Understanding Light for House Plants: Types of Light and How to Measure

In his best-selling book, The New Plant Parent: Develop Your Green Thumb and Care for Your House-Plant Family, Darryl Cheng encourages indoor gardeners to re-consider traditional house plant advice and instead think like a plant! He doesn’t rely on house plant lore or ‘tips and tricks’ but instead gives indoor plant parents the tools and science-based advice they need to grow a healthy and thriving indoor garden.

Darryl’s book is a comprehensive guide into all aspects of indoor plant care such as watering, fertilizing, soil, pests, propagation, and light. And it’s providing adequate light that is often the biggest challenge for indoor gardeners. The following excerpt from The New Plant Parent, used with permission from Abrams Image, explores the importance of understanding light and offers advice on how to better gauge light in your indoor living spaces.

The New Plant Parent book helps us to understand light for house plants — The New Plant Parent is a book that focuses on helping the indoor gardener understand a plant’s needs in terms of light, water, and nutrients.

House plants and light

Misunderstanding light intensity is the cause of most disappointing experiences when it comes to house plants. We have vague expressions that describe the amount of light a plant needs in order to thrive: there’s sun, partial sun, shade, bright indirect light, and low light. When it comes to house plants, with the exception of cacti and succulents and some flowering plants, most enjoy what horticulturists call “bright indirect light.” Plant-care advice tends to leave it at that, quickly moving on to the watering and fertilizing that we, the caretakers, must do for our plants. But what about the job that plants need to do? Their job of growing and living is powered by light! Unless they’re getting the right amount of light, all the water and fertilizer in the world will do them no good at all.

I often hear, “My room doesn’t get any sunlight.” But that room that “doesn’t get sunlight” probably has a window, right? How can you tell if your plants are getting the light they need from that window? I’ve thought about this a great deal, and here’s my answer: Only some plants need to see as much sun as possible, but all plants would benefit from seeing as much daytime sky as possible.

Understanding light

Why is light so poorly understood? Think about the environmental conditions that we share with our indoor plants. They generally enjoy the same temperature range that we do, and we’re not bad at determining when their soil is moist as opposed to dry, because we can distinguish between degrees of dryness pretty well by touch. Light, on the other hand, is something that animals experience very differently from plants. We humans use light to identify details in our surroundings, whereas plants use it to make their food. So, while we can see effectively into the far corner of a room, away from any windows, a plant living in that corner would be starving—and we would never hear its cries of hunger!

In fact, because we need to be able to see what’s going on in that corner to survive, evolution has ensured that we have a visual system that isn’t good at measuring light intensity—it’s optimized to make any scene look as bright as possible, no matter how much light there is. Our eyes can’t tell us how much light that plant in the corner is actually getting. So, if light is the prerequisite for proper plant care, we must become better at assessing it. It’s time to measure light.

What does this heartleaf philodendron see? (Photo credit Darryl Cheng)

The #WhatMyPlantSees Way of Assessing Light

Instead of asking, “How bright is it in this spot?” ask yourself, “What kind of light can my plant see from this spot?” Think about how it changes throughout the day and throughout the seasons. Get your eyes down (or up) to the level of the leaves and be the plant! Following a direct line of sight to the nearest window(s), try to identify the following types of light, in order of brightness. You can use this #WhatMyPlantSees checklist to develop an awareness of how much light a plant is getting in a specific place in your house.

learn about understanding light for house plants — Viewing from the top shelf, the light seen by the philodendron is entirely type 2b (re-flected sun) bouncing through the window and off the white blinds.You can probably tell that the philodendron is getting a relatively low level of bright indirect light compared to plants that are closer to the window, but it’s still more than what it would be getting if it couldn’t see a window at all. What about the monstera sitting up close to the window—what does it see? (Photo credit Darryl Cheng)

Type 1, Direct Sun: The plant has a direct line of sight to the sun. This is the most intense light a plant can receive, and most tropical foliage plants cannot tolerate it for more than three to four hours. Cacti and succulents, on the other hand, prefer it.

Type 2a, Filtered/ Diffused Sun: The plant has a partially obstructed view of the sun. For example, the sun might be shining through trees or through a translucent curtain.

Type 2b, Reflected Sun: The plant sees shiny objects or surfaces that receive direct sun, even if the plant itself cannot see the sun.

Monstera in a window — The monstera’s view is noticeably brighter because of the larger view of the blinds (upon which the sun shines— this is type 2b). Also, from this angle, some of the sky can be seen, giving type 3 light—light from the sky. (Photo credit Darryl Cheng)

Type 3, Sky Light: The plant sees blue sky on a clear day. This is an easy metric, because while the intensity of the light will change through the day, the amount of sky the plant sees from one position will not.

You’ll find that most house plants grow well in bright indirect light. A plant in bright indirect light must see any or all of types 2a, 2b, and 3 above. If there are extended times when the plant sees the sun (by getting type 1 light), then you should make sure the plant can tolerate direct sun. When you’re estimating light levels using this checklist, the size of your windows and the distance from the plant to the window matters. You can’t make your windows bigger, but you can move your plants. The best place for tropical foliage plants will be as close to the windows as possible, with a sheer white curtain to block and diffuse the direct sun—this results in their having the biggest view of the sky.

Houseplants in apartment — Here’s a room in a high-rise apartment, where large windows and few obstructions mean ideal light for most foliage plants. The windows on the far wall are facing west and on the right wall are facing north. (Photo credit Darryl Cheng)

Measuring Light with a Light Meter

You can learn a lot about the amount of light your different plants are getting by using the #WhatMyPlantSees checklist in The New Plant Parent. Over time, you’ll develop a sensitivity to the duration of light and distance from windows. At some point, however, you may want to measure light intensity to test your instincts, and for that you’ll need a light meter that measures foot-candles (defined as the brightness of one candle on an area of one square foot at a distance of one foot away). A light meter can demonstrate how rapidly brightness levels decline when you move a plant slightly farther from a window.

In the past, only serious growers would invest in a light meter (you can buy a good one for less than $50). Now there’s also an app for that. Smart-phone light meter apps—which range from being free to costing a few dollars—are not as accurate as dedicated light meters, but they are adequate to show you how light intensity varies from place to place. No one will tell you, “This plant must have exactly 375 foot-candles to grow well,” but you can learn a lot when you see light intensity dropping by a factor of ten as you walk from one side of your living room to the other. In the photographs for this chapter, I’ve alternated a dedicated light meter with a smart phone using an app, so you can see both in action.

Once you start measuring light, you’ll begin to feel more connected to your plants, as you get a sense of their most basic desire. You’ll know they’d starve when you measure only 30 foot-candles along a dark wall. You’ll smile as you know your plant is growing happily with 350 foot-candles near the window.

Aglaonema plant and light meter — Aglaonema is a typical “bright indirect light” plant. At the far side of the room, it still has a good view of the sky because of the floor-to- ceiling windows. On this clear day, I got a reading of 465 foot-candles—this is good light for the Aglaonema. (Photo credit Darryl Cheng)

Using a light meter

Here’s another checklist for bright indirect light, this time as measured by a light meter instead of using the #WhatMyPlantSees approach. Take your readings around the brightest time of the day, which is usually near midday, and try to balance readings for sunny and cloudy days. Hold the meter so that the sensor is next to one of the plant’s leaves, facing the nearest light source.

50–150 foot-candles:

This is “low light,” as in the commonly used phrase “tolerates low light,” but it really verges on “no light.” Among the plants you’re likely to own, only snake plants, pothos, some philodendrons, and ZZ plants will tolerate this level of light. When you get this reading, look up! For a location to be receiving only 50–150 foot-candles at noon on a clear day, the view is probably of a distant window or close to a window with major obstructions—either way, it is a constricted view of the sky.

200–800 foot-candles:

This level of light will yield satisfactory growth for all tropical foliage plants, and the “low-light” plants listed above will do much better in this light range. In this range, your plant can probably see a wide view of the sky or the sun shining on a white curtain, and watering can be done with little worry of root rot. Growth, water usage, and soil nutrient depletion will all be faster for a given plant at 400–800 foot-candles as opposed to 200–400 foot-candles. More light than this isn’t always better: Keeping your plants in the lower range of light intensity could make them more manageable, as they won’t need watering as frequently. You’ll sacrifice some growth, but the goal shouldn’t be growth just for the sake of it.

Checking light for houseplants with a light meter — These plants atop the wire shelf are grow- ing happily with 508 foot-candles right now. (Photo credit Darryl Cheng)

800–1,000 foot-candles:

A sunny window blocked by a sheer curtain will yield 800 to more than 1,000 foot-candles, and this is the high end of what is acceptable for bright indirect light.

8,000+ foot-candles:

Having a direct line of sight with the sun means very intense light. Only cacti and succulents enjoy this light level all day. A large tropical foliage plant could tolerate it for several hours, but smaller ones would prefer to be shielded with a sheer curtain.

Want to learn more about Darryl’s holistic approach to indoor plant care?

In his best-selling book, The New Plant Parent: Develop Your Green Thumb and Care for Your House-Plant Family, Darryl Cheng offers a new way to grow healthy house plants. He puts the focus on understanding a plant’s needs and giving it the right balance of light, water, and nutrients. We also recommend that house plant lovers follow Darryl on Instagram and check out his popular website, House Plant Journal.

For more information on growing indoors plants, be sure to check out our articles below: