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Propagation: Warm light/ Cool light question

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Watertown, WI
(Zone 5a)

January 20, 2011
4:03 PM

Post #8321169

I've read from several sources that mixing warm and cool fluorescent bulbs produces better results when starting seeds. I started all my seeds last year with cool bulbs and didn't really notice any issues other than that my seedlings didn't really take off until they got outdoors despite transplanting them to larger pots and gently fertilizing them (once they were far enough along) with fish emulsion.

This year, I went out and picked up a set of warm bulbs and rigged my shop light setup so that each light has one warm bulb and one cool one. Problem is, at least to me, that the light looks dimmer now and not quite as "outdoorsy" as the light I had with only cool bulbs. The light in my seed starting room (which is my office) is decidedly amber now rather than the bright-as-daylight lighting I had before.

Can anyone share their thoughts and experience with lighting? Should I go back to the brighter (cooler) bulbs or keep the warmer ones to increase the light spectrum (despite them being dimmer and more amber colored)?
Galesburg, IL

January 20, 2011
5:11 PM

Post #8321310

I'll try to answer some of your questions and offer some advise and my experiences. I have also seen the recommendation about combining the warm/cool lights in one fixture. Doing so will provide a more balanced range of light across the spectrum needed for plant growth. That is plant growth from seedling through flowering/fruit production.

Without going into specifics about the wavelengths of light needed for plant growth, blue light is needed for vegetative growth and red light is needed for reproduction. What we perceive as white light is a mix of all colors of the spectrum rather than an individual color or wavelength.

In very general terms newly germinated plants are going through rapid cell division and growth and their only concern is vegetative growth. The majority of light needed for vegetative growth is in the blue spectrum. "Cool" florescent light bulbs produce more blue light than in the other colors of the spectrum. That is not to say they don't produce any light in the red spectrum, they do, but a majority is in the blues. Warm lights provide more light in the red spectrum than blue and are better for flower/fruit production.

I use only cool white lights and I still get blossoms on plants that are inside longer than I would like, and I'm sure that seedlings would grow if all you used were warm lights. If you were in a laboratory, with replicated trials you could very easily demonstrate the advantages of each type of light, but it would probably be difficult to replicate the results in homes with all the other light that plants get from windows and other lights in the room. Since I primarily use my lights for seedling production, cool white is my choice. For those trying to overwinter plants under lights or produce flowers in houseplants in the winter I would probably switch to warm lights with more of the red spectrum. The recommendation of using one of each type of light is general for "plant growth" and most plants would grow from seedling to flower/fruit just fine using this combination. However, if you only use lights for one thing (seedling production), it is cheaper and easier to use one type of bulb and not worry about combining them.

Another note, what we perceive in light intensity is not the same that plants require. I have seen some types of lights (LED) that are made to emit very specific wavelengths for plant growth and you would have a hard time reading a book using only those lights. Plants under those lights however thrive because they are receiving exactly the wavelengths they need. Also, to humans, light in the blue spectrum appears much brighter than light in the red spectrum at the same intensity. So what you see is not necessarily what your plants are getting.

When you start discussing light spectrum and intensities, it can get confusing real quickly and I hope I have provided some good information without muddying the waters too much.
Watertown, WI
(Zone 5a)

January 20, 2011
8:05 PM

Post #8321583

trc - Thanks for the wealth of information!

Given what you've said, I think I'll keep my babies under the cool lights (which, now that I look at the box they came in says "daylight," while the other lights say "warm"). I'm starting some herbs and flowering annuals and I think I might use the mixed warm/cool lights on those. (I'm thinking of giving a few of the annuals to my dad to sell at the farmers market this spring, so a few buds/flowers would be a bonus.)

I'm going to have to look into the LED lights. I was reading a bit about them in another thread. Maybe next year's setup will include them. :)

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