Who needs green?
Of the four wavelengths of major importance in lighting for controlled environment agriculture, a wavelength in the green part of the spectrum (in the range of about 495 to 570 nm) does a few important things for you, but not much for the plants.
Green light is currently the least efficient to produce, relative to red, blue, and far red, and it also contributes the least to photosynthesis. At best, its energy is downconverted in the plant to activate the red photosynthetic pathway. It is often touted as being able to penetrate foliage to reach underlying leaves, but there are better ways to reach these parts of the plants. Also, in some cases green light actually inhibits plant growth.
Most LED lighting manufacturers use cheap white-light phosphor LEDs to produce green in their spectra, but these LEDs also emit blue, yellow, orange, and red in varying amounts, and their spectra degrade over time. People who want reproducible results should opt, instead, for narrowband green LEDs that do not corrupt the other parts of the spectrum and can produce calibrated outputs.
So, here is what you need green light for.
To get good color rendition for humans to inspect the plants and to have pleasant working conditions, green light in addition to red and blue is necessary, but it is only needed when humans are present and is wasteful of energy when humans are not present. The same is true for automated imaging and spectral reflectance measurements: the green light is needed only while measurements are in progress.
Some short-day plants, such as cannabis, will stop blooming or will hermaphrodize if their dark periods are interrupted with growth light. However, narrowband green light at about 520 nm can be used to allow night work and inspections during dark periods without producing these yield-ruining consequences.
Although the research has yet to be done, green light may have some useful effects on soil microbiome, beneficial fungi, etc., for which photosynthesis is not key. Also, there can be subtle effects on terpene expression that may in rare cases contribute value. So, well-controlled narrowband green light may be of use to researchers.
In conclusion, green light is inefficient and should be used sparingly, preferably in a well-defined narrowband format, for inspection, dark work, and research.
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