How to Measure Light for Plants in Your Home Before They Wither and Die

Photo: Stocksy / Rachel Gulotta Photography
It's hard to tell whether or not your plants are getting enough light. If your attempts to make that one corner of your living room greener keep ending in dead plants, that's a good sign the area doesn't get enough light. But it'd be nice to figure that out before the plants give their lives in the name of trial-and-error. Jesse Waldman, director of marketing and e-commerce at Pistils Nursery in Portland, Oregon, says understanding light intensity in your space can have a huge impact on the survival of your plants.

"When caring for a plant, there's different factors that you're balancing, but light is probably the most important," says Waldman. "The amount of light your plant is getting is going to dictate other adaptations to how you are caring for that plant. Each specific plant has a light requirement, but then most of them are able to kind of tolerate a little bit more or a little bit less with certain modifications."

Cacti likes lots of bright sunlight, explains Walman. If the bright light is indirect, a cactus will need less water than light that's direct. The more light a plant gets (within its tolerable range), the more it's able to photosynthesize and grow, says Waldman. The less light it gets, the slower it grows. "Also, in darker spaces the soil is more likely to stay wet for longer," he says.

Adam Besheer, co-owner of Greenery Unlimited, says you also have to consider the location of a plant in your space.

"Putting a plant in the same room as a bright window isn't the same as putting that plant in front of a bright window," says Besheer. "Corners are often popular places to put plants, but if you put a plant in a corner you're guaranteeing that at least half of that plant's leaves aren't getting any light."

There are a few ways to determine the light levels in your home. Keep reading to figure out which method works best for you.

How to how to measure light for plants with a meter

"The human brain can't account for brightness," says Besheer. "That's why your pupil automatically expands and contracts, and why being outside on a sunny day doesn't look like a poorly exposed digital photograph," says Besheer. "Simply put, we don't have a built-in relative measure of how bright something is, the same way we have a relative measure of say, a hot cup of water. You'll know if the water's too hot when it burns you."

Besheer says the best way to determine how much light your plant gets is by using a light meter. Light meters work by giving you a numerical value of the brightness of your space, measuring intensity in LUX (source of light of one candle and equal to one lumen per square meter) or, more commonly, in foot candles (a source of light equal to one lumen per square foot). In Greenery Unlimited's light meter guide, it explains that low light is between 25 and 75 foot candles, medium light is between 75 to 150 foot candles, and high light is over 150 foot candles.

Besheer recommends using either the Dr. Meter Digital Light Meter ($34) or downloading the LightMeter app, available on Android ($2) or iOS ($4). You'll notice that these tools are designed for photography, and Waldman explains that plant growers need much less precise light measurements than photographers. For that reason, he recommend apps like Plant Light Meter ($1) which give readings like "very low" or "medium."

How to determine light levels based on the direction your window is facing

If you don't want to use a light meter, Waldman explains that you can rely instead on cardinal directions. The sun rises in the east and sets in the west (in the Northern Hemisphere), which makes it easy to estimate how much light a room gets based off of what direction the window is facing.

The time of year also impacts how much light is coming through your windows. "During summer the sun is higher in the sky, and during winter it's lower in the sky so where the direct light beams fall in your house is going to change," he says. "Also, at least in Portland and many places around the country, you might have a lot more overcast weather in the winter so your bright exposure that used to be direct sun is probably going to be indirect." As we get into warmer months, some plants that you typically nestle up to the window may need to be moved back.

For the following explanations, Waldman says to assume that all the windows that we're talking about are unobstructed. No trees, no close apartment buildings, because that will diminish the level of light coming into the room.

North-facing windows

"North-facing light is the weakest light that we get because the sun follows the path of the south side of the sky in the Northern Hemisphere," says Waldman. "Northern light isn't bad. In fact, you can put plants really close to the window with north-facing windows and not have to worry about anything like burn. It tends to be really gentle, mild, good for tropicals. Good for plants that like low light or medium light."

East-facing windows

"East light is great for house plants. You'll get direct sun in the mornings, but the sun in the morning tends to be pretty gentle so even if your plant isn't a plant that really wants a lot of direct light, most of the time they can handle a bit of eastern direct light," says Waldman. "Then the eastern light stays in that bright, indirect place during most of the day in the afternoon when the sun is going down." Plants that enjoy lots of bright light like cacti probably wouldn't fair well on eastern light alone. But plants in the maranta or calathea families that like medium-to-bright indirect sun light can thrive.

South-facing windows

"South facing is like the most consistent light," says Waldman. "Almost any plant can go in a window with southern exposure, you just have to think about how close or far away from the window it needs to be to get the optimal amount of light." The sun is in the southern side of the sky pretty much all day, so plants in rooms south-facing windows will get a lot of sun. "If you have a cactus or succulents, and put that right in that window it's going to enjoy the direct sun," he says, "If you have you know like some tropicals that really like bright light like a Hoya, you can hang that in that window or a little bit ways back from the window." Plants that like bight indirect light can live a bit further back from the window so it's not in the path of direct beams.

West-facing windows

"Eastern light in the afternoons is really bight, and it's really hot in the summer," says Waldman. "It's definitely not a place to put something that's that tender, something that doesn't want direct sun. So for example, that calathea or maranta that would do well in that Eastern window, you put it in a Western window in that same direct sun for that couple hours in the afternoon, it's just gonna fry it." You can get away with keeping tropicals a bit back from the window so it doesn't get direct light, but anything that's directly in the window "should be like a cactus, succulent, or Bird of Paradise—something that's very well adapted to direct sun," he says.

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