The color of a bulb doesn’t matter as much; what matters is where on the photosynthetically active radiation window it falls. That’s why you want to see the plant light spectrum chart. The chart plots nanometer bands within the window of photosynthetic activity. It tells you what’s driving your plants’ photosynthesis, and what isn’t (just some pretty decoration).
Four hundred to seven hundred nanometers. This is the core range. This is the PAR window, the particular slice of the spectrum where plants use light to create their food. Everything above (far-red) and below (ultraviolet) is outside this range. Far-red actualy signals flowers and encourages stem elongation, but doesn’t directly power the chemical reactions necessary for building biomass.
Why Light Spectrum Matters for Plants
Chlorophyll a, as shown on the chart, has its strongest absorption in the deep red band around six hundred sixty nanometers. It’s strong again in the blue band around four hundred thirty nanometers. These bands, specifically, are critical for plant growth: You want photons in those parts of the spectrum to nourish your plants.
Many people will ignore green light because plants reflect it. But green photons penetrate more deeply into the canopy different than do red or blue light. They hit the lower leaves that might otherwise starve in the shade of upper branches. That is an important thing to consider for high-density growing. Many times a binary blue-and-red setup isn’t as good as a full-spectrum light. It balances the load on the whole plant structure. You get the benefits of depth penetration by green, along with the surface efficiency of red and blue.
A look at the visual data reveals trends toward efficiencies. Efficiency of photon use tops out (sharp peak) in the deep red sector; these are the photons that gets you the most bang for your buck. There’s good blue, as well, which helps maintain stem compactness and control plant shape. In between yellow-green, where chlorophyll a absorbs very little, efficiency falls off, but not to zero. It earns its keep. Filling in the blanks: Carotenoids and chlorophyll b contribute. By widening the net of available light, these accessory pigments provide biological backup and enable plants to live under different lighting conditions.
When selecting a fixture, don’t rely on what’s hyped by marketers; look at the spectral power distribution. High pressure sodium lamps, for example, tend toward heavy red/orange. They’re great for fruiting, but the lack of blue light will cause weak, stretched stems during the growing stage. Newer LEDs let us adjust the settings exactly, but that isn’t necessarily better. Why does sunlight work? Because it’s balanced. There’s a constant output of red (for yield), blue (for habit) and some green (for healthy canopy).
Don’t think you need a lab grade spectrum to get it done. You should of thought you need a mix of all three that mimics that balance. Don’t judge a light by its color. What matters is the photons emitted in that critical 400-700 nanometer range. And the number of those photons delivered is the flux. If possible, use a quantum sensor to measure the flux.
Plants use green light to penetrate deeper into the canopy to reach lower leaves, rather than just seeing red and blue. Humans tend to focus on green, but we should look more at numbers and less at the spectacle. That saves money and produces better plants.
The chart is the map. All you have to do is choose your route and believe the science rather than the spectacle. Light’s not decor; it’s energy.