Measuring Light for Your Indoor Plants
Plants need light for photosynthesis, the process within a plant that converts light, oxygen, and water into carbohydrates (energy). Without the right amount of those three things, your plant will eventually die. It sounds fairly simple, right? Yet somehow many of us struggle to achieve the right amount of light. The reason being is that we see light very differently than plants do. Visible light from the sun appears white to us, but it’s made up of multiple wavelengths (colors) of light. We have photoreceptors in our eyes, and plants have their own throughout their stems and leaves. Plants photoreceptors allow them to differentiate between red and blue, and allow them to see wavelengths in the far red and ultraviolet parts of the spectrum. Plants also see the direction light is coming from, can tell whether it is intense or dim, and can judge how long ago the lights were turned off. We on the other hand cannot. So it’s no wonder that we struggle to understand exactly what bright indirect light means, due to evolution, our eyes make any scene look as bright as possible but that doesn’t mean it is, at least not for a plant that needs light to make food and survive. So how exactly do we ensure if our houseplants are getting the right amount of light if our eyes are unable to measure the intensity of light?
Houseplants will show you that they’re not getting enough light, some easy to spot signs are; sparse or leggy growth, they begin to produce small and underwhelming leaves, they stop growing, variegation on leaves turn green, it no longer flowers, leaves will begin to lose their shape, and the soil remains moist for long periods and will eventually cause root rot. For a while, I was not aware that you could measure light to help guide you to provide the right amount for your houseplant. How easy would it have been if instead of saying place this plant in bright indirect light, we would instead say, this plant needs between 1,000 foot-candles to 3,000 foot-candles for around 8–12 hours a day. That will certainly take the guessing out and keep our plants from going through a slow death.
You’re probably wondering what exactly is a foot-candle and how do you measure it. A foot-candle is the imperial standard measuring system (feet, pounds, etc.) of a unit of measurement of brightness or illuminance. You will often find the same measurement in lux which is measured using the metric system (meters, grams, etc.). For conversion, 1 foot-candle = 10.764 lux. They have apps that you can purchase for as little as $2 that will let you measure brightness, or you can purchase a Lux meter for under $50. I purchased the Dr. Meter 1330B-V Digital Illuminance/Light Metter on Amazon for $34.99. Before I continue I’ll like to emphasize that Lux meters can detect and measure all white light, especially from the sun or incandescent bulb. If you are using red, blue, violet, etc., grow lights then I highly recommend you use a PAR meter. PAR is not a measurement of light, but rather represents the waveband in the light spectrum which is used by plants for photosynthesis. Although rather accurate a PAR meter is quite expensive, ranging anywhere from $150-$600. Therefore many choose the more affordable lux meter, and for a plant hobbyist that has anywhere from 1–30 houseplants in their home that only depends on the sun for light, a lux meter is enough.
My personal experience with a lux meter has made an enormous difference in my plant care. In March of 2021, I moved to my apartment, and unfortunately, all my windows face north and are blocked by a building. It still gives out some light and I assume it would be enough for some of my plants, however by May most of the plants inside my apartment had died or were on the verge of dying, the ones in the balcony were surviving but not thriving. I brought my Dr. Meter from my greenhouse and began measuring the light from the brightest spot of my apartment, the only spot where I could visibly see the sunshine at the brightest time of the day. It read 100 foot-candles… To give you a reference most of the plants in my possession require “bright indirect light” and bright indirect light is around 400-foot candles. It’s no wonder my plants weren’t growing and the soil was staying moist longer and that brought gnats! My first experience with gnats was a nightmare! I’ll do a blog on that next.
In my book New Plant Parent: Develop Your Green Thumb and Care for Your House-Plant Family by Darryl Cheng there is a chart on foot-candles. (By the way, if you’re looking for an informative plant book, I highly recommend this one.) In his chart, Darryl Cheng mentions that for “low light” plants 50–150 foot candles are tolerable. Before I move on, I want to quickly say that most low light “loving plants,” (big emphasis on the quotation marks,) don’t necessarily thrive in low light. For example, I see the snake plant listed as a low-light loving plant, but that’s not necessarily true. A snake plant may tolerate being in a low light condition only because it dies slowly and to us, it looks like it’s doing just fine. For medium light plants, 200–800 foot-candles are what most of your houseplants would prefer to be in. The low-light plants will do much better in this range. For the houseplants that require the high end of bright indirect light, 800–1,000 foot-candles, are ideal. Flowering begonias, and some succulents will do good in this range. For those with succulents and cacti that may require some direct light, 2,000–8,000 foot-candle are needed. For some comparison, outside in unobstructed sun, the foot-candle measurement would be about 10,000 foot-candles!
When you begin to measure foot-candles, I would look for the ideal foot-candle your plant needs on the brightest part of the day. For example, if your string of pearls requires 800–2,000 foot-candles, find a spot that will get that reading on the brightest part of the day as that will ensure you that the spot you chose, will give your plant the light it requires. If you measure at 800 foot-candles at 9 a.m you will realize that by 1 p.m that location will be much higher, and your houseplants will get sunburned if they receive too much light. Always monitor your houseplants and look out for sun damage. Take into consideration the time of year you’re measuring foot-candles, during the summer days keep the foot-candles on the lower end of the range your plant requires as the sun stays out for long periods of time. For instance, if your plant requires 800–2,000 foot-candles, a spot that gets around 800–900 foot-candles on the brightest part of the day will be ok. When measuring brightness with your lux meter, I encourage you to move a few feet away from a window so you can see your readings changing, add a sheer curtain and see how much that sheer curtain changed your measurement. On a cloudy or rainy day do some readings so you can see how clouds change the brightness.
In my personal experience, I’ve come to realize that most of my houseplants did so much better when I moved them to brighter locations. Using a lux meter has helped me better understand plant placement, and I no longer select a plant for a spot but rather select a spot for a plant. I use my lux meter to measure a spot where I would like to place a plant and depending on the foot-candles it gives me, I then decide what plant will do best in that location.