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Light: Difference between revisions
→Light schedules: Added "Moving away from 12/12 lighting" paper
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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, 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 ==
=== 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
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<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.
==Types of lighting ==
There are several light technologies that have differing efficiencies, spectrums, and form factors.
===CFL - Compact fluorescent lamp===
===HID - High-density discharge===
== 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.<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
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.
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Once a plant moves into the vegetative stage the maximum light intensity it can tolerate will be 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
=== 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.<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
<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>
The theoretical upper limit that a plant can tolerate before experiencing symptoms of poor health.
The Maximum
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>
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
== 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
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 C (100-280 nm)====
=== Low
Low-frequency light is constituted of red light in the visible spectrum, "far range" and
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>
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