What PPFD Do My Plants Need?
It depends on growth stage. Widely-used general reference bands: seedlings and clones want roughly 100-300 µmol/m²/s, vegetative leafy growth wants roughly 300-600, and flowering/fruiting high-light crops want roughly 600-1000+. These are general bands, not a species-specific guarantee — check your crop's published range, and remember PPFD changes with mounting height.
The one thing to know: PPFD (Photosynthetic Photon Flux Density, µmol/m²/s) measures how many photosynthetically-useful light photons actually land on your plant's leaf surface. 'Equivalent watts' and lumens measure neither photon count nor the specific light spectrum plants use — a fixture can carry a huge 'equivalent wattage' sticker while delivering mediocre PPFD at your actual mounting height. Read the manufacturer's PPFD-at-distance chart (or measure it yourself with a PAR meter), not the wattage-equivalence claim.
Why PPFD, not watts or lumens
PPFD (Photosynthetic Photon Flux Density) counts the photosynthetically-useful photons actually reaching your plant's leaf surface. Watts measure electrical draw; lumens measure brightness as perceived by the human eye — neither correlates reliably with the specific light spectrum and photon count plants use for photosynthesis. A fixture can carry a big "equivalent wattage" sticker while delivering mediocre PPFD at your actual mounting height. See Are "Equivalent Watts" Meaningless?.
How mounting height changes PPFD
PPFD falls off as you raise a fixture farther from the canopy, and rises as you lower it — sometimes risking light burn if pushed too close for the fixture's intensity. That's why a single headline PPFD number on a listing is less useful than a manufacturer's PPFD-at-distance chart, which tells you the actual reading at several specific mounting heights. If a listing has no such chart, measuring with a PAR/quantum meter at your own canopy height is the only reliable check.
PPFD is half the story — pair it with DLI
DLI (Daily Light Integral) accumulates PPFD over your total daily light hours and better predicts growth than a single PPFD snapshot, since the same PPFD run for 12 hours delivers roughly double the DLI of 6 hours. See the full reference table at The Garden Gear Numbers.
This page is general equipment-spec reference information, not plant-health or growing advice for any specific crop or property.
Frequently Asked Questions
PPFD (Photosynthetic Photon Flux Density) measures µmol of photosynthetically-useful light photons hitting a square meter per second. It measures actual usable light at your plant's leaf height. 'Watts' and 'equivalent watts' measure electrical draw or a marketing comparison to old lighting tech, not the light your plant receives.
Rough bands used across the industry: seedlings/clones want roughly 100-300 µmol/m²/s, leafy greens and herbs in vegetative growth want roughly 300-600, and fruiting/flowering stages of most light-hungry crops want roughly 600-1000+. These are general reference bands, not species-specific guarantees — always check your specific crop's published range.
Check the manufacturer's PPFD-at-distance chart (a reputable maker publishes PPFD readings at several mounting heights), or measure it directly with a PAR/quantum meter at your plant's canopy height. Never infer PPFD from a lumens rating or an 'equivalent watts' sticker — neither correlates reliably with PPFD.
Yes, significantly. PPFD falls off with distance from the fixture, so raising a light a few inches can meaningfully lower the PPFD reaching your canopy, and lowering it raises PPFD (and can risk light burn if pushed too close for the fixture's intensity). This is why a PPFD-at-distance chart matters more than a single headline PPFD number.
You need both. PPFD is an instantaneous reading; DLI (Daily Light Integral) accumulates PPFD over your total daily light hours and is the number that best predicts plant growth. Two setups with the same PPFD but different photoperiods deliver very different total light — see our DLI explainer for the calculation.