How to tell if a spot is bright enough for a houseplant

Why the usual way fails

Human eyes get used to brightness, so a spot a few steps from the window looks almost as bright as the sill even when it gets far less light. Plant labels say things like "bright, indirect light" without a number, which leaves you guessing. Light also changes through the day and with the seasons, so one look at noon tells you little about the morning or a grey afternoon. And a grow light that looks strong can be much weaker at the leaves than at the bulb, because light drops off quickly with distance.

How WristLux helps

WristLux lists plant placement and grow-light checks among the things it is for, and it has a plant mode. Hold your wrist where the leaves will be and read lux or foot-candles live from the watch's light sensor. Because a session keeps the minimum, maximum, average and duration, you can stay in one spot for a while and see how much the light moves, then do the same at the next spot. The numbers are approximate, so use them to compare places rather than as a lab measurement.

Step by step

1. Open WristLux on your watch and switch to plant mode.
2. Hold your wrist at leaf height in the spot you are considering, with your sleeve pushed back.
3. Stay there for a minute or two and note the session minimum, maximum and average.
4. Do the same on the windowsill and anywhere else you might put the plant.
5. Repeat at another time of day, for example mid-morning and late afternoon.
6. For a grow light, measure at the height of the plant, not right under the bulb.

Tips

• Look at the minimum as well as the average. A spot that gets one bright hour and stays dim for the rest of the day can still post a decent average, and the low number is the one that tells you that.
• Measure where the leaves are, not where the pot sits. On a shelf the top leaves can get much more light than the soil line, and the leaves are what the plant uses.
• Use hold when you cannot see the screen. If you have to reach behind leaves or into a corner, hold the reading, pull your arm back and read it in comfort while the meter keeps running.
• Glance at the Tile to remember the last spot. It shows the latest saved reading and how fresh it is, which helps when you are comparing a few places around the room.

Questions