2000 μmol/s<ref>Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment (Rodriguez-Morrison, 2021) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8144505/</ref>
 +
Yield appears to be linear with PPFD to around 2000 μmol/s<ref>Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment (Rodriguez-Morrison, 2021) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8144505/</ref><ref>High Light Intensities Can Be Used to Grow Healthy and Robust Cannabis Plants During the Vegetative Stage of Indoor Production (Moher, 2021) https://www.preprints.org/manuscript/202104.0417/v1</ref>. Above 500 μmol/s supplemental co<sub>2</sub> is advised to make optimum use of the light.<ref>Fluence Cannabis Cultivation Guide, 2020 - https://fluence.science/guides/cannabis-cultivation-guide/</ref>
−
<ref>High Light Intensities Can Be Used to Grow Healthy and Robust Cannabis Plants During the Vegetative Stage of Indoor Production (Moher, 2021) https://www.preprints.org/manuscript/202104.0417/v1</ref>
  
   
−
Temperature and light optima for photosynthesis were concluded to be at 25–30 °C and ∼1500 μmol m2/s respectively<ref> Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions (Chandra, 2008) https://dx.doi.org/10.1007%2Fs12298-008-0027-x</ref>
 +
Temperature and light optima for photosynthesis were concluded to be at 25–30 °C and ∼1500 μmol m<sup>2</sup>/s respectively<ref> Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions (Chandra, 2008) https://dx.doi.org/10.1007%2Fs12298-008-0027-x</ref>
   
 
=== Upper limit of light intensity ===
 
=== Upper limit of light intensity ===
−
The theoretical upper limit that a plant can tolerate before experiencing symptoms of poor health
 +
The theoretical upper limit that a plant can tolerate before experiencing symptoms of poor health.
⚫
Maximum ppfd cannabis can tolerate is around 2000 - 2500 μmol/s in perfect conditions<ref> Light dependence of photosynthesis and water vapor exchange... (Chandra, 2015)- https://doi.org/10.1016/j.jarmap.2015.03.002</ref><ref> Cannabis business times Oct, 2021 https://www.cannabisbusinesstimes.com/article/growing-under-high-light-intensities-lighting-report/</ref> and a DLI of around 40 - 60 moles
  
   
  ⚫
The Maximum PPFD cannabis can tolerate is around 2000 - 2500 μmol/s in perfect conditions<ref> Light dependence of photosynthesis and water vapor exchange... (Chandra, 2015)- https://doi.org/10.1016/j.jarmap.2015.03.002</ref><ref> Cannabis business times Oct, 2021 https://www.cannabisbusinesstimes.com/article/growing-under-high-light-intensities-lighting-report/</ref> and a DLI of around 40 - 60 moles.
−
Above (500?) μmol/s supplemental co2 is required [citation required]
  
   
−
Meta-analysis of plant morphology to light intensity<ref>A meta-analysis of plant responses to light intensity for 70 traits ranging from molecules to whole plant performance (Poorter, 2019) https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.15754</ref>
 +
Meta-analysis of plant morphology to light intensity.<ref>A meta-analysis of plant responses to light intensity for 70 traits ranging from molecules to whole plant performance (Poorter, 2019) https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.15754</ref>
   
 
== Measuring light intensity ==
  
== Measuring light intensity ==
−
[[File:Relative Spectral Power Distrobution for color temp.jpg|thumb|310x310px|SPD chart]]
 +
[[File:Relative Spectral Power Distrobution for color temp.jpg|thumb|310x310px|An SPD chart of common light spectrums]]
−
The amount of photosynthetically reactive light ([[Light#PAR|PAR]]) can be measured using a PAR meter or a lux meter combined with the chart of the light's spectral power distribution (SPD).
 +
The amount of photosynthetically reactive light ([[Light#PAR|PAR]]) present can be measured using a dedicated device known as a Quantum sensor. They are very accurate, however, as most smartphones have a lux meter, a much cheaper alternative is to use a "PPFD meter" app. A lux value can be combined with the grow light's spectral power distribution (SPD) to calculate the amount of PAR radiation. This is necessary because lux meters measure light in the frequency relevant to human vision and not photosynthesis.
−
A dedicated device may provide the most accurate results, but a cheaper alternative is to use a PPFD meter app on a smartphone. These apps use the phone's brightness sensor to find a lux value and ask the user to input the spectrum of the light they are using.
  
   
  +
Simply put, these apps use the phone's brightness sensor to find a lux value and ask the user to input the frequency of the grow light they are using. Often 3500K or 4000K for "full-spectrum lights", Samsung produces a number of LEDs in this range.
⚫
An example can be found at [https://www.waveformlighting.com/horticulture/convert-lux-to-ppfd-online-calculator waveformlighting]
  
  +
  +
Some examples are [https://play.google.com/store/apps/details?id=com.homestudio.ppfdmeter PPFD Meter] on Android or [https://apps.apple.com/us/app/photone-grow-light-meter/id1450079523 Photone] on iPhone (Better implementations may exist).
  +
  ⚫
A competent online implementation exists at [https://www.waveformlighting.com/horticulture/convert-lux-to-ppfd-online-calculator waveformlighting].
   
 
== Light spectrum ==
  
== Light spectrum ==
−
The color of light is dependent on the frequency of the light. see also [https://en.wikipedia.org/wiki/Electromagnetic_spectrum Electromagnetic spectrum]. Blue/purple light used to be standard in Cannabis growing but are slowly being replaced by full-spectrum lights. Throughout the plant's life cycle, it can be helpful to apply different ranges of the light spectrum.
 +
The color of light is dependent on the frequency of the light. see also [https://en.wikipedia.org/wiki/Electromagnetic_spectrum Electromagnetic spectrum]. Blue/purple light used to be standard in Cannabis growing but is slowly being replaced by "full-spectrum" lights. The light spectrum applied to a plant can affect its morphology and cannabinoid content. <ref name=":0">Magagnini 2018, The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L -http://dx.doi.org/10.1159/000489030</ref>
   
−
=== High frequency light ===
 +
=== High-frequency light ===
−
Approximately 3% of the light radiation from the sun is ultraviolet; in the range of 100-400 nm. The light is the ratio of 1:30 UVB to UVA, and all UVC is absorbed by the atmosphere, particularly the Ozone layer.
 +
Approximately 3% of the light radiation from the sun is ultraviolet; in the range of 100-400 nm. The light is the ratio of 1:30 UVB to UVA, and all UVC is absorbed by the atmosphere, particularly the Ozone layer. UV light rays can damage plants and stress them, triggering the creation of more cannabinoids such as THC, CBD, and CBG<ref>What light spectrum does weed need to grow? - Growing Cannabis 201: Advanced Grow Tips | Indica Institute - https://www.youtube.com/watch?v=NY0bB_Jmt5E</ref><ref>Pate, David W. (1983). "Possible role of ultraviolet radiation in evolution of Cannabis chemotypes" doi:10.1007/BF02904200</ref> <ref name=":0" />. It is common to find LED boards that include UV LEDs.
−
UV light rays can damage plants and stress them, triggering the creation of more trichromes<ref>What light spectrum does weed need to grow? - Growing Cannabis 201: Advanced Grow Tips | Indica Institute - https://www.youtube.com/watch?v=NY0bB_Jmt5E</ref><ref>Pate, David W. (1983). "Possible role of ultraviolet radiation in evolution of Cannabis chemotypes" doi:10.1007/BF02904200</ref> UV lights specifically for growing are available.
  
   
 
==== UV A (315-400 nm) ====
  
==== UV A (315-400 nm) ====
Line 93: Line 95:
 
==== UV C (100-280 nm)====
  
==== UV C (100-280 nm)====
   
−
=== Low frequency light ===
 +
=== Low-frequency light ===
−
Low-frequency light is constituted of red light in the visible spectrum, "far range" and infra-red.
 +
Low-frequency light is constituted of red light in the visible spectrum, "far range" and infrared.
   
 
Red light promotes cell elongation in plants, in cannabis, this manifests as stretching of the stem. While not visible to the human eye, far-red light is also photosynthetically active<ref>Dr. Bruce Bugbee (2019) - https://www.youtube.com/watch?v=sS7aAcacfgk&ab_channel=ApogeeInstrumentsInc.</ref>
  
Red light promotes cell elongation in plants, in cannabis, this manifests as stretching of the stem. While not visible to the human eye, far-red light is also photosynthetically active<ref>Dr. Bruce Bugbee (2019) - https://www.youtube.com/watch?v=sS7aAcacfgk&ab_channel=ApogeeInstrumentsInc.</ref>
   
 
Adding far-red light can increase cannabis yield by an average of 20%<ref>Fluence science - https://fluence.science/broad-spectrum-leds-cannabis/</ref>
  
Adding far-red light can increase cannabis yield by an average of 20%<ref>Fluence science - https://fluence.science/broad-spectrum-leds-cannabis/</ref>
−  
−  
   
   

Latest revision as of 15:06, 10 September 2023

In ideal conditions, cannabis can absorb 1500 - 2000 μmol/s. For context, in California, the sun can emit up to 2000 μmol/s around midday. This means that the light source should ideally be powerful enough to deliver this intensity over the area the canopy will occupy. Equipped with a powerful and efficient grow light, you may be tempted to use as much power as you can afford, to increase yield - but this is often not the best strategy; leaves can be damaged by overexposure to light (and heat) which will decrease yield. To avoid this, slowly increase the intensity of your light over several days, by increasing power draw by 5-10 watts per day, and observing the leaves regularly. Any yellowing leaves can be a sign of light stress. This may be accompanied by curling leaves which are a sign of heat stress, often caused by lights being too intense. Note also that seedlings require and are less able to make use of light than flowering plants.

Horticultural light metrics

There are many different metrics that can be used to measure the intensity and quality of light for photosynthesis in the cultivar. These metrics are important for assessing if a source is sufficient but especially notable when comparing two different lights.

Intensity

unit: μmol/s

Light intensity is quantified by the number of photons produced per second. Because the number of photons is so large they are measured in micro-moles (µmol). One µmol is actually 602 quadrillion photons. See: Measuring light intensity

Photosynthetic Photon Flux Density (PPFD)

unit: μmol/m2/s.

A measure of light intensity within the PAR range over an area. The maximum PPFD a grow light can produce is important to know as well as the size of the area it can deliver that PPFD.

Efficiency

unit: μmol/J

The light intensity a horticultural light produces per Joule of power it uses. At the time of writing, the most efficient LED package available is the LM301H by Samsung. It achieves 3.10 μmol/J @ 65 mA, 25°C. The efficiency of all LEDs will change slightly depending on the temperature and current supplied.

DLI (Daily light Integral)

unit: mols/m2

The amount of light energy in the PAR range a plant is receiving every 24 hours. For example, if you used a light with a PPFD of 500μmol/m2/s on a 20/4 lighting schedule that would result in a DLI of 43.20 mols. DLI is generally the most useful metric of light intensity the plant is exposed to as it takes the light schedule into account.

Full light intensity from the sun in summer reaches around 60 DLI[1]

DLI calculator

Horticulural light terms

PAR

PAR stands for Photosynthetic Active Radiation: light within the range of 400 to 700 nanometers that drives photosynthesis. For reference, the human eye can see light between 380 to 750 nm.

Types of lighting

There are several light technologies that have differing efficiencies, spectrums, and form factors.

CFL - Compact fluorescent lamp

HID - High-density discharge

LED - Light emitting diode

LEDs are the most compact and efficient light source but can be more costly than other means and require a driver.

LED Drivers

LED drivers can be categorized into constant voltage and constant current, both with dimmable options. If your panel has a recommended voltage, it's best to select a constant voltage driver. Dimmable drivers are more expensive but worth it because your plant requires more light at the flowering stage than at the seedling stage, so you may wish to use less power initially to conserve power. Meanwell drivers are the industry standard for efficiency and lifespan.

Light schedules

This refers to how many hours of light a plant is exposed to in a 24-hour period. Common light schedules include 24/0 (24 hours of light a day), 20/4 (20 hours light, 4 hours darkness), 18/6 (18 hours light, 6 hours darkness), and 12/12 (12 hours of light, 12 hours of darkness). Typically 12/12 is used when growing photo-flowers as this is very likely to initiate flowering in all strains, however, the light schedule has other effects on the cultivar. One 2023 paper has found that longer light periods can increase yield but decrease THC content.[2]

Auto flowers can be grown with lights on 24 hours a day (24/0), but constant light can leave the plant with no time to recover from deficiencies or stress in general. The absence of a dark period can also stave the plant's rhizosphere, negatively affecting growth[3]. It is a good idea to match the light schedule with the cultivar's circadian rhythm; Many growers recommend 20/4 or 22/2.

Ideal light conditions

Cannabis can tolerate an extreme light intensity. Many growers limit light levels early in the plants life but this is not nessesary[4].

Ideal intensity is around 1000 - 2000 μmol/s. 40 DLI is a good target although it is possible to go higher.

Generally the maximum light intensity a plant can tolerate becomes limited by other conditions such as CO2 level or nutrient deficiency. Note that light intensity is proportional to the rate of transpiration, meaning higher light intensity can raise the humidity of the environment as more water exits the plant. This creates another potential limitation to the maximum light that can be applied; the grower's ability to control the temperature and humidity.

Optimum light intensity

In this instance, "optimum light intensity" refers to the amount of light that provides the maximum yield from the cultivar.

The threshold for optimal growth and photosynthesis, from the vegetative stage, is a DLI of 65 moles.[5]

Yield appears to be linear with PPFD to around 2000 μmol/s[6][7]. Above 500 μmol/s supplemental co2 is advised to make optimum use of the light.[8]

Temperature and light optima for photosynthesis were concluded to be at 25–30 °C and ∼1500 μmol m2/s respectively[9]

Upper limit of light intensity

The theoretical upper limit that a plant can tolerate before experiencing symptoms of poor health.

The Maximum PPFD cannabis can tolerate is around 2000 - 2500 μmol/s in perfect conditions[10][11] and a DLI of around 40 - 60 moles.

Meta-analysis of plant morphology to light intensity.[12]

Measuring light intensity

An SPD chart of common light spectrums

The amount of photosynthetically reactive light (PAR) present can be measured using a dedicated device known as a Quantum sensor. They are very accurate, however, as most smartphones have a lux meter, a much cheaper alternative is to use a "PPFD meter" app. A lux value can be combined with the grow light's spectral power distribution (SPD) to calculate the amount of PAR radiation. This is necessary because lux meters measure light in the frequency relevant to human vision and not photosynthesis.

Simply put, these apps use the phone's brightness sensor to find a lux value and ask the user to input the frequency of the grow light they are using. Often 3500K or 4000K for "full-spectrum lights", Samsung produces a number of LEDs in this range.

Some examples are PPFD Meter on Android or Photone on iPhone (Better implementations may exist).

A competent online implementation exists at waveformlighting.

Light spectrum

The color of light is dependent on the frequency of the light. see also Electromagnetic spectrum. Blue/purple light used to be standard in Cannabis growing but is slowly being replaced by "full-spectrum" lights. The light spectrum applied to a plant can affect its morphology and cannabinoid content. [13]

High-frequency light

Approximately 3% of the light radiation from the sun is ultraviolet; in the range of 100-400 nm. The light is the ratio of 1:30 UVB to UVA, and all UVC is absorbed by the atmosphere, particularly the Ozone layer. UV light rays can damage plants and stress them, triggering the creation of more cannabinoids such as THC, CBD, and CBG[14][15] [13]. It is common to find LED boards that include UV LEDs.

UV A (315-400 nm)

UV B (280-315 nm)

UV C (100-280 nm)

Low-frequency light

Low-frequency light is constituted of red light in the visible spectrum, "far range" and infrared.

Red light promotes cell elongation in plants, in cannabis, this manifests as stretching of the stem. While not visible to the human eye, far-red light is also photosynthetically active[16]

Adding far-red light can increase cannabis yield by an average of 20%[17]


see also: How Lighting Affects Cannabis Cultivation - Cannabis Grow Lights[18]

References

  1. ↑ https://www.youtube.com/watch?v=ID9rE5JewVg&t=1170s&ab_channel=ApogeeInstrumentsInc. - Bruce Bugee Grow lighting myths
  2. ↑ Peterswald, T.J.; Mieog, J.C.; Azman Halimi, R.; Magner, N.J.; Trebilco, A.; Kretzschmar, T.; Purdy, S.J. Moving Away from 12:12; the Effect of Different Photoperiods on Biomass Yield and Cannabinoids in Medicinal Cannabis. Plants 2023, 12, 1061. https://doi.org/10.3390/plants12051061
  3. ↑ Karuna Chourey Ph.D. - Shango Los 2019 https://www.youtube.com/watch?v=u8XNN4V7qsU
  4. ↑ Best Grow Lighting for Cannabis with Dr Bruce Bugbee | Far red ePAR - https://youtu.be/VvBTsl_gScw?si=4kS8z9cSu9hprAMu&t=2771
  5. ↑ Daily Light Integral DLI Relation To Cannabis Yield (Meta-analysis, Matthew Debacco, 2021) https://www.youtube.com/watch?v=au7G-oVDeKg
  6. ↑ Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment (Rodriguez-Morrison, 2021) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8144505/
  7. ↑ High Light Intensities Can Be Used to Grow Healthy and Robust Cannabis Plants During the Vegetative Stage of Indoor Production (Moher, 2021) https://www.preprints.org/manuscript/202104.0417/v1
  8. ↑ Fluence Cannabis Cultivation Guide, 2020 - https://fluence.science/guides/cannabis-cultivation-guide/
  9. ↑ Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions (Chandra, 2008) https://dx.doi.org/10.1007%2Fs12298-008-0027-x
  10. ↑ Light dependence of photosynthesis and water vapor exchange... (Chandra, 2015)- https://doi.org/10.1016/j.jarmap.2015.03.002
  11. ↑ Cannabis business times Oct, 2021 https://www.cannabisbusinesstimes.com/article/growing-under-high-light-intensities-lighting-report/
  12. ↑ A meta-analysis of plant responses to light intensity for 70 traits ranging from molecules to whole plant performance (Poorter, 2019) https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.15754
  13. ↑ 13.0 13.1 Magagnini 2018, The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L -http://dx.doi.org/10.1159/000489030
  14. ↑ What light spectrum does weed need to grow? - Growing Cannabis 201: Advanced Grow Tips | Indica Institute - https://www.youtube.com/watch?v=NY0bB_Jmt5E
  15. ↑ Pate, David W. (1983). "Possible role of ultraviolet radiation in evolution of Cannabis chemotypes" doi:10.1007/BF02904200
  16. ↑ Dr. Bruce Bugbee (2019) - https://www.youtube.com/watch?v=sS7aAcacfgk&ab_channel=ApogeeInstrumentsInc.
  17. ↑ Fluence science - https://fluence.science/broad-spectrum-leds-cannabis/
  18. ↑ https://www.youtube.com/watch?v=tKzmx6XDOkE&t=235s&ab_channel=CannabisTech
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