Light Spectrum for Hydroponics — What Each Color Actually Does

Quick answer

Plants do not need every wavelength equally. Photosynthesis is driven mainly by blue (400–500 nm) and red (600–700 nm), with measurable contributions from green (500–600 nm) for canopy penetration and far-red (700–780 nm) for flowering signals. UV is optional for quality, not yield. Stop buying lamps by Kelvin; buy by PPFD and DLI on a real PAR spectrum chart.

The five bands that matter

Blue (400–500 nm) — compactness

Blue light suppresses internode elongation. Plants grown under blue-heavy spectra are shorter, leaves are smaller and thicker, and root mass is higher relative to canopy. [UCD-LET-01] If your seedlings are stretching, the lamp is too far away, the photoperiod is too short, or the spectrum is red-shifted.

Practical target: 15–25% of total photon flux below 500 nm during veg.

Green (500–600 nm) — penetration

Green photons reflect less off the upper canopy than blue or red and reach lower-leaf chloroplasts where photosynthesis would otherwise stall. Modern research treats green as fully photosynthetically active per photon — the old "plants don't use green" claim is wrong, just less obvious because leaves look green. [CORN-CEA-01]

You don't need to buy green diodes — white LEDs already emit a green-rich continuum.

Red (600–700 nm) — yield

Red is the most photosynthetically efficient band per photon. Most grow LEDs lean red because it's the cheapest way to push DLI up. Heavy red without enough blue produces tall, weak plants with thin leaves.

Practical target: 40–60% of photon flux between 600–700 nm during flower; ~30–40% during veg.

Far-red (700–780 nm) — flowering signals

Far-red shifts the phytochrome equilibrium toward the Pfr → Pr conversion, which:

• triggers flowering in short-day plants,
• increases stem elongation (good for tomato, bad for lettuce),
• enables the Emerson enhancement effect — far-red co-applied with red increases photosynthetic efficiency more than either alone.

Adding 10–20% far-red for the last 15 minutes of photoperiod ("end-of-day far-red") is a common trick to nudge fruiting crops toward flower without changing day length.

UV-A (320–400 nm) — quality

UV-A does not drive yield. It does drive:

• anthocyanin accumulation (red pigment in lettuce, basil),
• essential oil production in mint, basil, oregano,
• thicker cuticles (better post-harvest shelf life).

Dose carefully — 10–20 min/day of low-intensity UV-A is safe. Continuous UV-B causes tissue damage and reduces yield.

Kelvin temperature: useful proxy, bad spec

Lamp manufacturers label by color temperature (K) because consumers understand it. For plants:

Kelvin says nothing about UV or far-red. A 3000K white LED still emits across the full visible band — it just weights warmer.

How to read a real PAR chart

Real grow-light manufacturers publish spectral power distribution (SPD) plots: y-axis is relative photon flux, x-axis is wavelength 380–780 nm. What you're looking for:

1. A blue peak around 440 nm.
2. A red peak around 660 nm (not 630 — that's deep red, less efficient).
3. A continuous green/yellow plateau between the peaks.
4. Some 730 nm far-red if the lamp is sold for flowering.

If the chart shows only two narrow spikes (deep blue + deep red), it's an older "blurple" LED. They work but produce visibly purple plants under inspection, hide pest damage, and give worse outcomes for crops where canopy penetration matters.

Spectrum recipes by crop

Lettuce, basil, kale, spinach (leafy greens)

• 5000K white LED, ~20% blue, ~10% far-red.
• DLI target 14–17 mol/m²/day — see PPFD & DLI.
• Avoid heavy red. Excess red on lettuce causes tipburn risk and reduced anthocyanin in red varieties. [UCD-LET-01]

Tomato, pepper, cucumber (fruiting)

• 3000K warm white + supplemental 660 nm red and 730 nm far-red.
• DLI target 22–30 mol/m²/day.
• Add 15 min end-of-day far-red to trigger flowering response.

Strawberry

• 4000K neutral, balanced blue/red, optional UV-A in last 2 weeks for flavor and red pigment.

Mother plants and clones

• 6500K cool white, blue-heavy. Keeps clones compact, accelerates root strike.

Common mistakes

• Buying a "full spectrum" lamp that's actually warm white + red diodes. Check the SPD chart. If the brand won't publish one, walk away.
• Confusing watts with light. A 100W LED at 2.5 µmol/J pushes more usable light than a 200W cheap LED at 1.2 µmol/J. Compare PPF per watt (efficacy), not wattage.
• Ignoring distance. A great spectrum at 24 inches becomes a useless dim glow at 36 inches. Use the DLI calculator to lock in canopy intensity.
• Running blurple at the dinner table. If the grow is in your living space, white-led spectra preserve color vision and let you spot nutrient deficiency symptoms before they spread.

What to buy

If you're picking your first lamp, see how to choose a grow light. Short version: a 5000K white LED with a published SPD chart and ≥ 2.5 µmol/J efficacy outperforms 90% of "full spectrum" marketing claims at half the price.