Light intensity, PAR and DLI, is the quantity of light, how much energy the crop gets. Far-red is about quality: the color makeup of that light. It sits just past red, around 700 to 780 nanometers, at the edge of what we'd call visible, and it was ignored for years because the standard light meters didn't even count it. Yet the plant reads far-red with precision, and using it deliberately is one of the most underused levers in the room.
Plants sense far-red through a pigment called phytochrome, which flips between two forms depending on the balance of red and far-red light hitting it. The plant doesn't read far-red in isolation; it reads the ratio of red to far-red, and from that ratio it infers things about its world: whether it's in open sun or under a canopy, whether neighbors are crowding it, and what time of day it is. That single ratio drives decisions about stretching, branching, flowering, and more.
Quantity vs. quality.
This is why far-red belongs alongside intensity rather than buried inside it. You can give a crop exactly the right DLI and still get the wrong plant, leggy when you wanted compact, vegetative when you wanted flower, because the spectrum sent the wrong message. Quantity feeds the plant; quality instructs it. Far-red is the clearest single example: a small change in the far-red fraction can reshape a crop without changing how much total light it receives.
The shade signal the plant reads.
In nature, leaves absorb red light for photosynthesis and let far-red pass through and reflect, so the light under a canopy is rich in far-red, low in red. A plant sensing a low red-to-far-red ratio concludes it is being shaded by competitors and responds with the shade-avoidance reaction: it stretches stems to climb above the competition, raises leaves, and often shifts toward flowering. High red-to-far-red, the signal of open sky, keeps it compact and leafy. Growers use this directly: add far-red to deliberately stretch or trigger generative behavior, withhold it to keep transplants short and stocky. There's a bonus on the photosynthesis side too, the Emerson enhancement: far-red added alongside red drives more photosynthesis than either color does alone, so the right amount can lift growth as well as shape it.
A lever you set, not a number you watch.
Most inputs on this site are conditions you read and react to. Far-red is different: it is mostly a setpoint you choose, by crop and growth stage, the way you'd choose a temperature or a feed strength. Compact, stocky transplants want a high red-to-far-red ratio; a stretch phase or a push to flower wants added far-red; an end-of-day far-red pulse can be used to manage photoperiod and morphology. Because it's a setpoint, far-red is fundamentally a recipe parameter, part of the light recipe that moves with the crop's life, which is exactly where the open grow-recipe schema carries it: a far-red fraction defined per crop and stage, no lighting brand required.
How to read it.
The catch is that most inexpensive "PAR" meters stop at the red end and never report far-red, so a grower steering by a basic sensor is blind to the very lever the plant cares about. Reading it takes a sensor that resolves the far-red band: a quantum sensor with extended range (Apogee's ePAR sensors, for instance, extend to 750 nm to capture it) or a small spectral sensor that reports the red and far-red channels separately. Apogee is a welcome ingestion target here: its sensors offer open outputs (analog, 0–5 V, 4–20 mA, SDI-12, Modbus), so the reading lands in oat-ods through the gatherer like any other, and the red-to-far-red ratio becomes a number you can both verify and target.
The trap: flying blind on the lever the plant reads.
The trap is ignoring far-red because the meter doesn't show it, then wondering why a crop stretches or stays stubbornly compact under a "correct" light level. The other trap is adding far-red indiscriminately, stretching plants you wanted bushy. Far-red is a tool with a direction: decide the morphology you want, set the red-to-far-red ratio to match, and verify it with a sensor that can actually see that end of the spectrum.
Frequently asked questions.
What does far-red light do to plants?
Plants sense far-red light (about 700 to 780 nm) through the pigment phytochrome and use the ratio of red to far-red to judge whether they are shaded, crowded, or in open sun. A lot of far-red (low red-to-far-red ratio) signals shade, triggering stretching and often a shift toward flowering; little far-red (high ratio) keeps plants compact and leafy. Far-red added alongside red can also raise photosynthesis (the Emerson effect), so it shapes the plant and can boost growth at the same time.
What is the red to far-red ratio?
It is the balance between red light and far-red light reaching the plant, and it is the actual signal plants respond to rather than far-red on its own. Under a canopy, leaves absorb red and let far-red through, so the ratio drops and a plant reads it as being shaded. In open sun the ratio is high. Growers adjust this ratio deliberately to stretch or compact a crop and to influence flowering.
Why doesn't my PAR meter show far-red?
Because traditional PAR meters were built to measure photosynthetically active radiation from 400 to 700 nm, and far-red sits just past that range. So a basic PAR or DLI reading tells you the quantity of light but nothing about the far-red fraction the plant reads for shape and timing. Capturing far-red requires an extended-range quantum sensor (such as an ePAR sensor reaching to 750 nm) or a spectral sensor that reports the far-red channel separately.
Should I add far-red to my grow lights?
It depends on what you want the plant to do. Adding far-red encourages stretching and can push a crop toward flowering, useful for stem elongation or generative steering, while withholding it keeps transplants short and stocky. End-of-day far-red can be used for photoperiod and morphology control. Treat it as a setpoint chosen by crop and growth stage, not something to add indiscriminately, since the wrong amount stretches plants you wanted compact.