It’s no longer a surprise to most horticulturists that plants “see” light differently than humans. In fact, some of the light spectrum that the human eye is most sensitive to is used the least by plants. The light spectrum, which is a portion of the electromagnetic spectrum, is a representation of multiple electromagnetic waves with varying frequencies. These varying frequencies are how light colors are categorized. Light wavelengths are most commonly measured in nanometers (nm), which is equal to one-billionth of a meter.

The electromagnetic spectrum extends from very low-frequency radio waves to extremely high-energy gamma rays. The visible light spectrum is a small portion of the electromagnetic spectrum. Technically, “light” is visible electromagnetic radiation. Bracketed by invisible ultraviolet and infrared radiation, it is defined the Illuminating Engineering Society as “radiant energy that is capable of exciting the retina and producing a visual sensation.” The visible light range for humans falls between approximately 400 and 700 nm. Although the electromagnetic spectrum extends far beyond the visible spectrum of light, plants primarily use wavelengths in or close to the visible spectrum range. The photosynthetically active radiation (PAR) range for plants also falls in the 400-700 nm range.

By breaking down the light spectrum into bands determined by the wavelength, we can examine more closely how each band of light affects plant growth. In this article, we will go a little beyond the visible light spectrum and look at the 100-750 nm range. We’ll also look at three different plant pigments that capture some of this light.

100-280 nm: Ultraviolet-C (UV-C)

This shorter wavelength UV spectrum is absorbed by the ozone layer and does not affect plants in natural conditions. However, artificial UV-C radiation has shown potential in suppressing diseases on ornamental plants and extending the post-harvest life of cut flowers. The food industry has successfully used UV-C irradiation as a safe, environmentally friendly treatment for meats and horticultural products such as juices, fruits, and vegetables. Ultraviolet-C radiation has also been used as a pre-harvest treatment to make plants flower more quickly and to increase lateral branching. Experimentation has shown that a precise amount of UV-C exposure is critical. Too high an amount will burn the plants and too low will have little benefit.

It should be noted, however, that UV-C bulbs are harmful to both skin and eyes and should not be used in any fixture or application that could potentially expose humans or animals.

280-315 nm: Ultraviolet-B (UV-B)

Although it is outside the visible range of light humans can see, researchers have discovered that UV-B radiation can be beneficial to plants. Some plant varieties will naturally increase their production of essential oils when exposed to UV-B radiation. It is believed that many plants create essential oils as a defense mechanism against a variety of potential problems, including overexposure to UV radiation.

So, supplementing UV light in an indoor garden’s flowering room can trigger this natural response and increase a crop’s essential oil production. Metal halide-type lamps (metal halide, ceramic metal halide) naturally emit UV-B light, making them a good choice for UV supplementation. Keep in mind, however, that the glass in closed reflectors will stop most UV-B radiation from reaching the plants. Indoor gardeners with closed, air-cooled ventilation systems may want to supplement UV radiation by adding specific UV-B light fixtures. These are commonly sold in hydroponic, pet, and aquarium stores. An indoor horticulturist does not need a lot of UV-B light to get positive results; one-tenth the wattage of a growroom’s primary light fixture is good. Anymore is overkill. So, for each 1,000 watts, a grower should supplement up to 100 watts of UV-B light.

Finally, just as an excess of UV-C is dangerous for human skin, excess UV-B exposure can severely damage exposed plant cells by disrupting their DNA.

315-400 nm: Ultraviolet-A (UV-A)

The effect of UV-A on plants is not well-studied in comparison with UV-B, but it is still important, particularly given that horticultural luminaire manufacturers are beginning to include UV-A LEDs with wavelengths ranging from 365-380 nm.

Chlorophyll A, Chlorophyll B, and Beta-Carotene

Chlorophyll A, chlorophyll B, and beta-carotene are the three pigments found in plants that capture most of the light used for photosynthesis. It has been discovered that these pigments are most efficient at capturing light wavelengths that fall in the blue and red bands of light. Chlorophyll A, chlorophyll B, and beta-carotene are not very efficient at capturing green and yellow light.

400-430 nm: Violet

Both chlorophyll A and chlorophyll B absorb UV-A radiation and the violet range of light. Beta-carotene is much less effective at absorbing this range of light than the other two pigments.

430-500 nm: Blue

Peak absorption wavelengths for chlorophyll A, chlorophyll B, and beta-carotene fall in this range of light. Chlorophyll A’s peak absorption for blue light is around 430 nm, chlorophyll B’s peak absorption for blue light is around 453 nm, and beta-carotene’s peak absorption for blue light falls around 500 nm. The fact that the peak absorption for all three pigments falls in this wavelength range is why many LED manufacturers produce horticultural lighting systems that target this particular range of light. Metal halides, ceramic metal halides, and fluorescents also emit a good amount of light in this range.

It is important at this point to distinguish flux and energy. Light sources generate so many micromoles of photons per second, which is a measure of the flow (flux) of radiation. Each photon has a small amount of energy, and we can measure this instantaneous flux in watts. If we accumulate this energy for one second, we are measuring the energy in watt-seconds, or joules.

500-570 nm: Green

Even though chlorophyll A, chlorophyll B, and beta-carotene are not efficient at absorbing green light, plants do still respond to it. In fact, there are accessory pigments that harvest the light energy in this range and transfer that energy to chlorophyll, though not to the same degree of chlorophyll A, chlorophyll B, or beta-carotene.

570-590 nm: Yellow

Like with green light, chlorophyll A, chlorophyll B, and beta-carotene do not have a large response to the yellow light range. It is the accessory pigments in plants that harvest this range of light energy for photosynthesis.

590-620 nm: Orange

Both chlorophyll A and chlorophyll B absorb light from the orange band of the light spectrum. However, it is chlorophyll B that absorbs the most as it is most sensitive to the shorter lengths of red light wavelengths.

620-700 nm: Red

As previously mentioned, chlorophyll A and chlorophyll B have peak absorption ranges in both the blue and the red regions of the spectrum. Chlorophyll A and chlorophyll B have their peak absorptions of red light in the 620-700 nm range. Chlorophyll A’s peak absorption lies around 642 nm, while chlorophyll B’s peak absorption lies around 662 nm. High pressure sodium (HPS), double-ended HPS, and LEDs all target this the red range of light and most effectively match chlorophyll A and chlorophyll B’s peak absorption range to the red-light spectrum.

700-750 nm (730 nm): Deep Red

Discoveries in the way a plant rests and processes light energy have led to the use of specific wavelengths to help trigger a plant’s resting period. An intense exposure to far-red light at the start of the dark cycle in a flowering room reduces the amount of time the plants need the darkness. In other words, indoor horticulturists can use supplemental far-red lighting systems to stimulate the plant’s phytochrome and trick the plants into resting more quickly. This technique reduces the amount of darkness required per 24-hour cycle. This is an advantage for growers because they can then extend the light cycle without interrupting the plant’s flowering response. Additional light hours equate to more energy for the plant, which, in turn, creates more flower growth and larger yields.

Breakthroughs regarding how particular light frequencies affect plant growth are occurring all the time. Our increased understanding of PAR and the peak absorption rates of chlorophyll A, chlorophyll B, and beta-carotene have pushed horticultural lighting technologies to become more efficient and effective. Whether it be UV-B to increase essential oil production or deep red light to extend the lighting cycle in the flowering stage of growth, science has discovered that even light frequencies that fall outside of the PAR range can still be beneficial to plants (and the cultivator). Indoor horticulturists have ultimate control over the growing environment, including control over the light spectrum. As our knowledge of the light spectrum and how it affects plant growth continues to grow, there will be an increase in frequency-specific lighting systems used by indoor growers. As of now, LED lighting technologies hold the most promise for frequency-specific horticultural devices. It’s hard to say exactly what the future of horticultural lighting will hold, but one thing is certain: wavelength-specific lighting systems that enhance quality and increase yield will be embraced by the indoor horticulture community.