How Much Light Do Houseplants Need? PPFD and DLI Targets by Plant
Care labels say “bright indirect light” and leave you to guess. Here is what that phrase corresponds to in numbers, and how to tell whether a particular windowsill clears the bar.
The two numbers that matter
PPFD (µmol/m²/s) is the instantaneous intensity at the leaf. DLI (mol/m²/day) is the total the plant receives over the whole day. Both matter, and they fail in different ways:
- High PPFD, short exposure → a bright spot that still starves the plant.
- Moderate PPFD, long exposure → often perfectly adequate.
DLI = PPFD × seconds of light per day ÷ 1,000,000. There is a converter here if you would rather not do it by hand.
Ranges for common houseplants
These are general horticultural guidance bands, not measurements of your plant. Species tolerate wider ranges than any table suggests, and acclimation matters more than a precise target — a plant moved abruptly from low to high light can burn even inside its “correct” band.
| Plant | Typical PPFD band (µmol/m²/s) | Rough DLI (mol/m²/day) |
|---|---|---|
| Snake plant, ZZ plant, cast iron plant | 50–150 | 2–5 |
| Pothos, philodendron, peace lily | 100–200 | 4–8 |
| Monstera deliciosa | 150–300 | 6–12 |
| Fiddle-leaf fig, rubber plant | 200–400 | 8–15 |
| Calathea, maranta (prayer plants) | 80–200 | 3–8 |
| Herbs (basil, mint) indoors | 300–600 | 12–20 |
| Succulents, cacti | 400–800+ | 15–30 |
“Bright indirect light”, in the way most care guides use it, lands around 150–300 µmol/m²/s — comfortably inside a bright room near a window, but not in direct sun.
Why a window that looks bright often is not
Human vision adapts, so a room reads as “bright” across a range that spans more than an order of magnitude in actual intensity. A few reference points, on a clear day:
| Position | Rough PPFD |
|---|---|
| Outdoors, full summer sun | 1500–2000+ |
| Directly in a south-facing window | 300–800 |
| One metre back from that window | 100–250 |
| Three metres back, or a north window | 20–80 |
| Typical room lighting, away from windows | under 20 |
That last row is the one that surprises people: ordinary ceiling lighting contributes almost nothing photosynthetically, even though the room looks perfectly bright.
Checking a spot
Hold a light meter flat at the leaves, sensor facing the light, and let the reading settle. Take a few readings across the plant and at different times of day — the spread tells you more than any single number. If the readings sit below the plant’s band, the options are to move it closer to the window, move it to a brighter window, or add a grow light and check again at the new position.