The right light is absolutely essential for plant growth and is the most critical factor that affects their health indoors. Plants need light to produce energy and thrive. Having the right light for plants determines the amount of energy produced for growth.
A houseplant can get by without fertilizer for a long time, with the wrong soil for a long time too, or get by on haphazard watering for a while. But the wrong light will quickly cripple or kill a houseplant.
Alas, light is also one of the most difficult aspects of plant care to wrap one’s head around. So, let’s start with the essential measurements necessary to understand the light in your environment.
Measuring light
Light is generally measured in foot-candles or lux (lx). Much of botanical information, however, relies on foot-candle, where one foot-candle (ft-c) is the amount of light cast by a candle on a white surface one foot away in a completely dark room.
In outdoor settings, light levels on a bright day can range from 10,000 ft-c in open sunny areas to 250 ft-c or less in the shade of a large tree. While the exact measurement is probably not something the average plant owner needs to know, it is quite beneficial to have a general idea of how much light is present in a specific location in your home.
Photographers may have a handheld light meter, but it’s also easy to find the light in your environment with any number of free apps. Just search for “light meter” in your chosen app store and find one that doesn’t lock ft-c or lux behind an annoying paywall.
To measure how much light is coming through your window or hitting your plant shelf. Open your app or take out the light meter and a piece of paper. It’s easier than one might think figure out what kind of light is in a home.
Below is a chart of foot-candle (ft-c), and lux (lx) for reference.
| Light Level (ft-c) | Lux (rounded to nearest 10) | Light Levels |
| 40 | 430 | Low-light |
| 75 | 810 | Low-light |
| 150 | 1600 | Medium-light |
| 300 | 3200 | Medium-light |
| 600 | 6500 | High-light |
| 1200 | 13000 | High-light |
| 2400 | 25000 | High-light |
To calculate lux from feet candles or vice-versa, we use the following formula: lux = foot-candles x 10.764
To get a good sense of light throughout the day, it’s best to take measurements every few hours. That way, one can get a sense for how the light changes through the day, and it can vary a lot.
Light is affected by trees outside, roof overhangs, buildings and other structures, which may cast shade at certain times of the day. It is also affected by wall color. A white wall, for instance, will reflect more light back on a plant than a darker color. Curtains, window molding (and window cleanliness, wash your windows!) and all manner of indoor detritus will also affect light.
The chart below, taken from a FlorA Plant Care device, shows how much variance there is in light day to day through a week.
Light also changes seasonally. Anyone in a northern region knows well that a plant may thrive during the summer but struggle in the winter. (Winter also saps humidity from the air, a major issue for anyone growing humidity-loving plants in wintry areas. Read all about houseplants and humidity in the humidity guide.)
To get a good sense of light in an area, pull those lux or foot-candle measurements together and average them. Let’s say taking measurements every hour, a shelf gets 2,000 ft-c for a couple hours but only 75 ft-c for the other five hours of light. That averages out to about 625 ft-c, just barely hitting “high-light” levels. That may be OK for some high-light plants, but not enough for an exotic cactus or succulent that would thrive with five hours of 2,000 ft-c light.
Also note direct sunlight. East- or West-facing windows will likely get a lot of direct sunlight. For those desert succulents, that is perfect, but many plants will actually be burned by that much light and very quickly plant people will notice wilting leaves, dead spots and a sad plant.
When shopping for indoor plants, it’s best to select plants based on the approximate light levels in the location where you plan to place them. You can find information on the light requirements of a plant on its label. Sadly, most plant purveyors have two signs, “high light” and “low light,” and neither are exceptionally useful or sometimes even accurate. So it’s important to check a plant against your local environment’s light.
Light to thrive or light to survive
Houseplants are classified according to the amount of light needed for growth. Look for this information in general terms on the plant’s label:
- Low: minimum 25 ft-c – 75 ft-c or 75 ft-c – 200 ft-c for good growth
- Medium: minimum 75 ft-c – 150 ft-c or 200 ft-c – 500 ft-c for good growth
- High: minimum 150 ft-c – 1,000 ft-c or 500 ft-c – 1,000 ft-c for good growth
- Very high: minimum 1,000 ft-c or 1,000+ ft-c for good growth
In general, east-facing windows are going to be the best windows for most plants. They get direct morning light. Western windows are good alternative for plants that thrive with direct light as they get a good dose of direct sunlight in the hours before sunset.
But there is a lot to note about which window is best for which plant, especially when seasonality is calculated in.
Seasonality of light
Windows with eastern exposure within the home generally provide the best light conditions for most indoor plant growth because plants receive direct morning light from sunrise until nearly midday. Foot-candle readings at these windows can reach 5,000 to 8,000 ft-c. As the morning progresses, the direct sun recedes from the room. Southern and western exposures are interchangeable for most plants. In the winter, most plants, except those with a definite preference for northern exposure, can be placed in a room with southern exposure.
Windows with southern exposure give the largest variation of light conditions. The low winter sun shines across the room for most of the daylight hours.
In the summer, when the sun is farther north than it is in the winter, the sun rises at a sharp angle in the morning and is high in the sky by noon. Direct light comes into a south window only at midday. If there is a wide overhang covering the windows outside, the sun may not enter the room at all. The sun at noon on a summer day may measure 10,000 ft-c. Indoors, however, a southern window with wide eaves on the outside will receive about the same amount of light as a window with northern exposure.
Windows with northern exposure provide the least light. Because the United States is in the northern hemisphere, it receives most of its sunlight from the south. Out of the four exposures, the northern exposure receives the least light year-round.
That’s it, everything a plant parent needs to know about light when it comes to houseplants.
The Sprout Houseplant Tender makes this a lot easier. It monitors temperature every minute of every day, and sends alerts if a plant needs more light or might fare better in another window.