Far-Red Light - Somthing You Should Know About

I got a lot of this from Bruce Bugbee, Professor of Crop Physiology at Utah State and president of Apogee Instruments, makers of light sensors and meters, including red - Far-Red and PAR-far sensors.

One reason Far-Red light was neglected for so long is that, until LED’s, electric lights had no Far-Red. When the late Keith J. McCree of Texas A&M University defined the PAR region to be 400nm to 700nm, he had no single source of any chosen wavelength of light. He had to work with prisms and filters, which would have been remarkably difficult for Far-Red, by which we mean 700 nm to 760 nm. Today, you can buy LED’s made specifically to provide Far-Red. In addition, because of this limitation, he could not study the synergistic effects of multiple colors, and as we will see, synergism of low-energy Far-Red with high-energy red light in photosynthesis is an important issue. And McCree had no way of knowing that value could be assigned to Far-Red’s influence on photosynthesis.  

Far-Red light turns out to have important value to plants. So much so that there are serious suggestions to redefine PAR. The following seems like jumping ahead, but I think it’s an important illustration of the value of Far-Red light. It involves what looks like a paradox.

When plants are grown under LED grow lights (it doesn’t matter whether white or blurple), a light with added Far-Red providing 300 PPFD will, by measure of lettuce weight, outgrow a light providing 350 PPFD that has no Far-Red added.

A lot of the rest of this is about why.

Here are depictions of various light sources and the relative intensities of their spectra.

The first thing to notice is that sunlight is rich in Far-Red light. Traditional PAR light falls off sharply at Far-Red. Sunlight emits almost as much Far-Red radiation as red light. An interesting phenomenon is the spectrum shift of sunlight at dawn and dusk when the whole spectrum shifts in the red direction, toward longer wavelengths. You’ve seen it in the red dawn and dusk. Far-Red and red dominate. That has a practical application. Providing that spectrum shifted to red and Far-Red makes plants think night is falling. Plants will enter night mode, enhancing flowering.

We know a lot today about how plants use various wavelengths of light but still have a lot to learn about practical applications. Red has long been known to be essential for photosynthesis. Blue light inhibits cell expansion and so limits stem growth, preventing leggy unproductive plants.

Green light, which is heavily present in sunlight, was once assumed to be unimportant, since it obviously was reflected by green leaves. And while it does not penetrate as readily as red and blue, it actually penetrates quite well and contributes to photosynthesis.

Green photons have value to the plant but are also today being recognized as being of practical value to the grower. Grow lights with the green wavelengths filled in act as white light visually and make finding pests and identifying signs of plant disorders easier, as well as simply being more comfortable to work under. Blurple is still more electrically efficient for growing, but the gap is closing.

So what does Far-Red do? Far-Red is low energy but high effect because of the number of photons does not drop off as energy does. It only takes 10% added Far-Red to make a measurable difference. It also penetrates like green light. The value of Far-Red is prompting calls to redefine PAR based upon photons, rather than energy. Far-Red may be low-energy light, but one of the most important physical laws, the Stark-Einstein Law, holds that one photon excited one electron. Energy is not a factor.

You can express PPDF on the basis of energy or photons. We usually make PPFD measurements by referencing energy. It doesn’t matter with red and blue. In red and blue, energy PPFD is about equal to photon PPFD. But in Far-Red, photon PPFD is higher than energy PPFD. Our measurements mislead us. And that is why the lights with Far-Red measuring 300 PPFD outgrew lights without Far-Red measuring 350 PPFD.  

Here's the energy distribution with Far-Red.

Now look at the solar spectrum plotted by PPFD (photons) with McCree’s curve showing what was assumed of the effects from plotting by energy.

 

One of Far-Red’s key roles is that it gives the photosynthesis process a kick, boosting it over what would have been done with red alone. Far-Red can’t do the photosynthesis job alone, but it can greatly enhance the work red is doing. This was something McCree could not observe because he could not apply multiple wavelengths to leaves at once.

Far-Red also enhances cell expansion, the opposite effect of blue light. And Far-Red penetrates so well that Far-Red photon flux in shade is nearly as high as in open sunlight. Leaves absorb most of the red light but reflect or transmit the Far-Red.  So plants assume that getting more Far-Red than red means shade, and they invoke a shade response. What that response is depends on whether the plant is shade tolerant or a shade avoider. A shade avoider will try to grow up and out of the shade. Not a bad thing in come plants, but not very good at all in a tomato.

This chart plots both sunlight in the open and light under foliage shade. Note how little Far-Red is lost. So for plants, Far-Red without red means shade.

Shade tolerant plants try to grow bigger in the shade to capture more light. Lettuce is a shade tolerant. An added 10% of Far-Red increases lettuce growth dramatically due to the shade response and the synergism of Far-Red with red in powering photosynthesis. Good for spinach. Not so good for cucumber.

Tomato is a shade avoider and so grows taller under the influence of Far-Red. This is not necessarily a good thing for all purposes, but the following image from North Carolina State Horticultural Science is too good not to use as a dramatic demonstration of the effect. BR means BLUE+RED. BRFR means BLUE+RED+FAR-RED. Note that they are not “leggy,” just tall.

As we said earlier, Far-Red can be used in many plants in their early growth stages and can bring plants to flower sooner. That is perhaps its most significant practical application. And using Far-Red to induce the shade response is a benefit in shade avoiding plants where less compact plants are more useful for oil extraction. And the benefit for long-stem flowers is obvious. All of these effects can be quite striking.

I should point out that this does not produce pathologically leggy plants with skinny stems we see with a general lack of light. Stems in plants growing taller in shade response to Far-Red are simply taller, not skinnier. There is no weakness.

Far-Red is not measured by PAR sensors. Spectral radiometers do but are expensive. Apogee has a new sensor that covers Far-Red. The PAR-FAR sensor alone is $500.

Far-Red light has uses that are only now being explored. Plant reproduction follows seasons. Many plants regulate their flowering by the length of night. We talk about short-day plants and long-day plants, but we should be talking about long-night and short-night plants. Flowering of short-day plants can be inhibited by extended light hours of red with Far-Red. Remember, to plants, red plus Far-Red means daylight conditions. But shortening nights with Far-Red only does not inhibit flowering. In some species, it can enhance flowering.

In some long-day plants, using Far-Red light in a growing environment can shorten the time for flowers to set. Mixing in Far-Red increases plant size and boosts photosynthesis and improves energy production. And some plants have been found that will not flower at all if grown without Far-Red.

If you want to begin exploring, Amazon offers a number of Far-Red lights. I’m not going to pretend to expertise, and I notice no one likes to commit to just how to use them. But I would go back to the knowledge that dramatic size gains in lettuce were realized when “10%” Far-Red was added. But it seems clear that it does not require a great deal of additional light. (Sunlight contains about 20% Far-Red. Common LED’s around 2% to 3%.)

I’m not going to get into specific products. For one thing, like most emerging topics that are routinely hyped, product availability will likely be explosive. It’s a bit easier than choosing conventional grow lights in one regard. We are interested in a fairly narrow spectrum. But we are more dependent on what the seller tells us. Far-Red is almost invisible, and some lights are billed and red and Far-Red. We can, of course, see the red. How much is Far-Red, we can only guess. If you thought a PAR meter was expensive, try a Sekonic C-7000 device that will display a full spectrum by energy. But cost of lights is not extraordinarily high, so it’s not too expensive to play.

Sekonic C-7000 Spectromaster – $2,2000

To be clear, if you are using LED grow lights, either white or red-blue, you are already providing some Far-Red. But the comparisons I’ve discussed were made on the basis of adding Far-Red to those conventional LED lights. We often think little of the low wattage LED array bulb type lights for standard base sockets. They are inadequate as a basis for overall grow lighting. But because it doesn’t take a lot of Far-Red to make a difference, the low wattage specifically Far-Red arrays may do the job.

If you grow in a greenhouse or outdoors, you will not realize all the benefits of continuously adding Far-Red. The sun provides lavish amounts, and it passes easily through greenhouse material. But you can still make use of Far-Red in inducing night cycle effects and apply the shade effects. And because Far-Red is not energy-efficient, large growers desiring improved yields and profits may be better served by investing in conventional grow lights.

As a subject, Far-Red is likely to become somewhat buzzy and hyped much like beneficial microbes without much solid practical research. We may be left to find our own way for a while. But you should at least now know enough to investigate.