A light meter and some expert advice confirm that a shop light can be just as effective as a specialized grow light for indoor gardening

CLEVELAND, Ohio -- The hardest season for plants—and gardeners—in Northeast Ohio is this stretch until the last frost, usually sometime in May.

There are enough nice days here and there to tempt us into gardening, only to see young plants destroyed by frost, ice, and winds. To avoid heartbreak caused by spring’s wild temperature swings and freezing precipitation, gardeners have traditionally gotten a jump on the growing season by starting seeds in greenhouses, cold frames, or next to a window.

All these methods rely on sunlight, however, which is not a selling point of our part of the world. (Clearly, it is the ice cream.) Fortunately, with the advent of LEDs as a cheap and bright energy source that uses relatively little electricity, grow lights are a fantastic replacement for the greenhouses of yore. Now any little used corner or countertop can be an affordable indoor growing space. But can any old light promote plant growth?

Read all of Susan Brownstein’s gardening columns here.

To find out, I spoke with Roger Buelow, who lives in Northeast Ohio but travels the world as the chief technology officer for AeroFarms, an agriculture company that builds commercial indoor vertical farms. As CTO, Roger works with research and development teams to engineer the best possible grow lighting for indoor farming of greens and other food crops. I figured he would scoff at the lighting I set up for seedlings in our mudroom, which consists of an LED shop light and a couple of grow lights I picked up from Home Depot. Instead, he excitedly suggested we measure the light coming from these sources and find out if it had the right characteristics to replace the sun.

During a rapid-fire discussion about the lumens, photons, kelvins, and wavelengths that make a plant grow, Roger explained the main factors are an artificial light’s color and brightness. Visible light from the sun contains all the colors of the rainbow, but plant growth is most activated by red and blue light. (Indoor farming operations have an eerie magenta glow because industrial grow lights use only these wavelengths.)

Brightness is where natural sunlight often fails us in Northeast Ohio, but indoors, it not only depends on the lightbulb itself, but how close the bulb is to the plant. To take advantage of this effect, my lights are hung on chains only a few inches above my seedling trays, which I then shorten as the plants grow taller.

Professionals measure the combined brightness and color of artificial light with the impressive sounding units of “photosynthetic photon flux density,” or PPFD, which is simply amount of light hitting a plant that will be useful in making it grow. Much like animals need to consume a certain amount of calories to survive and thrive, plants need enough PPFD to perform photosynthesis. While commercial grow lights have up to several hundred or thousands of PPFD, a PPFD of about 150-250 is good enough for home gardeners.

Using Roger’s light meter, I measured the PPFD of my shop light, my grow light, and good old-fashioned sunlight. On a sunny winter afternoon, the light in my south-facing window was 470 PPFD, which sounds great until you realize that full sunlight in other parts of the world is around 2,000 PPFD, and sunny days can be seldom and short in winter. The shop light clocked in at a respectable 160 PPFD, and the grow light was about 200 PPFD.

What these readings do not account for is amount of time that the plant is exposed to the light source. With a sunny window or even a greenhouse, you are at the mercy of the length of the day, but artificial lights can be left on as long as you want. Roger advises that “most plants want a night,” but six to eight hours of darkness is plenty. I keep my lights on a timer and turn them on from about 6 a.m. to 10 p.m.

One more factor I had not considered for indoor growing was air flow. As you might remember from biology class, plants exhale oxygen and inhale carbon dioxide during photosynthesis. However, Roger explained, “Leaves have weak lungs.” The more light that the plant receives, the more oxygen it will exhale, and without sufficient fresh air, the oxygen will lay on top of the leaf and block carbon dioxide from reaching the plant. Roger recommends setting up a fan that provides just enough air flow to “get tremble on the leaf.”

Because I can control the length of time the lights are on and the distance from the lights to the plants, both the shop lights and grow lights will make my indoor seedlings grow. Now I won’t have greenhouse envy when I watch gardening shows.

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