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How Does the Far Red Light Spectrum Affect Plants?

far red spectrum led grow light

Recent advancements in LED grow lights have made it an exciting time to be an indoor grower. With cutting-edge spectrum variable lights, you can change the quality, flavor, potency, and yield of your crop just by adjusting the light spectrum over the course of the growth cycle.

Red light inspires flowering. Blue light ensures tighter internodes in vegetation and increases the concentration of oils and resins during flowering. And a blast of UVB in the finishing stage ensures heavy trichome production and greater potency.

But what about far red light? How does this less-discussed end of the light spectrum affect your crop?

Whether you’re growing at home or setting up a commercial operation, it’s important to understand how far red light can stunt growth . . . and to be aware of the one time when a blast of far red will serve you well.

First, let’s clarify what we’re talking about when we talk about far red light.

What is Far Red Light?

Most of the information you have about managing the light spectrum for your plants probably relates to PAR, or photosynthetically active radiation. PAR refers to the range of solar radiation (400-700 nm) that plants can use to create energy through photosynthesis. Generally speaking, this is the same range of radiation that is visible to the human eye.

While PAR is extremely important to plant growth, the overall development of your crops also depends on wavelengths that fall outside of the PAR range. UV, for example, cannot be used for photosynthesis, but plants can still sense it and respond to it. In fact, UV can be damaging for plants, just as it is for humans, and your crops defend themselves against those harmful rays by producing trichomes.

Far red light also falls outside the PAR range. With a wavelength of 700-850 nm, far red falls just between red and infrared light. Interestingly, your plants are extremely sensitive to the difference between red and far red light, and the balance between the two can give your crop very specific instructions on how to grow.

Here’s what we mean by that.

How Your Plants Perceive Far Red Light

 Your plants perceive red and far red light with the help of photoreceptors called “phytochromes.” Now, phytochromes have both an active form (far red absorbing) and an inactive form (red absorbing). Your plant switches between active and inactive phytochromes depending on how it perceives the red to far red ratio in its light source. You’ll see this ratio referred to as “R:FR.”

To put it in more direct, less scientific terms: when your plants perceive a higher ratio of far red light, they essentially flip a switch that cues certain biological functions.

And what are those functions? 

Well, that’s where things get interesting.

Plants Respond Differently to Far Red Throughout the Growth Cycle


The effect far red has on your plants changes depending on the maturity level of your crop. We’ll get into the details in a moment, but the headline is this. 

When your plants “see” more far red light than red light, they think they’re in the shade. 

It’s that simple. Everything we explain from here on out essentially comes down to that one idea. If you pump up the far red levels, your crops think they’re starving for sunlight.

Here’s what that does to their development:

The Effects of Far Red Light in Germination

This one is simple to explain. Seeds need sunlight to grow. Far-red light makes plants think they’re not getting sunlight. As a result, too much far-red on your seedlings may prevent germination altogether. 

The Effects of Far Red Light in Vegetation

If you’re getting stretch in the vegetation stage, too much far red could be the culprit. 

Because far-red makes your plants think they’re in the shade, they keep reaching upwards in search of the sun. Their leaves grow longer and wider and their stems elongate. The unhappy result is a long and lanky plant that’s too weak to hold up the healthy flowers you’re hoping to cultivate.

Another important downside of far-red is that it decreases chlorophyll and anthocyanins, both of which are responsible for your plants’ vibrant color. You also get a decrease in antioxidants, which protect both your plants and the people who consume them against free radicals.

Long story short: you cannot hope to turn out a decent product if your R:FR in vegetation is heavy on the far red. 

The Effects of Far Red Light in Pre-Flowering

 Okay, here’s where it actually gets fun. 

If you need your plants to get a move on with the flowering or you just want to increase yields, a temporary blast of far-red works wonders. Why? 

Using blasts of Far Red at the end of the day – 10 minutes while the lights as on and then to minutes while the normal lights are on – can keep plants flowering with less than the 12 hours of darkness that are normally required. Many growers claim that by using this technique they can get up to 20% more production per year by shortening the daily growth cycle this way. 20% of additional production can be huge! Once again, this is very strain-dependent. Different strains start flowering at different lengths of the dark period.

Another trick used by some growers is to use all far red at the end of the day and then wake the plants up with deep red in the morning. This puts the plants to sleep and wakes them up more efficiently maximizing daytime photosynthesis and nighttime metabolism.

Spectrum Control is Everything


There’s no doubt about it: indoor cultivation is a science. And as grow light technology advances, growers have more and more opportunities to play the scientist. Nurturing your crop is no longer about a flip of the switch—HPS lights on, HPS lights off. Now, with LED technology, you can guide your plants through their entire growth cycle, manipulate their chemical and flavor profiles, and—most importantly—ensure they grow up healthy and robust.

If you’re brand new to spectrum variable LED lighting, you can find more information about the best spectrum for your plants right here on our blog. We also invite you to learn more about the SolarSystem Controller, which allows you to pre-program your light spectrum for up to 365 days. But most importantly, please feel free to reach out any time if you have questions about LED grow lights or how you can give your plants the best possible care.

We’re always excited to speak with growers, and we wish you and your crop all the best. 


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