How to Measure Light for Plants
One of the biggest difficulties we notice among our clients is their ability to assess the proper lighting for their plants. It’s not easy. The human eye automatically compensates for brightness, which makes our ability to judge light levels deceiving. Indoor lights often complicate the issue because what looks “bright” may not be anywhere close when measured with an instrument. At Greenery Unlimited we’ve assessed thousands of spaces, and the first part of our job is always matching the right environment with the right plant. Below we will detail three basic methods for assessing light in your home or office: a quick, somewhat accurate eye test, a somewhat more accurate app that can be installed on your smart device, and instructions for using a physical instrument called a Light Meter that can be purchased for about $35.
We’ll delve into specifics below, but first it’s important we talk about a unit of measurement often used in conjunction with plants. Growing plants depends on giving them the right quality of light, which means providing them with a spectrum of light matching that of our galaxy’s sun, or at least a close(ish) approximation. However what’s much, much more important is providing them with a good quantity of light.
For hobbyists and interiorscapers like ourselves, we don’t need expensive tools like PAR meters or spectrometers. These devices that can be useful for commercially growing plants, but let’s be honest: most of you are just trying to keep your plants alive and happy enough to put out a new leaf every once in a while. For this purpose, talking about light in terms of foot candles will do the trick.
Although the term is anachronistic, foot candles are still a common form of measurement in the horticulture community. Dictionary.com defines a foot candle as the following:
- a unit of illuminance or illumination, equivalent to the illumination produced by a source of one candle at a distance of one foot ande qual to one lumen incident per square foot.
Foot candles are an imprecise form of measurement, but they give us a good idea of how much light a plant is getting. Once we have a hard numerical measurement of the intensity of light in a space, we can start looking at what plants would do well.
NOTE: If you’re used to working with lux or are using an instrument that only measures lux, please note there’s a direct conversion between the two units of measurement.
1 lux = .0929 foot-candle
So you can simply multiply your total lux by .0929 to get your foot candle measurement.
The Simple Method
Pick a spot in your house where you’re considering placing your plant. At the brightest time of the day, usually around noon, hold your hand up and look at the shadow.
High light: Crisp, well defined shadows and stark contrast.
Low light: Faint shadows, unclear outline.
This is by far the least precise way of getting a light reading, but it should give you a ballpark estimate of your lighting conditions. If you’re looking for a precise measurement and willing to take a couple of minutes to figure it out, read on.
The Slightly More Complicated Method
If you have an iPhone, Light Meter has a function for measuring foot candles. It costs $1.99 and is a good purchase if you’re unsure about the placement of a new plant. This guide will cover using this app, but really there are tons of light meter apps out there, and although most of them don’t measure in foot candles, they do measure in LUX, which is easy to translate to foot candles using the conversion rate listed above.
1. Select FC in the center ring. This stands for foot candles.
2. Select the OUTDOOR setting. Even if you’re indoors, this is the measurement you’ll be needing.
3. Point your camera. You’re essentially using your phone’s camera to measure brightness. It’s important that you point the camera IN THE DIRECTION OF THE LIGHT. Light Meter gives you the option of using the front or the back camera, and either option will work.
4. Tap the SUN button to measure the illuminance once, or tap and hold to enable realtime measurement mode.
5. Take note of the measurements. Write them down if you’re looking for a good spot. You can compare and contrast the best lit locations for your plant. Skip ahead to the How to Interpret Foot Candles section for information on how to utilize your reading.
The Way We Do It
A light meter is a great, reliable way to measure light. We use the Dr. Meter light meter, which has proven to be a reliable tool for the price. We’ll be walking through the steps on how to use this specific device, but it’s a simple process that should translate to all light meters on the market.
Remove the cap from the sensor.
Turn on the device. Press the LUX/FC button once. The screen will switch from LUX to FC on the bottom right.
Set your range. Push the RANGE button until you start getting a reading. If your reading is an unmoving "1 . " don’t panic. The RANGE button is simply moving the decimal point forward. You will probably need to press the button a few times to find the right range for your space. The brighter it is, the more times you’ll need to push this button.
The sensor will need to be angled towards the light, NOT TOWARDS YOUR PLANT. We don’t care how much light is bouncing off the leaves, we want to measure the light from the source. The method we use is to hold the light meter as close to the leaves of the plant as possible, with the sensor pointed away from the plant, in the direction of the incoming light. If your plant is against a wall, you can point the sensor against the wall to get an idea of how much light the back side of your plant is getting. This is why plants will often defoliate on the side that’s not facing the window, and why it’s so important to rotate your plants.
Write down your the value of your reading and continue to the next section for information on how to use Foot Candles to assess your space.
How to Interpret a Foot Candle Reading
There is no clear consensus on what levels of light individual plants need, and Foot Candle recommendations are all over the map. However, through our sister company Greenery NYC we have the advantage of monitoring plants over the course of years, and have come to rely on the following levels as the bare minimum in which a plant can survive. Many plants classified as “indoor” plants are actually tropical understory plants, which means they have adapted to the filtered light available on the jungle floor, so another note to consider is that too much direct sun can burn plants. However, like humans, plants are adaptable to different lighting conditions, and over time can adjust to a variety of environments. Think about a New Yorker taking a winter exodus to the beaches of Miami: without preventative measures like sunscreen, the sudden shift in environment can be damaging and in extreme cases deadly.
To see our recommendations, go to our Online Store and click the Light Requirements section on the Filter Bar. Plants listed as low light on our site can survive on 25 FC. High light plants need at least 150 FC. Remember there’s going to be variation in your readings, depending on the time of day, weather conditions, indoor lighting etc. But as a general rule, you should never place a plant like a fiddle leaf fig tree in an area with 20 FC. There are plants sold as interior plants that require even higher levels of light, such as aralias and podocarpus, but although these are marketed as indoor plants we don't consider them viable for long-term success except in areas with extreme levels of light, such as areas with an abundance floor to ceiling windows.
The same way humans need to eat organic matter to function, plants need light to photosynthesize. It’s their food. Without adequate lighting for your plant, it will become anemic and eventually die. Second to overwatering, inadequate lighting is the number one killer of plants, so be sure that if you get a plant that loves bright, indirect light, you put it in a place that’s going to nourish it for the long term.