Gardening Talk: How to get enough light on your seedlings in cloudy Michigan


I talked with a plant lighting expert about how to get sufficient light on our seedlings we have growing.

If you like gardening like me, you probably want to get the party started. Starting seeds now and in the past few weeks is a great idea. But the almost constant clouds of Michigan make it almost impossible to grow sturdy, dark green, healthy seedlings to transplant in our gardens.

We can add artificial light to our seed growing setting, but how do we know if we are giving the small seedlings enough of the right kind of light?

Erik Runkle, a professor in the Department of Horticulture at Michigan State University, says he’s had that same question many times already in the past few weeks. Runkle has been researching and teaching about supplemental lighting in horticulture for 25 years.

Runkle takes a very scientific discussion and turns it into some tangible actions for us home gardeners.

LED lights over a flat of newly germinated seedlings. (Mark Torregrossa | MLive.com)

He says seedlings want to get a certain quantity of light for a considerable length of time each day. Runkle advises plants like certain wavelengths of light, mostly blue light, green light and red light. Humans see green light, with not so much the ability to see blue sunlight and red sunlight. One important note: Runkle advises the white light we see from a florescent light has all three light colors.

Runkle gives us a starting point for an easy-to-find, easy-to-set-up plant lighting situation. Runkle starts with saying the LED tube lights made to replace florescent tube lights are the lights to use for plant lighting. LED lights put out some nice light, and use less electricity than florescent lights.

He advises four LED light tubes, jammed tightly together, would probably emit enough light to help the seedlings thrive. Typically those lighting fixtures and bulbs are 4 feet long. With this set-up, we could light two of the classic plastic flats. Each flat can hold 36 plants to 72 plants.

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LED lights over a flat of newly germinated seedlings. (Mark Torregrossa | MLive.com)

The lights should be at least 12 inches above the tops of the seedlings, and you could go as high as 18 inches above the plants.

Runkle says placing the lights too close to the plants may not give the seedlings a full spectrum of light.

The light that shines out to the side of our plants is wasted, so Runkle suggests placing a reflective material on three sides of our lit area. He says we can use white styrofoam or maybe even cardboard pieces covered with aluminum foil. This will reflect the stray light back onto the plants.

The supplemental lighting should shine for 16 hours to 18 hours a day.

I asked if it’s best to rush our trays of seedlings out into full sunshine on those rare sunny days. My thinking was the real sun is best. Runkle cautioned about that idea. The straight sun rays could be 10 times stronger than our artificial light, and could burn tender seedling leaves. He does want us to gradually introduce the larger plants to direct sunlight for a week or two before we plant them in the garden.

Without ample light, seedlings will stretch toward the sky and be very top heavy, weak stemmed and not healthy.

Runkle warns that ample lighting is only one part of raising healthy vegetable and flower transplants. The temperature in your growing area should be in the high 60s to low 70s, meaning a warmer part of your home.

Lighting is only one needed condition for healthy transplants. Of course, correct water with plants not drying out and not being flooded is a good goal. You’ll want to fertilize the seedlings once they are up and growing. I just use a general 20-20-20 liquid fertilizer at a half-rate. Since most planting soils are light and airy, they don’t hold a lot of nutrients. You’ll want to replace one of the weekly waterings with the liquid fertilizer.

What about the specialized grow lights? Runkle says it’s not a guarantee that grow lights put out better light for the plants. You may be spending more money for the same amount and quality of light. Many of the grow light companies eliminate the green light so it’s not too bright of a setting for us humans. Runkle’s 25 years of research shows plants also need green light. He comes back to you’ll probably have less money in your lighting set-up by putting four bright lights together about one foot above the plants.