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Far Red Light

Far Red Light

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Plant Science Topics:


-Phytochrome and balance of Pr:Pfr

-Promote flowering in short-day plants when used at end of each light cycle

-Stem elongation


Recommendations to Apply the Science:


-Earlier flowering in long day crops

-Stem elongation for cut flowers

-Stem elongation for grafting

-Less compact plants better for oil extraction

-How to reduce far red or select lights with correct balance of red to far red


Product References:


-powerPAR Far Red LED

-Sunburst MH

-Mini Sunburst HPS

-Phantom DE

-Solar System 550

-Solar System Far Red

-powerPAR Commercial 4 ft LED Fixture

-Active Aqua Stand, Light Hanger, Low Profile Legs, and Flood Tables



Barely visible to us, huge impacts on plant growth (gesture to show increase in height when saying big). This episode is going to be a bit technical, but please hang in there because learning the science and practical applications of far-red light can be a huge help to growers. I’m talking about faster flowering times, better cuttings, better grafted plants and all at very low light levels. Far-red lights offers huge payoffs with little input.




In episode 6 I introduce the plant pigment F, and mentioned how it regulates plant responses based on ratios of red to far-red light.


Well there are a lot of ways growers can manipulate this rati to their benefit, but before we get into the application, let’s do a quick lesson on the science.



Phytochrome is like a switch. It has two forms, like a light switch it can be on or off.


Let’s start it in the off position. This position is called Pr, or phytochrome red, in this form it absorbs red light. Graphic saying Pr absorbs red 600-700 nm


(turn on red light)


When Pr receives red light it turns on, to it’s active state, called Pfr, or phytochrome far-red.


Pfr is the active form of phytochrome, but, plant reactions actually occur when phytochrome is in either form Pr of Pfr, it’s a bit of semantics, but any who


Pfr, absorbs far-red light, (Graphic saying Pfr absorbs far-red 700-800 nm) and when it does, it switches off, back to Pr, which is called the inactive form.


Imagine there are a lot of of these switches in a plant, and the ratio of red to far red light will affect the ratio of phytochrome forms, which will affect phytochrome regulated processes.


Let’s look at the balance of red and far-red light in natural settings. First day time, in full sun, (jump cut)


So in general full sun, the dominance of red light will turn the Pr to Pfr, the active form. One of the most common phytochrome regulated processes that can be observed in full sun is the inhibition of stem elongation. Pretty much, compact plant growth. Let’s look at another situation…


Now let’s look at shade in a forest. The leaves above absorb a lot of the red light, that’s one of the main colors of light that powers photosynthesis, so they gobble it up. So what’ we end up with down here below the canopy is not much red, but a lot of far-red. This means that phytochrome is mostly in the inactive form. The far-red light turns the Pfr into the inactive Pr. If this is all getting a little confusing, just hang on for a second, we’re almost out of the woods… with phytochrome in the inactive form, it doesn’t inhibit stem elongation, so the plant elongates. Why? Well a plant in shade wants to reach out to get to the full sun, it wants that nice light, heavy with red that will power photosynthesis, so it grows tall to reach it. And let’s look at one more situation.


Sunset. During sunrise and sunset, the light travels further through the atmosphere and this changes the spectrum, you get more far-red. In this case, the far-red signals to plants that night has started. Earlier in this series we talked about photoperiod and how it really is the length of night that affects most of the photoperiod responses, well, growers can use far-red to hijack this process. Let’s get to the practical applications of far-red light.




Most important impact for many indoor growers in speeding up flowering time. Used end of day for both short day and long day flowering crops.

“I have consistently decreased flowering time by 10-14 days across many different strains. In doing so, I achieve a faster time to market while still achieving better than average yields,” Commercial Grower and Washington state licensee.


Shade avoidance, and show light change under plant canopy and how far-red penetrates canopy


Show far red and tomatoes and increase in stem elongation. Great to get more space for grafting or cutting clones

Stem elongation, due to cell elongation.  as shown earlier. Also good for cut flowers. (Get prop flower)


Some growers perfer less compact plants for oil extraction, sometimes easier to work with. And they can achieve better air flow through less compact plants.


Previous episode showed effects on germination


Let’s look at products for adding far red.


The applications of far-red can be considered in two ways, looking at increase far-red to get the morphology described earlier or balancing red to far-red to get normal growth.


Lights that deliver a good balance of red to far red include sunburst MH, or HPS, or DE HPS, or CMH or T5 or white LEDs. and many red LEDs don’t deliver any far-red which can also have advantages


To increase far-red, some options include powerPAR Far Red, or more powerful options like the California Lightworks far-red. Can be placed indoors or in a greenhouse. Used as a end of day treatment is most effective generally so it doesn’t complete with existing light, but during the day can also work under low light levels. When used as end of day is generally does not need high intensity, a little light can cover a wide area when positioned high above the canopy.


This has been episode 7 of Plants and Light thank you for watching.. For more information on horticultural lighting visit In the next episode it’s gonna get hot, with infrared light. I’m Farmer Tyler and the more you know, the better you grow!