Short answer
At the same PPFD, purple (red-and-blue) LEDs grow cannabis about as well as white LEDs: photon quantity and efficacy matter far more than colour [1]. White LEDs win in practice: they light lower leaves better, let you see the plant, and good white fixtures exceed 2.5–3 µmol/J [2]. Supported.
Why
Plants use photons across 400–700 nm. Across three cannabis studies, changing the blue fraction from 4 % to 20 % moved yield only about 12 %, and the authors concluded efficacy was more important for cost [1].
Different LED spectra do change plant shape and chemistry. In one trial two LED spectra gave more compact plants and higher cannabinoid concentrations than HPS, with equal total cannabinoid yield [3]. Spectrum is a tuning knob, not a yield switch.
Green light is not wasted. Leaves absorb less of it, so it travels deeper into the canopy and drives photosynthesis in lower leaves.
Pink-purple light hides deficiencies and pests. That alone is a reason to prefer white.
Older blurple fixtures were often sold on "equivalent watts" with low efficacy. Modern red-and-blue diodes can be efficient; check the numbers.
The numbers
| White full-spectrum LED | Blurple (red + blue) LED | |
|---|---|---|
| Yield at equal PPFD | similar | similar |
| Typical efficacy | 2.5–3+ µmol/J (good fixtures) [2] | varies widely; check the datasheet |
| Lower-canopy light | better (green penetrates) | less |
| Seeing problems | easy | hard |
| Blue fraction | ~5–15 % | often higher |
| Plant shape | normal | more blue = more compact [1] |
Verdict: a white LED is the better buy for almost every home grower; a blurple light with a good measured PPFD and µmol/J still grows good plants.
What to do
- Compare fixtures by µmol/J and PPFD map, whatever the colour.
- If you already own a blurple light, keep it and set it by measured PPFD.
- Inspect plants under white light every few days.
- Wear rated glasses; bright blue diodes strain eyes.
Common mistakes
- Choosing by colour story or "full spectrum" marketing.
- Diagnosing leaf colour under purple light.
- Believing a red-heavy "bloom" spectrum is required (Myth).
- Running a dim "veg spectrum": low light, not missing blue, causes stretch.
Related
- Full article: Light spectrum
- LED or HPS: which is better?
- What should I look for when buying a grow light?
- Terms: full spectrum · spectrum
