Kelvin (color temperature)
A unit describing the color of light emitted by a grow lamp, used to match light spectrum to plant growth stage.
Kelvin is a real, useful shorthand for the color of white light — lower K looks warm/red, higher K looks cool/blue. But for cannabis growing it's a crude proxy. Two lamps at the same Kelvin can have wildly different actual spectra and very different PAR output. Use Kelvin to pick the general vibe of a fluorescent or HID bulb, but for serious cultivation decisions look at the full spectral distribution and PPFD, not the color temperature on the box.
Definition
The kelvin (K) is the SI base unit of thermodynamic temperature [1]. In lighting, correlated color temperature (CCT), expressed in kelvins, describes the color appearance of a white light source by comparing it to the glow of an idealized black body heated to that temperature [2]. Lower CCT (≈2700 K) looks warm and reddish, like an incandescent bulb; higher CCT (≈6500 K) looks cool and bluish, like overcast daylight.
Why growers care
Cannabis responds to light spectrum at different growth stages. Blue-heavy light is associated with compact vegetative growth, while red-heavy light is associated with flowering and stem elongation responses mediated by phytochromes and cryptochromes [3] Strong evidence. Because CCT correlates loosely with the blue/red balance of a lamp, growers historically picked ~6500 K bulbs for veg and ~2700–3000 K bulbs for flower, especially with fluorescent (CFL, T5) and metal halide / high-pressure sodium fixtures.
What Kelvin does not tell you
CCT is a one-number summary of a complex spectrum and hides a lot:
- It is not brightness. A 6500 K bulb can be dim or blinding. Intensity is measured in lumens (human eye) or, for plants, PPFD in µmol/m²/s of photosynthetically active radiation (PAR) [4].
- It is not spectrum. Two lamps at 3000 K can have very different amounts of far-red, UV, or green light. LEDs in particular can hit a target CCT with narrow-band emitters that look identical to the eye but differ biologically [2].
- It is not a flowering switch. Plants flower based on photoperiod (hours of dark) in photoperiod-sensitive cannabis, not on bulb color Strong evidence.
For modern horticultural LEDs, manufacturers increasingly publish full spectral power distribution charts and photon efficacy (µmol/J), which are more meaningful than CCT alone.
Used in articles about
Kelvin shows up in discussions of Grow Lights, LED Grow Lights, HID Lighting, and Vegetative Stage versus Flowering Stage cultivation.
Sources
- Government Bureau International des Poids et Mesures (BIPM). The International System of Units (SI), 9th edition, 2019 — definition of the kelvin.
- Government International Commission on Illumination (CIE). CIE 015:2018 Colorimetry, 4th Edition — correlated color temperature.
- Peer-reviewed Magagnini, G., Grassi, G., & Kotiranta, S. (2018). The effect of light spectrum on the morphology and cannabinoid content of Cannabis sativa L. Medical Cannabis and Cannabinoids, 1(1), 19–27.
- Peer-reviewed Rodriguez-Morrison, V., Llewellyn, D., & Zheng, Y. (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science, 12, 646020.
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