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It's Seed Starting Time; Let the Light(s) Shine!


You Bet Your Garden
Question of the Week © 2018 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.

It's Seed Starting Time; Let the Light(s) Shine!

Q. Andy in Gloucester Virginia "on the north side of the York river" writes: "This is my first-time growing tomatoes from seed, and I'm pretty excited about it. I bought your "You Bet Your Garden Guide to Growing Great Tomatoes" book, have read it several times, highlighted parts, put sticky notes inside as place markers, etc. Now: In the book you talk about the light the seedlings need. I have two questions: 1: Does it matter whether the bulbs are florescent or LED? And 2: I found a setup that had four two-foot-long tubes in it. Can I use that since I do not have a lot of space for a four-foot bulb set up?"

A. That's a darn good question, Andy. We write what we know, and I began by starting my seeds under a basic shop light fitted with two four-foot long tubes with great results--so that's what I urge others to start with. But the fact that your two-foot-long set-up has four tubes could produce excellent results. I'm no lighting specialist (I don't even play one on TV) but it seems like your total of 96 inches of tubing might be fairly equal to my differently configured 96 inches. Heck, even two two-footers would be fine--IF you keep the plants directly under the lights; no containers off to the side. Every plant needs to be right under a tube.

The important thing to understand is that almost everyone needs artificial light to start seeds successfully. The exceptions would be people with a true sunroom that gets as much natural light as a greenhouse. Or an actual greenhouse (as long as it doesn't get cold inside at night). With your set-up, you'd arrange your containers in a square underneath the lights. With my longer bulbs, the plants are arranged in a railroad-car-like rectangle.

IMPORTANT: 1) the plants can't be on the outskirts of the lights. When you look down from the top, you should not see any containers sticking out. 2) the tops of the plants should be almost touching the tubes. Fluorescent light doesn't travel well--as least for plants. It's fine for us if we're reading under a fixture that's several feet above us, but the lumens necessary to grow short stocky starts (which is what you want) are only available an inch or less away from the tubes. Remember--the tubes are cool to the touch. I prefer to have my plants growing up into them than to be more than an inch away.

If your starts end up tall and lean, they didn't get enough light. Buy professionally grown transplants and get better lights for next year. Or grow your runts and see what failure really tastes like.

What about LEDs? Good question. I see LED tubes that are the same size and shape as my fluorescents and have high hopes for them. But I now have four existing fixtures, most of which hold four, four-foot tubes each (I start a LOT of seeds); and I have boxes of brand-new florescent tubes. So I'm not trying LEDs anytime soon. But I would love to hear from those who have.

Now let's drop back to before the lights. If you want decent starts, you must:

• Start with a bagged 'soil-free' mix. It may be called seed-starting mix, potting soil, professional mix or something similar. The principal ingredients will be milled peat (or coir), perlite or vermiculite, and compost or "composted forest products". Avoid 'miraculous' mixes that contain chemical fertilizers. If food is included, it should be natural, like worm castings, kelp and such.
If you start seeds in your garden soil, you will fail.
• Use the same kind of containers as the pros use; there's nothing better than the six-packs or four-packs that last year's purchased plants came in.
If you use kindergarten cute containers like old egg cartons you will fail.
• Fill the containers with your soil-free mix and then place them in an inch of water until they are saturated. Don't water from above.
• Place two seeds into each container or individual cell of a container. No more; no less.
• Place the containers into something with a low lip that holds water, like a baking sheet.
• Cover the whole Schmiege with plastic wrap or something similar and transparent.
• Place the whole Schmiege in the warmest spot in your house or on a professional heating mat designed for starting seeds.
• Add small amounts of water to the pan daily to keep the starting medium moist.
• As soon as you see the very first sprouts, remove all of your coverings and turn off any artificial heat. The other sprouts will soon appear.
• Place your set up directly under the kind of lights we just described. Continue keeping the medium moist, but don't overwater. No visible water in the pan.
• When most of the plants have four to six leaves, use a small pair of scissors to cull the weakest looking plant from each cell. (Yes, you must.)
• At three to four weeks of age, feed them a dilute solution of a gentle organic fertilizer.
• Do not install tropical plants like peppers and tomatoes outdoors until nights are reliably in the fifties.

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