UK indoor light & pet-safe plant finder
Tell us which way the window faces, how far the plant sits from the glass and what blocks the view. We estimate PPFD and daily light integral for British latitudes and match genera that will grow there, filtered for pets if you need it.
How the estimate is worked out
We start from typical photosynthetic light measured 50cm inside unobstructed windows in British homes, averaged over the hours of daylight: about 420 µmol/m²/s for a south-facing window in summer, 260 for west, 220 for east and 90 for north, and higher under a roof light because it sees the whole sky. Those are averages across sunny and overcast days, not the brief peaks a sunny afternoon can reach.
Light then falls away with distance from the glass. A window is a large source, so the fall-off is gentler than the inverse-square law for a bulb, but in a room with a ceiling and walls it is still steep; we use an exponent of 1.6, which matches the readings we have taken in terraced houses and flats. A net curtain or frosted glass removes about 30%; a building, tree or deep window reveal about half.
Season scales both intensity and day length. A British June gives roughly 16 useful hours, spring and autumn about 12, and December barely 8 at a fraction of the intensity. Daily light integral is PPFD × hours × 3,600 ÷ 1,000,000. Each genus in our range carries the PPFD band it grows well in; a genus “will thrive” when your estimate sits inside that band, and “will cope” when it reaches at least 60% of the lower limit. Humidity removes a genus when your room is more than 10 points drier than its minimum, and the pet filter shows only genera that are non-toxic to cats and dogs.
For a winter boost, a full-spectrum LED bar delivering 100–150 µmol/m²/s at leaf level for 12 hours adds 4–6 mol/m²/day, enough to keep most aroids growing through to March. See our winter care protocol.
Light, explained
Still unsure? Email care@growtropic.co.uk with a photo of the plant and its roots — the nursery team replies Monday to Thursday.
What is PPFD and why does it matter?
Photosynthetic photon flux density is the number of light particles in the photosynthetically useful range landing on a square metre each second, measured in µmol/m²/s. It is the light a leaf can actually use. Lux meters measure brightness as the human eye sees it, which overstates yellow-green light and understates red and blue.
What is DLI?
Daily light integral is the total light a plant receives across a day, in mol/m²/day. It combines intensity and day length, which is why the same window gives a Monstera a DLI near 10 in June and below 2 in December. Most aroids need about 6–12 to grow steadily and stall below 3.
Why does distance from the window matter so much?
Light from a window spreads and falls away rapidly as you move into the room. In typical British rooms the drop is steeper than most people expect: a plant 2m back from an east window gets roughly a tenth of what it would get on the sill. Moving a plant a metre closer often does more than any fertiliser.