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How to judge plant light without a PAR meter

A PAR meter measures the light plants use, at one spot, at one moment. It is the honest way to get a PPFD reading from your own shelf. Most people with a few houseplants do not own one, and do not need to. This page is about the tools you already have: your hand, your plant, a phone used carefully, and the chart in the box. What PPFD and DLI actually mean is covered separately in PPFD, DLI and distance.

The shadow test

Hold your hand about a hand's length above the spot where the plant sits, during the brightest part of the day, and look at the shadow it casts.

• A sharp, dark shadow with crisp edges means strong, direct light. That is the kind of spot sun-loving plants want.
• A clear but softer shadow means bright light that is not direct, which suits most foliage houseplants.
• A faint, fuzzy shadow means low light. Shade-tolerant plants can cope. Many others will stretch.
• Barely any shadow at all means very low light, the kind of spot where a lamp starts to make sense.

Do it at different times of day and on more than one day. A spot that casts a sharp shadow at noon in June may cast almost nothing at three in the afternoon in December. The winter gap, including UK day length and north-facing rooms, is in do I need a grow light for a UK winter.

The same test works under a lamp. Hold your hand where the leaves are. A sharp shadow directly under the fitting that fades to a vague one at the edge of the shelf tells you the edge plants are getting much less than the middle ones.

The shadow test is a comparison, not a measurement. It tells you whether one spot is brighter than another. It cannot tell you a number.

Let the plant tell you

The plant is the most reliable meter you have, if you give it a few weeks:

• Too little light: longer gaps between leaves, smaller new leaves, a lean towards the window or the lamp, and variegation fading to plain green.
• Too much light: bleached, yellowed or crisp patches on the leaves facing the light, often on the top leaves nearest a lamp.
• About right: new leaves the same size as or larger than the old ones, compact growth, and no steady lean.

Light is not the only cause of those symptoms. Overwatering, cold draughts and a cramped pot can all look like light problems. Change one thing at a time and wait.

A phone lux app is not a PAR meter

Phone apps that measure "light" use the camera or the ambient light sensor. They report lux, which is brightness weighted to human eyesight, not the light plants use. A phone lux app is not a PAR meter, for several reasons:

• Wrong measure. Lux is weighted towards green-yellow, where human eyes are most sensitive. Plants use a broader range, including blue and red. Two lamps with the same lux reading can deliver quite different amounts of light to a plant.
• Different spectra confuse it. A purple red-and-blue lamp can read low on a lux app while delivering useful light to the plant. A cool white tube can read high.
• Uncalibrated sensors. Phone sensors are not built or calibrated for this. Different phones can give different readings in the same spot, and the sensor's direction and position matter.
• Converting lux to PPFD is guesswork. You will see conversion factors online. They depend on the light source's spectrum, and they are approximations at best.

That said, a phone app can still be useful as a rough comparison, the same way the shadow test is. Measure two spots with the same phone, held the same way, under the same lamp or the same window, and the higher reading is probably the brighter spot. Just do not read the number as PPFD.

Maker distance charts are indicative

Many grow-light listings include a chart showing PPFD at different distances. These are useful, and they are a better starting point than guessing. They are also indicative, for reasons the listings page goes into:

• The chart is usually measured directly under the centre of the lamp. The edges are lower.
• It reflects that maker's conditions, often with reflective walls nearby. An open room can read lower.
• It is for that model at that setting. A different size in the same listing is a different lamp.
• Nobody checked your shelf.

Use the chart to set a starting distance, then adjust by watching the plant. If the chart says a distance and the plant bleaches, move it further. If it stretches, move it closer, or lengthen the day.

Hours matter as much as brightness

A spot can pass the shadow test at noon and still be a poor spot overall, because the light only lasts a few hours. What the plant receives over the whole day matters, which is why a lamp on a timer can help a spot that is bright but brief. A lamp still needs a night off. The ranges and the dark period are in hours and a dark period.

When a real meter is worth it

If you are running a larger setup, comparing several lamps, or chasing a specific target for a demanding plant, a proper PAR meter, or a quantum sensor, removes the guessing. For a few houseplants on a sill or a shelf, the shadow test, the plant and the maker's chart are usually enough.

Questions

Can I use my phone to measure light for plants?

Only as a rough comparison between spots. A phone lux app measures brightness as people see it, not the light plants use, and phone sensors are not calibrated for it. It is not a PAR meter.

What does the shadow test tell me?

Whether one spot is brighter than another, and roughly whether light is direct, bright indirect or low. It does not give a number.

Should I trust the distance chart on a grow-light listing?

Use it as a starting point. It is usually measured under the centre of the lamp in the maker's conditions. Watch how the plant responds and adjust from there.