# Light spectrum: blue, red, far-red and UV, what's real

> Photon quantity (PPFD/DLI) and fixture efficacy matter far more than spectrum. Less blue slightly increased cannabis yield (about 12 % from 4 % to 20 % blue). Far-red stretches plants; its yield effect is contested and seems to trade off against cannabinoid concentration. UV has repeatedly failed to raise cannabinoids in controlled trials and can cut yield at high doses.

- Canonical: https://genetixbay.com/knowledge/environment/light-spectrum.html
- Category: Environment · Level: Intermediate
- Evidence: Contested · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Veg, Flower · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**Does UV light increase [THC](https://genetixbay.com/knowledge/glossary/thc.md)?** No, UV light does not reliably increase THC; the claim is a myth. Three independent controlled trials across two labs found UV-B, or UV-A plus UV-B, gave no commercially useful rise in yield or [cannabinoids](https://genetixbay.com/knowledge/glossary/cannabinoid.md), one cultivar lost yield and THC, and the highest dose cut yield. GenetixBay's position matches the trials: UV makes little or no visible difference to cannabis.

**Is far-red worth adding to my LED?** Adding far-red to a cannabis LED is contested and not a proven win. One study found a lower red to far-red ratio raised inflorescence yield but lowered cannabinoids and [terpenes](https://genetixbay.com/knowledge/glossary/terpene.md), another found more height and less yield, and end-of-day far-red helped only some high-THC lines. Far-red reliably [stretches](https://genetixbay.com/knowledge/glossary/stretch.md) plants, so test it side by side with your cultivar, measuring grams and lab potency.

**What spectrum is best for flowering cannabis?** For flowering cannabis, a broad white LED spectrum with roughly 5–15 % blue works well; a red-heavy 'bloom' spectrum is not required, and HPS at about 4 % blue also flowers fine. Research shows photon quantity and fixture efficacy in µmol/J matter far more than colour, though yield fell about 12 % as blue rose from 4 % to 20 %.

**Do I need a different spectrum for veg and flower?** No, cannabis does not need a different spectrum for veg and flower; one broad white spectrum works for both unless you have a specific reason. More blue gives shorter, more compact plants, but stretch in veg is mostly caused by low light intensity, not by missing blue. Running a dim 'veg spectrum' is a common mistake; raise [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md) instead.

**Is a full-spectrum white LED as good as HPS?** Yes, a full-spectrum white LED is as good as HPS for cannabis yield at the same PPFD, according to research comparisons, and better on running costs. Good current LED fixtures exceed 2.5–3 µmol/J against about 1.7 µmol/J for HPS, and they put less heat load on the leaves. Choose a fixture on efficacy and its PPFD map, not on 'full spectrum' marketing.

**Do LEDs need more [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md)?** No, LEDs do not create a need for more Cal-Mag; that is a myth. Calcium and magnesium demand in cannabis follows the cultivar, the water source such as [RO](https://genetixbay.com/knowledge/glossary/ro-water.md), coco buffering, root-zone pH and [transpiration](https://genetixbay.com/knowledge/glossary/transpiration.md). Cooler leaves under LED can lower transpiration and calcium delivery, so correct leaf [VPD](https://genetixbay.com/knowledge/glossary/vpd.md) and check water, medium and pH before reaching for a supplement.

**Short answer.** Buy photons, not colours. Across three studies, yield **fell about 12 %** as the blue fraction rose from 4 % to 20 %, but the authors concluded that **fixture efficacy (µmol/J) matters more** for cost-effective cannabinoid production [1]. **UV** did not deliver commercially useful gains in yield or cannabinoids in three independent controlled trials and cut yield at high doses [2][3][4]. **Far-red** is genuinely contested: one study saw more yield with lower cannabinoid concentration [5], another saw the opposite on yield [6], and end-of-day far-red helped some high-THC lines [7].

## What's real, what isn't

| Claim | Status | Detail |
|---|---|---|
| More photons = more yield | *Supported* | See [PPFD and DLI](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md). |
| Lower blue fraction slightly increases yield | *Supported* | ~12 % linear drop from 4 % to 20 % blue [1]. More blue gives shorter, more compact plants. |
| A "bloom" red-heavy spectrum is required for flower | *Myth* | Broad white LED spectra with ~5–15 % blue flower fine; HPS (~4 % blue) also works [1]. |
| UV-B raises THC or visibly improves buds | *Myth* (house position: little or no visible difference, matching the trials) | No commercially relevant increase in yield or cannabinoids from 287 nm UV-B; one cultivar lost yield and THC [2]. UV-A + UV-B: no effect on total THC [3]. High-CBD cultivar: +3–13 %, not significant; highest dose cut yield [4]. |
| Far-red increases yield | *Contested* | Lower R:FR (3 vs 12) increased inflorescence yield but decreased cannabinoids and terpenes [5]; an earlier experiment found low R:FR increased height and reduced yield [6]; end-of-day FR raised yield in some high-THC lines and was strongly genotype-specific [7]. |
| Far-red stretches plants | *Established* | Shade-avoidance via [phytochrome](https://genetixbay.com/knowledge/glossary/phytochrome.md); consistent across studies [5][6]. |
| Green light is wasted | *Myth* | Green penetrates deeper into the canopy and drives [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) in lower leaves. |
| White LED vs HPS at the same PPFD | *Supported* | Similar yield; LED wins on efficacy and leaf heat load [1][8]. |
| LEDs need more Cal-Mag | *Myth* | The light source doesn't create a Ca/Mg need. Demand follows cultivar, water (RO), coco buffering, root-zone pH and transpiration. Cooler leaves under LED can lower transpiration and Ca delivery; fix leaf VPD, not the bottle. See [Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md). |

## Practical choices

1. **Choose by µmol/J and uniformity first.** Good current LED fixtures exceed 2.5–3 µmol/J; HPS is ~1.7 µmol/J [8].
2. **Keep one broad white spectrum** for veg and flower unless you have a specific reason.
3. **Far-red supplement:** if you try it, run it as an end-of-day (EOD) 10–15 min treatment or a modest FR fraction, on a side-by-side trial with your own cultivar, measuring both grams and lab potency. Expect more stretch.
4. **UV:** not recommended as a potency tool. If you use it, protect eyes and skin (UV-B and UV-C are hazardous); the evidence does not justify the cost or risk.

## Common mistakes

- Choosing a fixture on "full spectrum" marketing instead of efficacy and PPFD map.
- Adding UV and switching off a good fixture for the sake of a colour story.
- Running a "veg spectrum" at low intensity: low light, not blue, causes stretch.
- Judging spectrum trials by eye: purple-pink light hides [deficiencies](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) and pests.
- Adding Cal-Mag "because it's an LED". Check water, medium, pH and leaf VPD first ([Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md)).

## Safety

UV fixtures and bright blue LEDs can damage eyes. Wear rated glasses when working under lights, never look into UV sources, and switch UV off before working in the canopy.

## Evidence

The blue-fraction result is *Supported* (three studies, one lab) [1]. The UV result is *Supported* in its negative direction by three trials across two labs [2][3][4]; the house position is the same: UV-B makes little or no visible difference. Far-red is *Contested*: results depend on cultivar, timing and ratio [5][6][7].

## Sources

1. Westmoreland FM, Kusuma P, Bugbee B, 2021. Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids. PLoS ONE 16(3): e0248988. https://pmc.ncbi.nlm.nih.gov/articles/PMC7987162/
2. Rodriguez-Morrison V, Llewellyn D, Zheng Y, 2021. Cannabis inflorescence yield and cannabinoid concentration are not increased with exposure to short-wavelength ultraviolet-B radiation. Frontiers in Plant Science 12:725078. https://doi.org/10.3389/fpls.2021.725078
3. Llewellyn D et al., 2022. Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content. Frontiers in Plant Science 13:974018. https://doi.org/10.3389/fpls.2022.974018
4. Westmoreland FM, Kusuma P, Bugbee B, 2023. Elevated UV photon fluxes minimally affected cannabinoid concentration in a high-CBD cultivar. Frontiers in Plant Science 14:1220585. https://doi.org/10.3389/fpls.2023.1220585
5. Kotiranta S, Sarka A, Kotilainen T, Palonen P, 2025. Decreasing R:FR ratio in a grow light spectrum increases inflorescence yield but decreases plant specialized metabolite concentrations in Cannabis sativa. Environmental and Experimental Botany 229:106059. https://doi.org/10.1016/j.envexpbot.2024.106059
6. Kotiranta S et al., 2024. (Low R:FR increased height and reduced inflorescence yield.) Industrial Crops and Products. https://doi.org/10.1016/j.indcrop.2023.117772
7. Peterswald TJ et al., 2025. The effects of far-red light on medicinal Cannabis. Scientific Reports. https://pmc.ncbi.nlm.nih.gov/articles/PMC12092679/
8. Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. https://doi.org/10.1038/s41438-020-0283-7