Kelvin (color temperature)
SI unit measuring light color appearance; used to compare lamp spectra in horticulture.
Kelvin describes how warm or cool a light looks to the eye—useful shorthand for ballpark spectrum. But it masks the actual spectral mix; two lamps at 3000 K can deliver very different wavelengths. Modern growers who know what they're doing check full spectral charts and photon efficacy, not just a single number.
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) appears warm and reddish, similar to an incandescent light bulb. Higher CCT (~6500 K) appears cool and bluish, similar to overcast daylight. CCT alone does not fully describe a lamp's spectrum—it is a single-number summary of a complex distribution of wavelengths.
Why growers care about kelvin
Cannabis responds to different wavelengths at different growth stages. Blue-enriched light (shorter wavelength) is associated with more compact vegetative growth, while red-enriched light (longer wavelength) correlates with flowering responses and stem elongation, mediated by phytochrome and cryptochrome photoreceptors [3] Strong evidence.
Because CCT correlates with the approximate blue–red balance of a lamp, growers historically selected ~6500 K bulbs for the vegetative stage and ~2700–3000 K bulbs for flowering, particularly with fluorescent fixtures (CFL, T5) and older high-intensity discharge lamps (metal halide for veg, high-pressure sodium for flower). This convention persists partly because these ballast types were limited to fixed color points.
Modern full-spectrum LEDs give growers finer control over individual wavelengths and make kelvin less central to light choice.
What kelvin does not tell you
CCT is a one-number summary and obscures critical information:
- It is not brightness or intensity. Two lamps at 6500 K can differ by orders of magnitude in photon flux. Brightness for human vision is measured in lumens; for plants, the relevant measure is PPFD (photosynthetic photon flux density) in µmol/m²/s of photosynthetically active radiation (PAR, 400–700 nm) [4] Strong evidence.
- It is not spectrum. Two lamps at 3000 K can contain very different proportions of far-red light, UV, or green wavelengths. LED fixtures in particular can achieve the same CCT using narrow-band emitters that look identical to the human eye but differ substantially in their effects on plant physiology [2] Weak / limited.
- It is not a flowering trigger. Cannabis flowers based on photoperiod—the duration of continuous darkness per day—in photoperiod-sensitive varieties, not on bulb color temperature Strong evidence.
For more meaningful light selection, growers should consult the lamp's spectral power distribution (SPD) graph and photon efficacy (µmol/J), which directly show how many photons a lamp produces per joule of electrical energy.
Modern practice
Professional horticulturists and informed home growers increasingly rely on spectral data and PPFD measurements rather than CCT alone. Most LED manufacturers now publish full SPD curves and photon efficacy ratings, making side-by-side comparison more transparent than it was with fluorescent and HID fixtures.
Kelvin remains a useful quick reference for general color warmth but is not a substitute for understanding what spectrum a specific lamp actually delivers.
Sources
- Government Bureau International des Poids et Mesures (BIPM). The International System of Units (SI), 9th edition. 2019.
- Government International Commission on Illumination (CIE). CIE 015:2018 Colorimetry, 4th Edition. 2018.
- 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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