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Greenhouse growers of short day plants will also need to use blackout systems to ensure successful flowering of short day plants like cannabis. Supplemental lighting is a different type of lighting that can help support greenhouse plants when days are very overcast and there is not sufficient natural light. Photosynthesis is an important metabolic process in plant growth. More than 90% of the dry matter of a plant is created from photosynthetic conversion of atmospheric carbon dioxide (CO2). Elevations in CO2 can accelerate plant growth and improve photosynthetic productivity.

Doubling CO2 concentration has been shown to increase total crop yield up to 30%. In order to convert maximum amounts of CO2 plants need to have good lighting and proper temperature to fuel the process. Photosynthetic uptake of CO2 and production of plant material in cannabis are optimized when temperatures are kept around 25°C. Plants can differentiate light based on its electromagnetic wavelengths. Light that is not visible to humans, like ultraviolet (UV), are visible to plants. Plants can also tell the time of the day from light, and based on the length of daylight, the can tell the season. In order to do this, they must be able to sense brightness and intensity of light, light exposure length, and location of light sources. Phototropism is a common phenomenon in plants where they bend towards light sources. There is evidence that blue light wavelengths help the plant locate the light source. Plants do not have a nervous system to translate light into pictures like humans, but they are very much able to “see” in a way that is as complex as human sight.

Cryptochromes are blue light receptors that plants and humans both have. Blue light is not photosynthetically active, but it helps cue the plant’s internal clock. This molecule becomes primed when it is exposed to red light, then phytochrome can absorb the next wavelength which is far red light (nearly infrared). Phytochrome regulates many stages of plant development like germinations, stem and leaf growth, and flowering.There is also evidence that phytochrome helps the plant sense temperature. Light is essentially waves of electromagnetic radiation which are measured on the electromagnetic spectrum. There are three ways to measure radiation on the electromagnetic spectrum: frequency, wavelength, and photon energy. Frequency is measured in hertz (Hz) and corresponds to wavelength which is measures in nanometers (nm) or micometers (μm). Light can have low or high frequency and wavelengths can be long or short. Based on the range of light’s frequency, it can be divided into different bands in a spectrum. Photon energy is the amount of energy carried by a single photon. Photons are packets of energy that have no mass, they are measured as an electronvolt (eV) or as joules or microjoules (1 joule = 6.24 x 1018 eV). Their energy is proportional to the electromagnetic frequency, and inversely proportional to the wavelength of light. If a particular frequency is high, the photon will have high energy. If a wavelength is long, the photon will have lower energy. Photons with the same color of light will have the same frequency, and the same energy. Intensity of the radiation is not captured by photons, so light from a campfire and light from the sun will have the same photon energy. The electromagnetic spectrum contains high frequency and high energy radiation like gamma rays, and radiation with low energy photons like radio waves. Photosynthetic light falls mostly in the visible light spectrum between 400 and 700 nm. Ultraviolet (UV) radiation falls between 100 nm and 400 nm. UV-C light at 100 nm to 280 nm is very damaging for plants, but UV-A (315 nm to 400 nm) will not damage plants. For cannabis, UV-B light between 280 nm and 315 nm may stimulate increases in THC production. PAR, PPFD, and DLI are common lighting terms that cannabis growers will encounter. Since light is not tangible, it can be hard to understand how it is measured. Measurements of light describe its different qualities like color, intensity, and the amount of energy it can deliver to the plant. PAR are the wavelengths of light that can be used in photosynthesis. PAR sensors can be used at different levels of the cannabis plant canopy if the grower wants to measure how much useful light is penetrating through the leaves.

The measure used for PAR is watts per square meter (W/m2). PAR tells growers the wavelength or color of photosynthetic light, but to describe the photon energy, or intensity of the light, another measure is needed. Photosynthetic photon flux density (PPFD) is expressed as μmol∙m-2∙s-1, which is micromoles per square meter per second. PPFD describes the number of energy particles (photons) in the photosynthetic active radiation (PAR) range that fall on a one square meter area in one second. Low PPFD irradiance may limit photosynthesis at the leaf surface. High PPFD irradiance in excess of photosynthetic needs at the leaf surface may be a threat to plant metabolism. A 2010 study found that the most productive PPFD for cannabis is 1500–2000 μmol∙m-2∙s-1 with environmental conditions at 25–30°C with CO2 concentration elevated to 750 ppm.

Under these indoor conditions photosynthesis in cannabis was optimized. Lighting with intensities of 1500–2000 μmol∙m-2∙s-1 can draw a lot of electricity and be very expensive to run.

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