The answer in one paragraph
Spectrum sets plant shape first and chemistry second. Blue light at 400 to 500 nm keeps internodes short, thickens leaves, and raises chlorophyll per leaf area. Red light at 600 to 700 nm carries most of the photosynthetic drive and lets stems stretch. Far-red at 700 to 750 nm triggers shade avoidance and can add mass when mixed with red. Green at 500 to 600 nm passes through upper leaves and reaches the lower canopy. Ratios matter more than single bands. Almost every fixture sold for cannabis today uses white LEDs rated 3000K to 5000K, plus added 660 nm and 730 nm diodes.
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Band by band
- 400 to 500 nm, blue: short internodes, compact plants, higher leaf mass per area.
- 500 to 600 nm, green: penetrates upper leaves, useful in deep canopies.
- 600 to 700 nm, red: peak photosynthesis range for most plants; 660 nm diodes are common.
- 700 to 750 nm, far-red: shade signal; adds stem stretch and can raise yield when the red to far-red ratio drops.
- Below 400 nm, UV: some trials report higher THC and terpene counts. Results vary by cultivar and dose.
Numbers in a flowering room
Targets from published cannabis trials and extension guides:
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- PPFD: 600 to 1000 µmol per square meter per second at canopy top.
- DLI: 20 to 40 mol per square meter per day.
- Photoperiod: 12 hours light, 12 hours dark.
- Canopy temperature: 24 to 28 C (75 to 82 F) at ambient CO2.
These ranges shift with cultivar, room design, and CO2 level. Reports from commercial rooms put intensity and DLI above spectrum as the main yield lever. Head-to-head controlled trials that separate the two are few.
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Why convenience decides the spectrum
A grower picks a spectrum once, at purchase. After that the spectrum is fixed. Timers, dimmers, and app controls change intensity and schedule, not the red to blue ratio. That leaves a short list of defaults: 3000K white for flowering, 4000K to 5000K white for veg, plus 660 nm and 730 nm diodes. Tunable multi-channel fixtures exist. They cost more, need a controller, and demand a test plan to justify the price. Most buyers skip that step. Spectrum choices now come from the fixture catalog, not from crop trials.
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How to test spectrum in your own room
- Fix everything else: same clone, same medium, same DLI, same canopy temperature.
- Split the room or run two tents at the same wattage and different spectra.
- Measure at canopy height with a quantum sensor, not a phone app.
- Log internode length at day 21 and day 42.
- Dry, cure, and weigh. Send samples to one lab for cannabinoids and terpenes.
- Run the test twice. One run is noise.
What is not settled
Dose-response curves for UV and far-red in cannabis are thin. Most published work covers a few cultivars and one or two spectra. Numbers from one room do not transfer to another. A grower who wants a specific ratio should ask for the spectral power distribution chart and check the diode count, not the marketing image.