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The Influence of Spectral Light Quality on Cannabis Plant Growth

By Alex Fraser
Published: February 17, 2021 | Last updated: May 4, 2021
Key Takeaways

With a plethora of grow light options on the market, knowing what light to buy can be confusing. Alex Fraser explains how the different spectral light ranges impact cannabis plant growth and health.

Expert growers have always known that light plays a crucial role in plant growth and development, but until recently there weren’t many options regarding the color spectrum of lighting technology available. With the rise of affordable, high-efficiency LEDs the choices are practically endless, and manufacturers have rushed in to supply nearly every thinkable flavor of lighting, all claiming to have the ideal spectrum for fast growth and heavy yields. Fixtures featuring tunable spectra allow the grower to dial in custom light recipes, which often only adds to the confusion over what is best for the plant.

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To understand how spectrum influences plant health, it’s important to have some basic knowledge regarding light and photosynthesis. In the simplest terms, plants use energy harvested from sunlight to combine CO2 and water to produce sugars and oxygen. These sugars are used by the plant for energy and to build biomass. Rigorous university studies have shown that it takes a minimum of 10 photons (particles of light) to absorb one molecule of CO2. It turns out this rule holds true regardless of the color of photons the plant receives as long as they are within the visible range (400-700nm). In other words, at a given intensity, plants can perform about the same amount of photosynthesis under any color of light.

Spectrum plays a very important role in the shape and structural development of the plant; this relationship is known as photomorphogenesis and it is the subject of many ongoing studies. Although plant structure is separate from biomass production and yield, the shape of the plant can have an impact on how the plant receives light, and so the two are often interdependent.

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Graph of the visible light spectrum.The visible range of the light spectrum is between 400-700nm.

Spectral Light Ranges and Plant Development

Let’s take a look at the different spectral ranges of light and how they affect plant development.

Infrared (800nm-1mm) — The majority of photons produced by traditional light sources such as high pressure sodium (HPS) and metal halide (MH) are in the form of infrared (IR), which accounts for their inefficiency compared to modern LEDs. While infrared photons can’t be harvested for energy, they have the effect of warming up plant surfaces which increases transpiration and speeds up some metabolic processes. While it’s probably more effective to control plant canopy temperature through well-designed HVAC, many experienced growers have tailored their growing style to the effects of heavy IR. Understanding this concept is one of the biggest challenges for growers transitioning from HPS to LED.

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Far Red (700-800nm) — Most of the photons in the far-red range don’t contribute directly to photosynthesis, but combined with red photons they can increase the overall energy harvested by the plant through the Emerson Effect. Far red light also has an important role in shade-avoidance response. When plants are exposed to high ratios of far-red light they sense they are being shaded by taller plants. In response, plants increase cell division, causing stems to stretch and leaves to expand. While these traits aren’t always desirable, the ability to control shade response effects could help shorten veg times and increase yields in certain cultivars. 730nm far-red diodes, which stimulate both the Emerson Effect and shade-avoidance response, are readily available and relatively inexpensive.

Red (600-700nm)Red light doesn’t seem to steer plant structure and development, but when it comes to biomass production, red is a rock star. The 660nm “deep red” is the most efficient diode currently available in terms of energy used to photons produced. It’s not surprising many LED grow light manufacturers pack their fixtures full of these highly efficient red diodes.

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(Read also: Far-Red Lighting and the Phytochromes)

Green/Yellow (500-600nm) — Green is the most misunderstood color when it comes to plant lighting. Indoor cannabis growers have long known that green light can be used to illuminate grow spaces during the dark cycle without reverting their plants back to a vegetative state. That’s because green light doesn’t activate the plant’s light detection receptors, called phytochromes. However, as stated before, all photons in the visible light range count nearly equally toward photosynthesis, and green light is no exception.

Green light plays a powerful role when it comes to canopy penetration. While red and blue photons are quickly absorbed in the upper leaf layers, many green and yellow photons pass through the leaves to be absorbed by lower growth. The green/yellow-heavy ratio supplied by HPS lamps is one reason that technology has always been known for good penetration. Human eyesight is most sensitive to light at a wavelength of 555nm, so green light is important for visual purposes as well. Anyone who has worked under purple lights knows how difficult it can be to diagnose potential problems.

Blue (400-500nm) — Conversely to far-red light, blue light slows cell division and expansion. While that might seem like a bad thing, this effect helps produce sturdy, compact plants, and it’s the reason many growers love MH lamps for vegetative growth. Plants grown without any blue photons tend to exhibit weak, stretchy growth. One possible concern with blue light is its effect on the human circadian rhythm. Some studies have shown disruption of sleep cycles in subjects who were exposed to high levels of blue light. Growers who work in their gardens late at night should take this into consideration.

Ultraviolet (10-400nm) — While infrared light is what makes sunlight feel warm on your skin, it’s the high-energy photons in the ultraviolet (UV) range that damage skin cells, causing sunburn. Although ultraviolet photons don’t contribute much to photosynthesis, they do cause some interesting responses for resin and anthocyanin production. Many of these effects vary wildly from species to species and even cultivar to cultivar. Ultraviolet light also plays a role in pest and disease control.

Read also: Red Light, Blue Light: Balancing LED Efficiency with Performance

The Perfect Plant Spectrum Doesn’t Exist

If the quest for the perfect plant spectrum has taught us anything, it should be that there is no such thing. Each color range should be used as a tool to steer plant growth and development, but the effects on workers should also be taken into account. Today’s high-performance plant genetics were bred for decades under sunlight and HPS lamps, and experienced growers know how to get high yields in those conditions.

With all this said, grow light manufacturers should listen to the needs of growers and avoid trying to reinvent the wheel with a Holy Grail spectrum.

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