PAR range
~400–700 nm
Leafy PPFD
200–400 µmol/m²/s
Fruiting PPFD
Often 400–700+ µmol/m²/s
Lettuce DLI
Often ~12–17 mol/m²/day
Photoperiod greens
12–16 hours typical
Ignore alone
Lumens for plant growth
PAR: the light plants can use
Photosynthetically Active Radiation (PAR) is light in the approximate 400–700 nanometer waveband that drives photosynthesis. Grow lighting discussions should start with PAR, not with how bright a fixture looks to human eyes.
Human brightness metrics (lumens, lux) weight greens/yellows heavily because our eyes are sensitive there. Plants care about photon flux usable for photosynthesis. A lamp can look dimmer to you yet outperform a “bright” shop light on crops.
Full-spectrum LED grow lights aim to deliver PAR efficiently with spectra that also support compact morphology and useful secondary plant responses. Exact recipe spectra vary by brand; adequate PPFD and even coverage matter most for beginners and intermediates.
PPFD: intensity at the canopy
Photosynthetic Photon Flux Density (PPFD) measures how many photosynthetically active photons hit a square meter per second, in µmol/m²/s. It is the practical intensity number at leaf level.
Leafy greens often target roughly 200–400 µmol/m²/s. Culinary herbs may sit in a similar band. Fruiting tomatoes and peppers frequently need 400–700+ µmol/m²/s in productive indoor systems, with advanced grows going higher when CO₂, irrigation, and genetics support it.
Manufacturer PPFD maps show intensity at defined hanging heights across a grid. Use them — center readings alone hide weak edges. A PPFD calculator helps estimate averages from maps or plan photoperiod math toward DLI.
DLI: intensity integrated over the day
Daily Light Integral (DLI) is the total photosynthetically active photons delivered per square meter per day, in mol/m²/day. Roughly, DLI ≈ PPFD × (hours of light) × 0.0036.
Many lettuce crops perform well around roughly 12–17 mol/m²/day. You can reach a similar DLI with moderate PPFD for more hours or higher PPFD for fewer hours — within reason. Plants still need dark periods for respiration and proper development; 24/0 lighting is rarely ideal for quality greens.
If plants stretch, DLI or PPFD may be too low. If leaves bleach, taco, or look stressed at the top, intensity may be too high or the fixture too close. Adjust height, dimmers, or hours systematically and log results.
PPF, efficacy, and reading a spec sheet
Photosynthetic Photon Flux (PPF) is total PAR photons emitted by the fixture per second (µmol/s) — a whole-lamp output. PPFD is what lands on the canopy. High PPF spread over a huge area yields low PPFD; the same PPF focused on a 2×2 foot tent yields high PPFD.
Efficacy (µmol/J) describes how many photosynthetic photons you get per joule of electricity. Higher efficacy means less heat and power cost for the same photon output — valuable indoors.
Beware marketing that only quotes watts or “equivalent watts.” Ask for PPF, efficacy, and PPFD maps at realistic mounting heights for your footprint.
Photoperiods and crop examples
Leafy greens commonly run 12–16 hours on. Basil often enjoys strong light in a similar photoperiod. Strawberries and fruiting crops have more nuanced day-length and intensity needs by cultivar — match lighting to the plant guide, not only to a generic chart.
Use timers. Random manual switching creates uneven DLI and confuses troubleshooting. Coordinate lights-off with your schedule for inspection — many growers prefer checking plants at the start of the light cycle.
In multi-tier racks, upper tiers may run hotter and receive different intensity if bars are identical — verify each level’s PPFD rather than assuming uniformity.
Hanging height, uniformity, and heat
Follow manufacturer hang height as a starting point, then refine with plant feedback and measurements. Too close: bleaching and heat stress. Too far: stretch and slow growth. Overlapping multiple fixtures improves edge uniformity for wide benches.
Modern LEDs emit less radiant heat than HPS but still heat the room and can warm upper leaves at short distances. Manage air temperature and VPD alongside light — high light increases water demand from the reservoir.
Reflective walls help a little; they do not fix a fixture that is undersized for the footprint. Size lights to the canopy area you intend to cover at target PPFD.
Choosing lights for hydroponic systems
For a first DWC lettuce bucket, a quality full-spectrum LED bar or small panel that can deliver ~200–300+ PPFD over the plant is enough. For a 2×4 or 3×3 tent of mixed herbs, choose a fixture with a published map hitting your leafy targets across the footprint.
Fruiting gardens need more power, stronger mounting, and often dimming to raise intensity through veg into flower/fruit. Pair higher PPFD with correct EC, calcium supply, and airflow to avoid tip burn and blossom problems.
Budget for a timer, proper hang hardware, and optional dimming/controller before buying a second novelty light. One correctly sized fixture beats two random bulbs.
Common lighting mistakes in hydroponics
Using kitchen lumens as a buying guide. Running 20 hours at very low PPFD and wondering why plants stretch. Mounting lights for human head clearance instead of canopy distance. Ignoring edge plants that receive half the center PPFD.
Raising EC to “push” plants under weak light — that burns them. Or blasting seedling cubes with fruiting-crop PPFD on day one. Ramp intensity as plants establish.
Measure, adjust, log. Lighting is measurable engineering, not vibes — and hydroponics already asks you to measure water; add photons to the same discipline.
Frequently asked questions
Are lumens useless for grow lights?
Lumens measure human-perceived brightness, not photosynthetic photon delivery. They are a poor primary spec for plant growth. Use PAR-based metrics: PPF, PPFD, and DLI.
How many hours a day should hydroponic lettuce get light?
Typically 12–16 hours, with PPFD around 200–400 µmol/m²/s, aiming for a DLI often near ~12–17 mol/m²/day depending on cultivar and quality goals.
Do I need UV or far-red diodes?
Not for a successful first leafy garden. Adequate PPFD from a solid full-spectrum LED matters more. UV/far-red are advanced spectral tools once basics are dialed in.
Why are my plants tall and floppy under an LED?
Usually insufficient PPFD/DLI, lights hung too far, or genetics seeking light. Increase intensity or duration modestly, lower the fixture carefully, and ensure EC and temperature are not also stressing plants.
Related tools & plant guides
- Grow Light Coverage Calculator — Determine the recommended coverage area for your grow light based on wattage and hanging height.
- Humidity Calculator — Get recommended humidity and VPD ranges based on your plant growth stage and leaf temperature.
- Hydroponic Lettuce guide — pH 5.5–6.5 (aim 5.8–6.2), EC 0.8–1.2 mS/cm
- Hydroponic Basil guide — pH 5.5–6.5, EC 1.0–1.6 mS/cm
- Hydroponic Tomatoes guide — pH 5.5–6.5, EC 2.0–3.5 mS/cm (stage-dependent)
- Hydroponic Peppers guide — pH 5.5–6.5, EC 1.8–2.8 mS/cm
- Hydroponic Strawberries guide — pH 5.5–6.2, EC 1.0–1.8 mS/cm
Keep measuring
Pair this guide with free hydroponic calculators, crop targets in plant guides, and meter picks in product reviews.