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A Primer to Supplemental Lighting for Indoor Seed Starting

Perhaps my least favorite part of winter is waking up to darkness in the morning. This morning, as I led my half-asleep six-year-old down the steps into the living room, we were greeted with streams of light coming through the windows. After the short days of winter and several days of cloudy, wet weather, the sun was a welcome sight. I'm not the only one welcoming the longer days and more sunlight; plants also need adequate light.

Many gardeners have begun, or will soon begin starting seeds indoors for the upcoming growing season. We are so excited to get our tomatoes and peppers off to a quick start. We carefully plant the seeds, gently water, and tuck them in under a plastic cover to keep the humidity up. Every day brings new hope of germination as we peek under the lid, excepting to find the tips of the new plant pushing their way through the soil.

When the day comes, the gardener fills with joy. Seedlings have begun to emerge! The cover quickly is removed, but now what? It's too early to plant outside; the plants would get hit by a frost and perish. Now that the plant is stuck inside we need light and lots of it.

Starting seed indoors can be fraught with many perils for the plants. The one I most encounter is inadequate light. Some crops like lettuce can get by sitting on a bright windowsill. Still even in the case of lettuce and other greens, adding supplemental light would be beneficial.

Unfortunately, we just can't match the light intensity of the Sun, whether we put the plants under a grow light or in a bright window. And when we finally can transplant outside, we have leggy, floppy plants. Yes, "leggy" is a technical term in horticulture that describes elongated internodes (the section of stem between the leaves), due to the absence or low intensity of light. Okay, so the actual technical term is etiolation, "leggy" just sounds better.

At the garden center, the healthier tomato is often a shorter, stockier plant. The plant's stature indicates it received adequate light while growing at the nursery. So what is a home gardener to do when it comes to providing light to their seedlings?

A quick lesson in the physics of light, aka photons. The Sun beams photons at the Earth, which have varying wavelengths. These wavelengths are best observed in a rainbow; where on one side you can see red and orange light (longer wavelengths) and at the opposite are blue and violet (shorter wavelengths). Plants absorb mostly the red and blue ends of the spectrum, but not much green light, reflecting it instead, hence why plants are green.

To have a good seed starting light, you need to provide red and blue light. Some bulbs are cool-colored (blue) while others considered warm-colored (red). Use both warm and cool colored bulbs to offer a broader spectrum of light. A gardener could also opt for bulbs or tubes specially designed to give off a broad range of light these are called grow lights. Grow lights cost a bit more but may be worth it if you are a competitive gardener.

For my indoor seedlings, I turn to a simple fluorescent shop light. I prefer to use T5 fluorescent tubes as these are relatively affordable, while delivering ample light more efficiently than the older fluorescent bulbs (T12 and T8). Fluorescent tubes also produce very little heat. Therefore, I can lower the light fixture so the tubes are only a few inches from the tops of the plants. Tube light output diminishes with use. Most hardcore gardeners replace their tubes every year. I also use an automatic timer to provide the plants with 16 hours of supplemental light per day, with 18 hours being the maximum for seedlings.

Light Emitting Diodes (LEDs) are highly efficient at converting electrical energy into light energy. Commercial greenhouse growers are learning the ins and outs of growing with LEDs resulting in uniform, compact and sturdy seedlings. The commercial success for growers has increased manufacturing of LEDs, which is driving down the price of these fixtures, making them more affordable for homeowners. I hope to replace my T5s with LEDs in the coming years.

While my tomato seedlings sit under the fluorescent, I am basking in the glow of the Sun. Both plants and I are energizing for the coming gardening season.

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