To measure whether a corner is truly low light for a houseplant, put a light meter at leaf height and record readings at several times of day. Avoid judging the entire room by one quick reading beside the window. What matters is the light that reaches the plant where it actually sits, for long enough across an ordinary day.
The two numbers you may see: lux and foot-candles
A basic light meter usually measures illuminance in lux or foot-candles. One foot-candle is approximately 10.76 lux. A reading of 1,000 lux is about 93 foot-candles, not 1,000 foot-candles. You can use either unit if you keep it consistent.
The University of Minnesota Extension indoor plant guide gives roughly 50–250 foot-candles as one practical low-light category, though requirements vary by species and light duration. Treat this as a rough placement guide rather than an exact pass/fail test: people use the term “low light” differently, and a meter designed for human-visible brightness is not a precise plant-growth-light instrument.
What a meter looks like
Many meters have a separate white sensor dome that measures light, a display and a button to switch between lux and foot-candles. Place the sensor in the same plane as the leaves, rather than taking a reading from the other side of the room. The photographed meter below shows the sensor and display as separate connected parts.
A five-minute home measurement routine
1. Put the sensor where the leaves grow. Move decorations aside temporarily, but do not move the entire plant into a brighter spot just to test it. Hold the sensor facing the main light source, without your hand casting a shadow.
2. Write down the unit and time. A reading of “120” is useless if you cannot tell whether it means lux or foot-candles. Note whether blinds are open, whether lamps are on and whether the Sun is shining directly into the window.
3. Repeat at morning, midday and late afternoon. A north-facing window can be fairly stable, but an east-facing one may receive a short burst of stronger morning light. A single midday number can overstate a dim corner’s daily light supply.
4. Compare locations. Test the windowsill, a nearby shelf and the back of the room. Light can fall substantially with distance from a window or after passing through curtains, insect screens and roof overhangs.
5. Check the plant over subsequent weeks. Track new leaves, leaf spacing and soil drying speed. A measurement is a clue, not a substitute for noticing how your particular plant responds.
Why a phone app may give a different number
Phone light sensors were commonly designed to adjust screen brightness rather than measure plant light. Case placement, camera exposure and calibration can affect app estimates. A phone can sometimes help compare brighter and darker positions in the same room, but do not treat its reading as laboratory-grade, especially under colored grow lights.
How to interpret the reading without overwatering
In dimmer conditions many houseplants photosynthesize and grow more slowly, which generally means they use water more slowly, too. Check the potting mix before watering rather than following exactly the same watering calendar you used in summer. Low light does not make a plant immortal in darkness: a windowless room without an appropriate grow light is not a sustainable permanent location.
Frequently asked questions
Is 100 lux enough for a pothos?
It is very dim compared with many common plant-light recommendations. A pothos may tolerate dim conditions for a time, but growth may slow or stall. Check actual new growth and consider moving it nearer a window or adding suitable supplemental light.
Should I measure at night?
You can, if an artificial light stays on as part of the plant’s normal day. Otherwise nighttime darkness is expected; what matters is the strength and duration of the plant’s useful light exposure.
Can I use a lux reading to calculate exactly how much a plant will grow?
No. Lux describes light weighted for human vision, while plant responses depend on the spectrum, intensity, total daily exposure and the species. A specialized plant-light meter can assess photosynthetic photon flux density more directly.
Related plant care
If the plant looks stretched rather than simply slow, see why pothos vines develop long gaps between leaves in low light. For more guidance on choosing plants for shady corners, read Penn State Extension’s low-light houseplant discussion.
Leave a Reply