Newbie question re lights for seed starting

newbieroseloverNovember 26, 2004

Hello everyone,

I'm a regular on several other forums but just found yours, and there seem to be alot of experts here! Hope you can help me. I'm planning to use a downstairs room that stays consistently dark and cool to start perennial seeds in January, to set out in my garden in the spring. We live on a mountain at 3300-foot elevation, so I'll be trying to get a headstart on the season.

Anyway, I have a large homemade light box that was used to 'expose' materials painted with photographic emulsions (an oldfashioned printmaking technique). I'm wondering if this setup can be used to germinate seeds. It has 6 ballasts fitted with a total of 11 light tubes (one broke). Each tube measures about 22" long and is marked

GE 20W Black Light F20T12-BL

Will these lights give the correct spectrum that the seeds need? If so, how close should it be suspended above the flats? If not, what should I replace them with?

Thanks very much in advance for enlightening this newbie!

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lightmaster(z8 Salem, Ore.)

You do not want to use blacklights for plants. They put off light in the far end of the violet spectrum, not very much for plants. If I were you I would but some inexpensive shoplights from HD and buy T8 tubes (one 4100 Kelvin; one 3000 Kelvin.) This will give an even balanced amount of light.

If you need more info...let me know!

    Bookmark   November 27, 2004 at 7:41PM
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I'm a newbie too. Isn't Kelvin a measure of temperature? 3000 Kelvin seems pretty hot to me.

    Bookmark   November 30, 2004 at 10:53AM
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It's also a measure of the color of light, though it doesn't take into account the overall spectrum per se. "3000K" is the color of light emitted by a (theoretical) dark body heated to a temperature of 3000K. (As a measure of heat, it is indeed quite hot, a bit more than 2700 degrees Celsius.) You can think of it as a numerical quantification of terms like "red hot" and "white hot." It's a little counterintuitive since red is considered "warmer" than blue but is achieved at a lower physical temperature, but what can you do?

I have to disagree that mixing 3000K and 4100K is terribly useful, they're both on the "warm" end of the spectrum. Also, since seedlings should be grown as close to the lamps as possible, you lose the benefit of any mixing of the different colors of light anyway because the lamps have to be at some distance for the light to "mix." If you use two different lamps an inch or two from the seedlings, one set of seedlings will basically get 3000K light, the other 4100K light.

For starting seeds, warm light is probably a better bet anyway - you want to keep the seedlings stocky. Bluer light tends to encourage taller, thinner growth, redder, shorter, squatter growth.

Finally, don't start your seedlings too early. If they get rootbound in their pots, they'll be slower to establish in the ground, even if they have more foliage.

    Bookmark   November 30, 2004 at 2:42PM
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Thank you very much for your help, everyone!

    Bookmark   November 30, 2004 at 7:43PM
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DanaNY(z6 Astoria, NY)

Actually, it's the opposite. Cool (blue spectrum) is better for stocky foliage growth and starting seeds. Warm (red spectrum) is better for flowering and fruiting. You'll definitely want to go with cool bulbs for growing seeds.

    Bookmark   December 4, 2004 at 5:56PM
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In my experience, cool white fluorescent lamps alone used for garden plants promote legginess. They're not bright enough to keep the plants short with intensity alone, whereas a greater relative amount of red keep foliage growth slower. Which is what you want for garden plants to be transplanted outdoors. Tall, leggy plants that superficially look "nicer" than their stumpier cohorts often lag behind the latter once they're transplanted outdoors into conditions very different from the ones they were started in.

    Bookmark   December 4, 2004 at 10:24PM
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DanaNY(z6 Astoria, NY)

It's pretty well established that the blue spectrum is better for vegetative growth and the red spectrum is better for flowering. You are more likely to get leggy plants if you start seeds under warm bulbs rather than cool. It's the cool bulbs that keep growth more compact. The article below explains it perfectly (read vegetative and flowering sections).

A lot also depends on the lumen output of the bulbs you're using (not all bulbs are created equal), how long you have the lights on, and the distance you have the plants to the lights. If any of those conditions are lacking, your plants will be stretching for more light and become leggy.

Here is a link that might be useful: Blue & Red Spectrum

    Bookmark   December 6, 2004 at 3:51PM
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It's really not worth arguing over, but first, the gross oversimplification that "blue encourages growth and red encourages flowering" is just that, a gross oversimplification.

Second, as I said, my experience is different. I've been growing one or another sort of plant - from seedlings for transplant to orchids that grow under lights year round - under a fairly wide variety of fluorescents for more than 25 years at this point and I was commenting from my experience. Either way, if you're doing it right and not starting them too early, it's not going to make a world of difference either way, there just isn't that big a difference among ordinary fluorescent lamps. If you're interested in comparing results, try one lamp of each in a two lamp fixture. I won't be a properly controlled experiment, but kept properly close to the seedlings, there will be a difference in the seedlings response to each lamp. Decide for yourself which gives you better results.

    Bookmark   December 8, 2004 at 12:58PM
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