A light's spectrum (spectral power distribution) is how its output is spread across wavelengths, usually shown as a graph from about 300 to 800 nm. Plants read spectrum through photoreceptors: phytochromes respond to red and far-red, cryptochromes and phototropins to blue and UV-A, UVR8 to UV-B. Spectrum steers shape, stretch, leaf thickness and timing; the total number of photons does most of the work on yield.
In practice
Growers talk about "veg spectrum" (more blue, compact plants) and "bloom spectrum" (more red). With modern white LEDs, the differences between fixtures are smaller than the marketing suggests. Westmoreland and colleagues (2021) found that lowering the blue fraction modestly raised cannabis yield, but that photon efficacy mattered more for cost. Established.
Practical rules: a broad white spectrum with extra red is a sound default for both stages. Low red to far-red ratio promotes stretch. Extra UV-B produced little visible change in controlled trials.
Bands commonly cited: UV-B 280–315 nm, UV-A 315–400 nm, blue 400–500 nm, green 500–600 nm, red 600–700 nm, far-red 700–750 nm (some definitions to 800 nm).
Don't confuse with
- Colour temperature (K): a single number describing how warm or cool white light looks; two lamps with the same kelvin can have very different spectra.
- Intensity (PPFD): how much light, not which wavelengths.
Far-red is light from about 700 to 750 nm, just beyond visible red; it adds to canopy photosynthesis when mixed with other light and triggers shade responses like stretch.UV-A is ultraviolet light from 315 to 400 nm; plants sense it much like deep blue, and adding it gives no proven gain in yield or potency.UV-B is ultraviolet light from 280 to 315 nm; added to grow rooms in hopes of more resin, it gives little or no visible difference in controlled trials.The stretch is the rapid increase in height in the first two to three weeks after switching to flower, when many cultivars grow 1.5 to 3 times taller.An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.Photon efficacy is how many micromoles of PAR photons a fixture produces per joule of electricity (µmol/J); higher means more light for the same power bill and heat.The red:far-red ratio compares photons near 660 nm with those near 730 nm; plants read a low ratio as shade and respond by stretching.Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.Full spectrum is a marketing term for a grow light with broad output across the visible range, usually a white LED, sometimes with added red, far-red or UV.Blue light is the 400–500 nm band; it keeps plants compact with shorter internodes and thicker leaves, but a high blue fraction slightly lowers cannabis yield.Photosynthetically active radiation: light in the 400–700 nm waveband that plants use for photosynthesis, counted in photons rather than brightness.
