Far-red is the waveband from roughly 700 to 750 nm (some definitions run to 800 nm). It sits outside classical PAR, and the eye barely sees it. On its own it drives little photosynthesis, but mixed with shorter wavelengths it boosts photosynthesis through the Emerson enhancement effect. Zhen and Bugbee (2020) showed far-red photons up to 750 nm were about as effective as PAR photons for whole-canopy photosynthesis in several crops. Established outside cannabis.
In practice
Far-red is also a signal. A low red:far-red ratio reads as shade and promotes stem elongation and leaf expansion. Some growers use a short burst of far-red at lights-off (end-of-day far-red) to speed the switch to darkness signalling; cannabis-specific evidence for meaningful gains is limited. Peterswald and colleagues (2025) reviewed and tested far-red in medicinal cannabis with mixed outcomes on yield and quality.
Add far-red if you want faster canopy closure and can tolerate stretch. Skip it if you are short on height. Fixtures with far-red are often rated in ePAR (400–750 nm), which inflates the headline number compared with PAR-only ratings.
Don't confuse with
- Infrared (heat): thermal infrared is far longer wavelength (above about 3,000 nm); 730 nm far-red LEDs do not heat plants meaningfully.
- Red (660 nm): the most efficient photosynthetic band; far-red balances it.
