Dr. Bruce Bugbee from Utah State University proposed that the growth of cannabis is affected by 9 environmental factors, namely, light, CO2, water, nutrition, temperature, humidity, root-zone temperature, oxygen, and wind. Now I want to talk about light.

Light plays an important role in the entire life cycle of plants. The mechanism is that plants complete the photosynthesis process through the excitation of external photons, thereby affecting their growth, development, morphology, establishment, fruit coloration, material metabolism accumulation, gene expression, etc.

There are three fund-amental dimensions of light: light duration, light quantity and light quality. Multiple studies have shown that the control and intervention of photoperiod, light intensity, and light quality can achieve the purpose of controlling the yield, flowering, and morphology of the plants.

 Photoperiod

Photoperiod refers to the interval in a 24-hour period during which a plant or animal is exposed to light. Photoperiodism can also be defined as the developmental responses of plants or animals to the relative lengths of light and dark periods. They are classified under three groups according to the photoperiods: short-day plants, long-day plants, and day-neutral plants.

Cannabis is a short-day plant. This means that they have evolved the ability to synchronize their physiological flowering response to seasonal changes in day-length to optimize reproductive success.

Cannabis plants have two important phases: the growth stage and the flowering stage. 

While cannabis plants are in the growing stage, keep them under at least 18 hours of grow light, 6 hours of darkness (also known as 18/6), or 24 hours (24/0) of indoor light.

For an indoor grower, when a plant is about half the final size you want it to be, you should change it over to the "Flowering" stage. This is the stage when your plant starts growing buds. You do this by changing your light.  In the flowering stage, most indoor growers choose to have their grow lights on for 12 hours and then off for 12 hours (12/12 ) 

What you also need to know is that some plants, such as autoflowering cannabis, produce flowers based on their age and not light. 

Light intensity

Light intensity refers to the amount of visible light received per unit area, which is used to indicate the intensity of light and the amount of illumination on the surface area of an object.

The light intensity has a great influence on the photosynthesis of organisms. When you give a plant more light, this can theoretically increase photosynthesis and result in more biomass.

All Cannabis can be treated as high light intensity tolerant since all have been bred outside for an extended period of time during the history of the plant's domestication process. For the most part, an outdoor grown plant is been bred and allowed to be able to handle the intensities of natural sunlight typically is a lot higher than what plants would be experiencing in an indoor environment.

Some indoor growers increase the light intensity with age if the plant requirements are kept optimum.

Light intensity for plant growth is typically measured as photosynthetic photon flux density (PPFD). The unit is umol/m2s. Generally, PPFD is greatest around the center and decreases along the sides and in the corners. When light leaves a source, it spreads out in all directions as a sphere. As the sphere of light spreads out in all directions, it is carrying the same amount of power, but its area is increasing. This means that its intensity is decreasing as it spreads out. PPFD can be measured with a quantum sensor. In growth rooms, PPFD is best measured with growth chamber doors closed, often at the top of the plant at the height of the uppermost fully expanded leaves. It is best practice for the user to be some distance away from the sensor head, enough to ensure they themselves are not blocking or shading any of the PPFD. Many indoor growers choose 700-1000umol for their cannabis cultivation, but studies have shown that cannabis yield increases linearly as PPFD reaches 1800 μmol·m -2 ·s -1, and when PPFD exceeds 1800 μmol·s m -2 ·s -1, and the yield decreases linearly.

All plants have a point where light becomes excessive and will damage the plant – referred to as the light saturation point. Therefore, it is important to know the optimal DLI for the crop. Daily light integral (DLI) is the amount of PAR received each day as a function of light intensity and duration(day). The unit is mol/(m^2 d). If plants can acclimate to a given PPFD, then DLI, through photoperiod adjustment is often a better predictor of overall plant growth. The DLI value required by cannabis plants varies during the growth cycle. Because cannabis seedlings require less light, whereas a cannabis plant flourishes when supplied with a lot of light in its growth stage and flowering stage.

Calculating the DLI needed for cannabis can help you grow cannabis better.

The formula for calculating DLI is: μmol m-2s-1 (or PPFD) x (3600 x photoperiod) / 1,000,000 = DLI (or moles/m2/day).

Assuming PPFD is 1000umol/m^2/𝑠 and photoperiod is 12 hours, then DLI= (3600*1000umol *12h) /1,000,000 =43.2mol/(m^2d)

Light quality

Light quality is one of the conditions that affect plant photosynthesis. Light quality is the wavelength (in nanometers, nm) within the electromagnetic spectrum. In other words, light quality refers to the spectral distribution of light, or the relative number of photons of blue, green, red, far-red, and other portions of the light spectrum emitted from a light source. Blue light, green light, and red light comprise the spectrum of light primarily used for photosynthesis.

Different light quality or wavelength has significantly different biological effects, including different effects on plant morphological structure and chemical composition, photosynthesis and organ growth and development. 

1）Blue light(445-500 nm)
Blue light with shorter wavelengths can provide energy and catalysis for chemical reactions in photosynthesis, while inhibiting plant elongation and promoting cell differentiation. Blue light is one of the necessary conditions for plants to thrive.

2）Red light (R, 620-700 nm)
The wavelength of red light is longer, which mainly provides power for plant photosynthesis, which is conducive to plant photosynthesis and chlorophyll formation, and is conducive to the growth of stems and promotes plant flowering. 

3）Green light(G, 500-580 nm)
Green light has always been a controversial light quality, and some scholars believe that it can inhibit plant growth, cause plants to dwarf, and reduce vegetable yields. However, there are also many studies on the positive effect of green light on vegetables. A low proportion of green light can promote the growth of lettuce; adding 24% of green light on the basis of red and blue light can promote the growth of lettuce.

4）Far Red(FR, 700-775 nm)
Far-red promotes extension growth. Although we can barely see far-red radiation, it has a major effect on extension growth, meaning it influences the size of leaves, the length of stems, and ultimately the height of plants. But there really isn’t much photosynthetic activity occurring by far-red light alone. There is a big decrease in photosynthetic activity when the light goes above 700 nanometers, which is the far-red light region. That is the reason that those light wavelengths have been ignored. But the photosynthetic rate is boosted when red, blue and far-red light are combined. Far-red, blue and red light have a synergistic effect. 

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