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The Definitive Guide to Grow Light Spectrum

AChoosing the right grow light spectrum for your commercial operation can be a challenge. Many LED grow light suppliers have conflicting information on the topic due to bad marketing or simply a lack of knowledge in plant and light research.
In this article, our light spectrum experts break down what light spectrum is, how plants respond to light, and how light spectrum influences plant growth.

What is Grow Light Spectrum?

Light spectrum is the range of wavelengths produced by a light source. When discussing light spectrum, the term ‘light’ refers to the visible wavelengths of the electromagnetic spectrum that humans can see from 380–740 nanometers (nm). Ultraviolet (100–400 nm), far-red (700–850 nm), and infra-red (700–106 nm) wavelengths are referred to as radiation.
As growers, we’re most interested in the wavelengths that are relevant to plants.  Plants detect wavelengths that include ultraviolet radiation (260–380 nm) and the visible portion of the spectrum (380–740 nm) which includes PAR (400–700 nm), and far-red radiation (700–850 nm).
When considering light spectrum for horticultural applications, greenhouse and indoor environments will differ.  With indoor environments your grow light’s spectrum will account for the total light spectrum that your crop receives.  Whereas in a greenhouse you must consider that your plants are receiving a combination of grow light and solar spectrum.
Either way, the amount of each waveband that your crop receives will have significant effects on growth.  Let’s learn more about how this works.

How Do Plants Respond to Light?

Plants use light for photosynthesis and photomorphogenesis. Photosynthesis is the process by which plants and other organisms convert light energy into chemical energy. Photomorphogenesis refers to how plants modify their growth in response to light spectrum.
One example of photomorphogenesis is a plant bending toward a light source. Light also affects plants’ developmental stages, such as germination and flowering.
The light that plants predominately use for photosynthesis ranges from 400–700 nm. This range is referred to as Photosynthetically Active Radiation (PAR) and includes red, blue and green wavebands.
Photomorphogenesis occurs in a wider range from approximately 260–780 nm and includes UV and far-red radiation.

Why Use Light Spectrum When Growing Crops?

Plants have photoreceptors that can trigger different growth characteristics when activated by photons of specific wavelengths.  So by controlling your light spectrum you can affect powerful changes in plant growth.

Growth characteristics that can be affected using spectrum are listed below:

  • Fruiting
  • Flowering yield
  • Rate of growth
  • Fresh weight
  • Compactness
  • Root development
  • Plant health
  • Color
  • Flavor
  • Nutrition

It's important to note that activating plant response using light spectrum is one component of a larger process and results are heavily dependent on many factors such as light intensity, photoperiod, growth environment, plant species, and even plant variety.

How Does Each Light Spectrum Affect Plant Growth?

Although results are dependent on other factors, there are general rules of thumb that you can follow when using light spectrum to elicit different plant responses.
Outlined below is an overview of how each waveband is used for horticultural purposes so that you can trial light spectrum strategies in your own growth environment and with your chosen crop varieties.

How to Use Light Spectrum for Plants?

  • Red Light: More red light tends to induce more biomass growth and stretching.  Red light is often applied to bulk up plants in early development or to stretch plants when longer internodal spacing is desired.
  • Blue Light: Higher ratios of blue light are a powerful tool for improving plant quality.  Improvements to biochemical processes often occur when more blue light is present, resulting in better nutrition, color, root development, and overall quality.  Deploying higher ratios of blue light often means less total PPFD to the crop, so these strategies should be used strategically and sparingly.
  • Green Light: We understand that green light is important to photosynthetic efficiency and plant development, though these processes are still being explored.  Adding supplemental green light is most important when there is no sunlight to provide adequate green light to crops.  The best pink LED grow lights take this into consideration and offer an adequate amount of green within their pink spectrum.

About UV and RI spectrum

UV Light Wavelengths (100–400 nm)

The UV waveband is outside of the PAR waveband and may offer new applications for horticulture that have not yet been well defined.

It’s easiest to think of UV radiation in similar terms to its effect on human health.  We all know that you can get sunburned from extended exposure to UV light, while short exposures often result in a tan instead.  In this respect, plants and people react similarly to UV light.

Like people, plants can become damaged from exposure to UV radiation. Plants also naturally elicit protective compounds to mitigate UV tissue damage.  In response to UV, plants may turn darker or more purple. Studies have shown that UVB light can enhance essential oil content and phenolic compounds in some herb species.

Potential of UV light include increased leaf coloration and thickness, as well as resistance to environmental stress, pests, and fungus. The amount of UV required to achieve these potential benefits is not well defined. Additionally, the dangers associated with UV are not yet well quantified

Far-red Light Wavelengths (700–850 nm)

Far-red light is at the far end of the red spectrum between 700–850 nm. Studies have found that plants respond to wavelengths up to 780 nm. Recently there has been increased attention and research related to the potential for far-red light to increase and control growth. Far-red light can induce a shade avoidance response, which results in extension and stretching (read below for more information on shade avoidance).

Far-red light also promotes flowering in long-day plants and leaf expansion, which increases the available surface area to capture photons for photosynthesis. There have also been recent reports that far-red can increase efficiency of the PAR waveband associated with photosynthesis.

It’s important to consider how phytochromes, a class of photoreceptors, perceive the ratio of red to far-red radiation (R:FR). Phytochrome-mediated regulation is a complex process that can have a profound impact on extension growth and flowering. The ratio of blue to red light present can also impact how plants respond to far-red radiation.

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