Measuring Light for Your Indoor Plants: Why Your Eyes Aren't Enough

Plants need light for photosynthesis, the process they use to capture light energy and convert carbon dioxide and water into carbohydrates. Without enough light, a plant cannot produce enough energy-rich compounds to support its growth and maintenance.

It sounds fairly simple, right? Give your plant light and it grows.

Yet somehow, many of us struggle to determine whether our houseplants are actually receiving enough light.

Part of the problem is that humans perceive light very differently from plants.

Visible sunlight appears white to us, but it contains many different wavelengths of light. Plants have photoreceptors that respond to different portions of the light spectrum, allowing them to detect information about their light environment. Plants respond not only to the intensity of light, but also to its direction, duration, and spectral composition.

In other words, plants "see" their light environment very differently than we do.

Our eyes are also remarkably good at adjusting to different light levels. A room can look perfectly bright to us while providing surprisingly little light for a plant.

So when a plant care guide tells you to place your plant in "bright indirect light," how are you supposed to know whether the spot you've chosen is actually bright enough?

One solution is surprisingly simple: measure it.

How Can You Tell if Your Plant Isn't Getting Enough Light?

Houseplants often give us clues when they aren't receiving enough light. Depending on the species, you may notice sparse or elongated growth, smaller leaves, slower growth, reduced flowering, changes in leaf shape, or loss of variegation.

You may also notice that your potting medium stays moist much longer than it did under brighter conditions. Lower light can reduce a plant's water use, which means the potting medium may take longer to dry. When combined with other conditions, prolonged saturation can increase the risk of root problems.

The difficulty is that by the time some of these symptoms become obvious, your plant may have already spent weeks or months growing under less-than-ideal conditions.

For a long time, I didn't realize that I could actually measure the light around my houseplants.

Think about how much more useful it would be if, instead of simply being told to put a plant in "bright indirect light," you had a measurable range to use as a starting point.

That's where a light meter can be incredibly useful.

What Are Foot-Candles and Lux?

Light intensity around houseplants is often described using foot-candles or lux.

Both are measurements of illuminance. Foot-candles are commonly used in the United States, while lux is the corresponding SI unit.

1 foot-candle is approximately 10.764 lux.

You don't necessarily need expensive equipment to start experimenting with light. There are smartphone apps that estimate illuminance, or you can purchase a dedicated lux meter.

I purchased a Dr. Meter digital illuminance meter, and using it completely changed the way I thought about plant placement.

There is, however, an important limitation you should understand.

Lux Isn't Actually a Measurement Designed for Plants

Lux and foot-candles measure light according to human visual sensitivity. They weren't created to measure photosynthesis.

Plants don't have human eyes, so a lux meter doesn't directly tell us how many photosynthetically useful photons a plant is receiving.

You may also encounter the term PAR, or photosynthetically active radiation. PAR describes the region of the light spectrum traditionally considered most relevant to photosynthesis, approximately 400 to 700 nanometers.

When plant scientists and professional growers want to quantify the photons reaching plants within this range, you'll often see measurements such as PPFD, or photosynthetic photon flux density.

So why am I talking about lux meters at all?

Because for the average houseplant grower using predominantly white light, especially sunlight, a lux meter can still be an inexpensive and useful tool.

It won't tell you everything about your plant's light environment, but it can help you compare locations objectively and recognize differences that your eyes may completely miss.

And that is exactly what happened to me.

The Light Problem I Couldn't See

In March 2021, I moved into an apartment where all of my windows faced north and were partially blocked by another building.

The apartment didn't look particularly dark to me. Light still entered through the windows, and I assumed it would be enough for at least some of my plants.

By May, many of the plants inside my apartment had either died or were struggling. The plants on my balcony were surviving, but they weren't exactly thriving either.

So I brought home the light meter I had been using in my greenhouse.

I went to what looked like the brightest location in my apartment, at the brightest part of the day, and measured it.

About 100 foot-candles.

Suddenly, a lot of what I had been seeing made sense.

The plants weren't growing well, and their potting medium was remaining moist much longer than I expected. Those conditions also contributed to another problem I had never dealt with before: fungus gnats.

My first experience with fungus gnats was a nightmare, but that's a story for another article.

The important lesson was that my eyes had convinced me that the room was bright. The meter showed me just how little light was actually reaching my plants.

What Does "Low," "Medium," or "Bright Indirect" Light Actually Mean?

This is where things get a little tricky.

Terms like low light, medium light, and bright indirect light are useful descriptions, but they aren't standardized scientific measurements.

Different growers, authors, and plant-care resources may use somewhat different foot-candle ranges for these categories. That's why I don't recommend treating any single chart as an absolute rule.

One resource that helped me when I first started measuring light was Darryl Cheng's New Plant Parent: Develop Your Green Thumb and Care for Your House-Plant Family. Cheng provides approximate foot-candle ranges that can be useful as practical starting points for understanding indoor light.

And there is another distinction that's particularly important:

A plant that tolerates low light is not necessarily a plant that prefers low light.

Snake plants are a perfect example.

You'll frequently see snake plants described as "low-light plants." They certainly tolerate lower-light environments better than many houseplants, but tolerance doesn't necessarily mean optimal growth. A snake plant in very low light may survive while producing little new growth.

That's an important distinction when reading any plant care guide.

Don't just ask:

"Can this plant survive here?"

Ask:

"Can this plant grow well here?"

Light Intensity Is Only Part of the Story

There's another reason we shouldn't become too attached to one perfect foot-candle number.

Plants experience light over time.

A plant receiving a particular light intensity for a short period isn't necessarily receiving the same total amount of light as a plant exposed to that intensity for many hours.

This is one reason seasons can change plant care so dramatically. Day length changes throughout the year, and depending on where you live, the angle and intensity of sunlight reaching your windows can change too.

Plant scientists and growers can describe the total amount of photosynthetically active light a plant receives over an entire day using something called Daily Light Integral, or DLI.

That's a topic deserving an article of its own.

For now, the important thing to remember is:

Light intensity matters, but so does how long your plant receives that light.

Don't Measure Your Plant's Light Just Once

If you buy a lux meter, one of the best things you can do is experiment with it.

Take a measurement in the morning. Measure the same location again around midday and later in the afternoon.

Move a few feet away from the window and watch how quickly the reading changes.

Put up a sheer curtain and measure again.

Measure on a sunny day and then repeat the experiment on a cloudy day.

Try different windows.

And repeat some of these measurements during different seasons.

You may be surprised by how dramatically the amount of light reaching the same location can change.

Window direction, latitude, season, clouds, nearby buildings, trees, curtains, and your plant's distance from the window can all influence the amount of light it receives.

That's why I wouldn't recommend finding one reading at 9 a.m. and assuming that represents the plant's entire day.

Instead, use your meter to begin understanding the light environment of that location.

And, of course, continue watching your plant.

A meter is another tool. It doesn't replace observation.

A Light Meter Changed How I Choose Where My Plants Live

In my own experience, many of my houseplants performed much better once I moved them into brighter locations.

But the biggest change wasn't simply that I started giving my plants more light.

I started thinking differently about plant placement.

Instead of deciding that I wanted a particular plant in a particular corner and then trying to make the plant tolerate that location, I started measuring the environment first.

I could measure a location, observe how its light changed throughout the day, and then choose a plant whose needs were compatible with that environment.

That eventually changed one of my basic rules for growing houseplants:

I no longer select a plant for a spot. I select a spot for a plant.

And if there's one thing I hope you take away from measuring light, it's that.

Your eyes can tell you whether a room looks bright.

Your plant can eventually tell you whether the light is working.

A light meter can help you figure it out before your plant has to.