How to Measure Light for Your Indoor Plants - Backyard Boss
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How to Measure Light for Your Indoor Plants

Proper and adequate light is essential for indoor plants. Without it, they can droop, change leaf color, and even die, regardless of how well you otherwise care for them. Plants need light to produce chlorophyll, which reflects green light, making plants appear green. They absorb red and blue light for photosynthesis, creating food and new growth.

So, how much light is too much? How much is too little? By learning how to measure it, you can create better lighting conditions for your indoor plants, promoting stable chlorophyll production and boosting their health.

Types of Light: Artificial vs. Natural Light

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The less light a plant receives, the less chlorophyll it produces. A plant with less chlorophyll reflects very little green light and absorbs very little red and blue light. When this happens, your plant’s leaves turn yellow or brown, and the plant eventually dies.

Conversely, excessive light causes chlorophyll to break down and reduces photosynthesis, which can also cause premature plant death. Both artificial and natural light transmit energy called photons.

Plants focus on light wavelength (long and short) and intensity. Chlorophyll absorbs red regions from the light’s long wavelength and blue regions from the short wavelength.

Understanding Wavelength

light wavelength
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Flashlights, lamps, and light bulbs are popular artificial lights people use to help plants make chlorophyll when they don’t have access to natural light. Their operating power is expressed in wattage or watts. However, while indoor plants can thrive with artificial lights, not all are equal.

Artificial lights don’t have enough blue and red regions to kickstart chlorophyll. On the other hand, natural light is radiant in red and blue. So what’s the solution if you don’t have access to natural light? Grow lights!

You can select and control wavelengths for optimum plant growth by picking the best grow light.

Understanding Intensity

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Light intensity has a direct effect on photosynthesis. For example, low intensity won’t kickstart photosynthesis, while high will begin negatively interacting with chlorophyll molecules when the light reaches the plant’s foliage.

Regardless of where you place your plants outdoors, they’ll all receive an equal amount of blue and red light from natural sunlight, but the same isn’t true for plants growing indoors or under artificial light.

When it comes to artificial light, plants nearby receive a good amount of red and blue light than those placed farther away

How to Measure Light for Your Indoor Plants

Since the human eye automatically adjusts to different light levels, it isn’t a reliable way to measure light. Instead, invest in a digital light meter. If you don’t have access to advance light meters, here are three more ways to measure light.

Method 1: Determining Light Levels With Foot-Candles 

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Place a lit candle 1 foot away from the area where you place your plants. Now carefully observe your room. Is anything obstructing light from reaching your plants? Things like curtains, furniture, screens, or pillars are indoor obstructions, and large trees are outdoor obstructions.

Keeping the light conditions and obstructions in mind, categorize your space according to light intensity. For example, which part of the room:

  1. Receives strong non-obstructive natural and artificial light?
  2. Gets dappled natural and artificial light?
  3. Receives very little to no light?

Categorizing helps you understand how much light your indoor plants are getting versus how much they need. While this is not an exact measurement, placing them in categories helps you estimate the number of foot-candles your plants are receiving. 

Categorizing Popular Indoor Plants With Foot-Candles

To help you calculate, 1 foot-candle (FC) is 1 lumen (amount of light emitted per second) per square foot or 10.76 lux.

Light Intensity

No. of Foot-candles Description of Light Intensity at Mid-Day

Indoor Plants that Prefer this Light Intensity

Low light 25 to 100 FC Low light usually occurs in rooms with north-facing windows. These areas are generally highly shaded and artificially lit with overhead lights.
Medium-light 100 to 500 FC Areas with moderate light intensity don’t receive direct sunlight but are usually near shaded east or west-facing windows. East-facing windows receive light in the morning, and west windows are lit by the afternoon sun.
High light 500 to 1000 FC Areas with high light intensity are usually near south-facing windows, as these windows let in the most sunlight.
Direct sunlight >1000 FC Indoor plants that receive direct sunlight usually face windows with no obstruction, both outside and inside. This light exposure is characteristic of sunrooms or an unshaded south-facing window.

Method 2: Calculate the Wattage Per Square Foot of the Growing Area

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This method will help you determine the wattage you need for your growing area. After categorizing your plants under low, medium, and high light, use these guidelines to calculate approximate wattage:

  1. Low light: 50 to 250 FC ≈ 10 to 15 watts
  2. Medium-light: 500 to 1000 FC ≈ 15 watts or more
  3. High light: >1000 FC ≈ 20 watts or more

To know how much light you need, multiply the value of watts your plant needs by the square feet of the growing area. For example, peace lilies need 25 to 100 foot-candles, which is 10 to 15 watts per square foot.

Knowing how much wattage your plants need helps you invest in the right grow lights for healthy plant growth. When selecting grow lights, pick one that emits blue and red lights to mimic natural light.

Note: If your plant doesn’t respond well to the artificial light conditions you’ve made according to the calculation, expose it to natural light to see how it responds. Also, check other factors affecting its growth, like temperature and soil moisture and quality.

Method 3: Use Phone Apps to Measure Light Intensity 

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For this method, you’ll need a smartphone and access to the Lux Light Meter Pro app that works on a smaller scale. Check out Google Play Store to download for Android devices and the Apple App Store for Apple devices.

Upon opening the app, you’ll see the light meter measures in either lux or foot-candles. The app uses your mobile device’s ambient light sensor or auto brightness. It can measure anywhere between 0 to 1 lux or 0.0 to 0.1 foot-candles. 

To take your first reading:

  1. Face your front camera to the source of light (an open window or a light bulb).
  2. Shortly after, you’ll see a minimum, average, and maximum reading on the app.
  3. Record the average reading as the light intensity measurement.

At the bottom of the app, you’ll see a “store” icon that you can use to save your measurements. Storing measurements allow you to compare light intensity across different parts of your home to choose the best spot for your indoor plants.

Note: Phones may differ in their auto-brightness settings, so measuring light intensity with the same phone is good for precise comparison.

Watts Up!

It’s essential to figure out how much light your indoor plants need for healthy growth. Plants need light to produce chlorophyll and kickstart photosynthesis, but not all plants need high light intensity. Categorizing plants according to the amount of light they need will make indoor gardening easier and help your plants thrive.

Leave your experiences, thoughts, and questions in the comment section! And share with friends and family who might find this helpful.

Happy gardening!