How to Measure Light for Plants


One of the most biggest difficulties we notice among our clients is their ability to asses 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 NYC we assess a lot of spaces, and a big part of our job is figuring out the right space for a plant.

 
 

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


We’ll delve into specifics below, but first it’s important we talk about a unit of measurement often used in conjunction with plants. 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:

    noun, Optics.

    • 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. We keep an informal list of foot-candles required to grow specific plants, and for new site visits will cross reference this list when choosing a plant for a specific location. Our team uses a light-meter, which is a device with the specific function of measuring light. While we realize buying a light meter is not a practical consideration for most of our clients, we have found a solution that’s almost as good.

 
 

Get the App


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 convert to foot candles.

 
 

SET UP

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 bet lit locations for your plant.

 
 

What Does it Mean?


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. It will not survive.

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.