Light meters have become standard kit for the serious indoor plant person. The phone-based apps and the dedicated PAR meters are both useful when used properly. They are misused constantly, and the misuse leads to plant problems that get blamed on the wrong things.

What the units actually mean

Lux is the measurement most consumer light meters report. It is a measurement of light intensity weighted to human visual perception. It is not what plants actually use to photosynthesise.

PPFD — photosynthetic photon flux density — is the measurement that matters for plants. It captures the light wavelengths that drive photosynthesis. The PPFD meters cost more but produce numbers that are directly useful.

Converting lux to PPFD requires a multiplier that depends on the light source. Direct sunlight at around 50, fluorescent at around 65, LED at around 70 to 80 depending on the spectrum. The conversion is approximate but useful when only a lux meter is available.

What numbers different plants actually need

Low light tolerance plants — pothos, sansevieria, ZZ plant — survive on PPFD as low as 50 to 100. They will not grow much but they will live.

Medium light plants — most aroids, peace lily, prayer plant — want PPFD between 100 and 250 for healthy growth.

High light tolerance plants — bird of paradise, fig species, most succulents — want PPFD between 250 and 600 for the growth they are capable of.

Direct outdoor sunlight at midday is around 2000 PPFD. Indoor windows in southern-facing positions in summer might reach 800 to 1200 PPFD a few feet from the window. By the time you are six feet from the window, you are at 200 PPFD or less.

Where the measurements go wrong

Time of day matters enormously and most measurements miss this. A measurement at midday tells you nothing about the light available at 9am or 4pm. The plant cares about the integrated light over the day.

Seasonal variation matters more than people expect. Sydney’s winter direct sunlight is meaningfully weaker than summer direct sunlight. A plant position that gets adequate light in January gets inadequate light in July.

Plant placement direction matters. Light meters measure light arriving at the meter. Plants need light arriving at the leaves. A leaf turned away from the window gets less light than the meter reports.

How to actually use a meter

Measure at the position of the plant, at the orientation of the foliage, at the time of day the plant is mostly active, on a day representative of the season. Take measurements at three or four times of day to estimate the integrated light over the day.

Combine the meter readings with observation of the plant. Light is one input. Growth pattern, leaf colour, internode length, and chlorosis patterns provide additional information that the meter cannot capture.

What to ignore

Phone-based light meter apps that report lux without specifying which sensor they are using are imprecise enough that the absolute numbers are not reliable. The relative numbers — this position is brighter than that position — are usable.

Manufacturer claims about “low light tolerance” are marketing more often than horticulture. Many plants sold as low-light tolerant will survive but not thrive in genuinely low light positions. Survival is not the same as health.

When the meter is most useful

Two situations. Setting up grow lights, where the meter helps you set the distance and intensity correctly. Diagnosing problems with established plants, where a meter reading from the plant’s current position can confirm whether light is the limiting factor.

For day-to-day plant care of healthy plants, the meter mostly confirms what observation has already told you.