Blue light covers roughly 400–500 nm. Plants detect it with cryptochromes and phototropins, which control stomatal opening, phototropism (bending towards light) and stem elongation. More blue generally means shorter internodes, smaller and thicker leaves and more compact plants. Blue photons drive photosynthesis a little less efficiently than red ones per photon, and blue diodes are typically less efficient to produce.
In practice
Westmoreland, Kusuma and Bugbee (2021) grew cannabis under fixtures with blue fractions from about 4% to 20% of photons. Lower blue gave slightly higher yield and slightly taller plants, while cannabinoid concentration barely changed. They concluded fixture efficacy matters more than the blue fraction. Established.
Use more blue (around 15–20% of photons) when height is your problem, for example in a short tent or with stretchy cultivars, and in veg if you want tight plants. Most white LEDs already give 10–20% blue, which is fine for the whole run.
Units and formula
Blue fraction (%) = photons 400–500 nm ÷ total PAR photons × 100. A spectroradiometer or the fixture's spectral datasheet gives this.
Don't confuse with
- Colour temperature: "cool" 5,000–6,500 K white has more blue than "warm" 3,000 K white, but kelvin is not a blue percentage.
- UV-A: 315–400 nm, sensed partly by the same receptors.
Myth check
"Veg needs a blue lamp and flower needs a red lamp" is outdated HPS-era advice. A single broad white spectrum works for both. Supported.
