Grow lights emit specific wavelengths of light to stimulate plant growth. This targeted illumination is known as Photosynthetically Active Radiation (PAR), spanning the 400 to 700 nanometer (nm) range of the electromagnetic spectrum. The red light portion of this spectrum, typically covering 600 to 700 nm, plays a crucial role in plant development. Understanding this function is fundamental for indoor growers looking to optimize plant health and yield.
How Plants Use the Red Light Spectrum
The primary function of red light (600–700 nm) is to fuel photosynthesis. Chlorophyll pigments within plant cells are highly efficient at absorbing this color, particularly deep red light (around 660 nm), which is readily taken up by both Chlorophyll a and Chlorophyll b.
Chlorophyll a is the main pigment initiating the light-dependent reactions of photosynthesis. This absorption drives the conversion of carbon dioxide and water into sugars, supplying the plant with energy for metabolic activities. Red light photons are considered the most photosynthetically efficient of all visible light wavelengths, making the red spectrum foundational for maximizing plant biomass and energy production during all growth stages.
Red Light’s Specific Role in Flowering and Fruiting
Beyond energy production, the red light spectrum acts as a signaling mechanism dictating a plant’s reproductive schedule. Red light regulates photoperiodism, the plant’s response to the relative lengths of day and night. This control triggers the plant to transition from its vegetative stage to its reproductive stage.
The presence of red light signals a long day or short night, a necessary condition for flowering in many species. This signaling role promotes the formation of buds, flowers, and subsequent fruits, playing a decisive role in determining the final harvestable yield. Growers often increase the proportion of red light to intentionally induce blooming.
The deep red wavelength around 660 nm activates the light-sensing protein called phytochrome, converting it into its biologically active form, Pfr. Pfr initiates molecular events that promote flowering and fruit development. This hormonal regulation also influences stem elongation and bud formation.
The Relationship Between Red and Far-Red Light
Red light’s signaling function is linked to its interaction with far-red (FR) light, typically between 700 and 800 nm, just outside the visible spectrum. Plants use the phytochrome system to detect the ratio between Red (R) and Far-Red (FR) light. This system acts as a biological switch, monitoring light quality.
The phytochrome protein exists in two reversible forms: Pr and Pfr. When the plant absorbs red light, the inactive Pr form converts into the active Pfr form. Conversely, far-red light converts Pfr back to the inactive Pr form. This R:FR ratio indicates whether the plant is in full sunlight (high R:FR) or shaded by other vegetation (low R:FR).
A high R:FR ratio signals open space, encouraging compact, healthy growth and inhibiting excessive stem stretching. A low R:FR ratio, perceived when a plant is under a canopy that has absorbed red light, triggers a shade avoidance response. This response causes the plant to rapidly elongate its stems in an attempt to reach unfiltered light.
Implementing Red Light in Indoor Growing
For indoor cultivation, red light is rarely used alone, as a pure red spectrum results in weak, overly elongated plants. Instead, it is combined with blue light (400–500 nm) to achieve balanced, vigorous growth. Blue light suppresses the stretching effect of red light, leading to shorter, stockier stems and thicker leaves.
Growers adjust the spectrum based on the plant’s life cycle, increasing red light when the plant is ready to flower. The vegetative phase uses a lower red-to-blue ratio, while the flowering phase often utilizes a higher ratio, sometimes reaching 80% to 90% red light. Far-red light can also be added at the end of the light cycle to simulate sunset, accelerating the transition to the flowering stage in photoperiod-sensitive plants. This precise control allows indoor growers to manipulate plant architecture and optimize the timing and quantity of the final harvest.