Far-red radiation — often called far-red light — can be defined as photons with wavelengths from 700 to 800 nanometers (nm). Humans can barely see far-red radiation. Far-red promotes the growth of plants.
Far-red radiation has a major effect on the growth of plants. It influences the size of leaves, the length of stems, and the height of plants. Plants perceive light quality through the pigment phytochrome, which exists in two different forms. One form is called the red-absorbing form (PR) and the other is the far-red absorbing form (PFR). There is an overlap in the absorption of PR and PFR. Therefore once plants are exposed to light, phytochrome always exists in these two forms. The proportion of red and far-red radiation affects how much phytochrome is in one form or the other. Plant elongation increases if the amount of far-red radiation increase, plants are more compact if the amount of red light increase. Sometimes in flower growing an elongation response is desirable but in the production, while often it is not. There are different possibilities to reduce the elongation: A. limiting the density of hanging baskets overhead, B. using wider plant spacing, C. supplemental lighting that emits little or no far-red, D. using spectral filters that reduce transmission of far-red from the sun.
For some flower plants, far-red light promotes flowering. When the natural days are short, low-intensity lighting is often delivered to promote flowering of long-day plants. For some long-day plants, flowering is quicker when photoperiodic lighting includes both red and far-red radiation. In that case, lamps that emit red and far-red radiation are advised.
Unfortunately, there is no way to deliver light that is effective at promoting flowering without the accompanying of elongation of plants. The far-red light increases total biomass as well as leaf elongation in red leaf lettuce and increase fresh weight, dry weight, stem length, leaf length and leaf width in baby leaf lettuce. However, it can decrease pigment content in both red leaf lettuce and baby leaf lettuce. Addition of far-red light will result in taller sweet pepper plants.