Lux, PAR, and Foot-Candles: A Plain-English Guide to Plant Light
Ask a plant store employee how much light a monstera needs and you'll get "bright indirect." Ask a horticulturist the same question and you'll get "around 200 PPFD, with a daily light integral of 10–12 mol/m²/day." Same plant, very different answers.
The gap between casual plant advice and measurable plant light is confusing for most hobbyists — and it matters more than any other environmental variable for long-term plant health. Here's a plain-English breakdown of the three units you'll run into, what each measures, and which one actually predicts how your plant will grow.
Lux (and why your phone's light meter is lying to you)
Lux is a measure of perceived brightness to the human eye. It's weighted heavily toward the wavelengths we see best — greens and yellows in the middle of the visible spectrum.
That weighting is the problem. Plants use mostly red and blue light for photosynthesis; green light is partly reflected (which is why leaves look green). A lux reading systematically overestimates how useful the light is for a plant, because it emphasizes the wavelengths the plant rejects.
If you've ever downloaded a "light meter" app for your phone, that's what it measures — and that's why plants don't always thrive in a spot your app rates as "bright."
PAR and PPFD (the numbers that actually matter)
PAR stands for Photosynthetically Active Radiation — light between 400 and 700 nanometers, the range plants can actually use for photosynthesis. PAR is a range, not a number.
PPFD (Photosynthetic Photon Flux Density) is the number: how many photons in the PAR range hit a square meter per second, measured in µmol/m²/s. That's the unit you'll see on commercial grow light spec sheets and research papers.
Rough targets by plant type:
- Low-light tolerant (pothos, snake plants, ZZ): 50–150 PPFD
- Medium light (most tropicals, philodendrons, calatheas): 100–250 PPFD
- Bright indirect (monstera, fiddle leaf fig, alocasias): 200–400 PPFD
- Full sun lovers (succulents, citrus, most vegetables): 400–800+ PPFD
The University of Georgia's greenhouse research extension publishes detailed PPFD tables for commercial production if you want species-level data.
DLI: The Daily Total That Predicts Growth
PPFD tells you light intensity at a moment. DLI (Daily Light Integral) tells you how much total light a plant got over the whole day, measured in mol/m²/day.
DLI is what actually predicts growth. A plant getting 300 PPFD for 12 hours fares better than one getting 500 PPFD for 4 hours — even though the peak is lower — because the total daily light delivered is higher.
Typical DLI targets:
- Most houseplants: 6–12 mol/m²/day
- Herbs and leafy greens: 12–17 mol/m²/day
- Flowering/fruiting plants: 20+ mol/m²/day
Foot-Candles (you'll still see this in older books)
1 foot-candle ≈ 10.76 lux. Same perceived-brightness problem as lux, just in imperial units. If you have an older gardening book, this is the unit you'll find. Convert to lux and mentally downgrade how useful the number is.
Why Continuous Measurement Changes the Answer
Light levels in a home change dramatically across the day and across the year. A spot that reads 400 PPFD at noon in July might only hit 80 PPFD at the same hour in December. And cloudy days can knock measurements down by 70% compared to sunny days.
A one-time reading tells you almost nothing. A continuous reading — integrated across the day into a DLI number — tells you whether your plant is actually getting what it needs.
That's why PlantSense includes a dedicated PAR sensor instead of a standard lux sensor. It measures light in the range plants actually use, records it throughout the day, and reports DLI directly. Instead of guessing whether "bright indirect" describes your living room, you get a number you can compare to the plant's actual requirement.
The Practical Takeaway
If you want to use the light info in care guides:
- Don't trust phone lux readings as a stand-in for plant light.
- If a care guide lists a PPFD range, that's the useful number.
- If you only have access to lux, a very rough conversion for typical daylight is lux ÷ 50 ≈ PPFD. (The actual factor varies with light source.)
- For long-term health, DLI matters more than peak intensity.
Better light data is the fastest way to end most plant-care guessing. It turns "I think this spot is bright enough" into a number you can act on.