You can’t eyeball plant light. Human eyes adapt so smoothly that a spot with a fraction of a window’s light still reads as “bright enough” — which is why so many plants decline in spots their owner was sure about. The good news: the instrument you need is already in your pocket, and the whole thing takes about ten seconds a spot.
The method
- Pick a clear day, and one hour. Measure every spot at roughly the same time — late morning to early afternoon is ideal — so you’re comparing like with like.
- Set the phone down at the exact spot. Not near it — on the sill, on that shelf, in that corner, at the height the pot will actually sit, lying flat and screen up.
- Read the tier, not the third digit. Phone light sensors are rough, and that’s fine. You’re sorting the spot into a tier — dim, workable, bright, direct — not measuring to the lux. Compare the pattern across your spots.
Why rough numbers are enough
For deciding where a plant can live, the tier a spot falls into matters far more than the exact reading. Two phones will disagree on the absolute number and agree completely on the pattern — every step from the window costs most of the light. That pattern is the whole insight. Six spots in one measured room ran from 20,000 lux at the glass to 18 in the corner — you don’t need precision to act on a gap like that.
No phone handy? Use the shadow
The shadow test gets you the tier with nothing at all — the shape of the shadow a spot casts tells you most of what you need to know.
