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. |
| 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; consistent across studies [5][6]. |
| Green light is wasted | Myth | Green penetrates deeper into the canopy and drives photosynthesis 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. |
Practical choices
- Choose by µmol/J and uniformity first. Good current LED fixtures exceed 2.5–3 µmol/J; HPS is ~1.7 µmol/J [8].
- Keep one broad white spectrum for veg and flower unless you have a specific reason.
- 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.
- 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 and pests.
- Adding Cal-Mag "because it's an LED". Check water, medium, pH and leaf VPD first (Cal-Mag).
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].
