Color Temperature & Bulb Choice

ultra_violetDecember 7, 2013

I usually haunt the African Violet forum but am coming over here to get enlightened about color temperature.

Most African Violet growers including myself use one fluorescent tube each of warm white (3000K) and cool white (~4100K), which is generally standard in the world of AVs. Until recently I had mistakenly thought that warm white and daylight bulbs (5000/6500K) were the same thing and apparently they aren't (and that daylight is actually closer to cool white).

Because daylight bulbs are similar to natural sunlight (~5000K), why wouldn't growers use daylight tubes exclusively instead of continuing to mix a warm and cool tubes?


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In my opinion (I might be wrong`I'm not an expert) people who use warm 3000K bulbs just use them because most continue to pass on misinformation that you need warm tubes in order for plants to flower. I use only 6500K tubes and my plants are flowering and fruiting just fine.

Maybe someone already experimented to see the difference in plant growth between 3000K and 6500K and it would be nice the results but my bet is that 6500K will do much better job than the warm bulbs.

The only reason why I might see using warmer bulbs is for appearance of plants. Under 6500K you loose your red and green appearance. Since they are flowers obviously appearance is very important when you look at them. That's my theory

    Bookmark   December 8, 2013 at 11:42AM
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Hi ultra_violet,

I tend to agree with slimak (but IâÂÂm no expert either). I think a lot of what us gardeners do is done because of what weâÂÂve heard or read that other seemingly successful gardeners have done in the past. Most of it seems to work pretty good, but we really need to try doing a little experimenting on our own to see if thereâÂÂs a better way that works for us.

In the past, IâÂÂve read about many gardeners who mix one cool white and one warm white bulb (mostly because someone suggested that to them) to provide a wide spectrum of light for their plants. However, looking at the visible spectrum graphs for some of the newer fluorescent bulbs available to us these days, it does seem that many of them can indeed provide a light spectrum similar to that of the sun.

Personally, I use T8, 6500K bulbs with great success. Since I only grow flower seedlings (until theyâÂÂre large enough to transplant outside), IâÂÂm really only interested in vegetative growth. However, sometimes if the weather delays the transplanting IâÂÂve had plants start to bloom before I could get them out :-) If I really wanted the plants to flower, I think IâÂÂd use 5000K bulbs. Those bulbs have a much better balance of blue and red light.

Hope this is helpful,


    Bookmark   December 8, 2013 at 1:43PM
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seysonn(8a WA/HZ 1)

I am also using the same T8, 6500k bulbs in 48" shop light. This is the most economical choice(me thinks). The fixture + bulbs cost about $21. The 6500K bullbs(Philips Daylight Deluxe) costs a little more but probably is a better choice. I have two sets but right now I am using one for test growth. At 32W per bulb , 64w for two, if consumes about ONE kilo watt per 16 hours. It can accommodate about 24 small 3.5" pots. Combining two of them I have more than enough space to grow seedling for all my early spring planting.

    Bookmark   December 8, 2013 at 4:51PM
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Furthermore I simply believe that Iight spectrum has no real effect on flowering and it's not a trigger. When and why does a plant flower ? As far as I know is because it's first of all mature enough to do so. I haven't had any seedlings flower even though they are in the same environment as more mature plants that are flowering.

Secondly, plants flower when they sense they will die soon. That's the whole point of flowering so they could grow seeds and reproduce. I've noticed this to be true. Changes in temperature played a big role in my lettuce for example. When the temps got up to 85 degrees they bolted and created seeds. Size of pots mattered when it came to my tomatoes. I started Tiny tims at the same time. I transplanted one into a one gallon pot and the other stayed in a 4" smaller pot. Obviously later there was a difference in plant size but what happened was that the plant in smaller pot flowered faster than the one in the bigger pot. So I come to a conclusion that the root constraint in the 4" pot triggered faster flowering. In both cases plants sensed that "this is it guys, we're going to be compost soon! " and they hurried to create flowers so they could reproduce.
I know that the length of night time matters for some plants to but I was to lazy to try it out :)

Different varieties have different triggers but at the end I don't believe that light spectrum is one of them. Light spectrum has more to do with growth of the plant. As you most likely know plants prefer to absorb certain wavelengths of light to drive photosynthesis, 600-700 nm on the red end and 400-500 on the blue end mostly. Since photons of blue light have more energy than red light it simply drives photosynthesis better. More fuel more production.

I don't remember but I think the T5ho 3000K fluorescent tubes peak at 610nm which is just at the bottom of the slope of most benefitial red light spectrum so that's why I think that it's a waste of money to try to use them. I can't find the PAR peaks for the 6500K tube but I know that it's in the sweet spot of the blues for plants. If the 3000K light produces light that the plant doesn't really use than why have there in the first place.

As far as using 5000K compared to 6500K tubes I don't really know which is better since I haven't use the 5000Ks but I assume that 6500K has more peaks on the blue scale therefore delivers more usable photons to the plants but that's just an quick assumption and not an experiment conclusion.

Either way I suggest you just experiment with different bulbs, make notes and come to your own conclusions what works better. There is just so much misinformation on the subject in my opinion and people repeat it over and over again that it's best just to try things out and see what works best . Manufacturers and retailers are there to make money so just keep that in mind too. Sorry about the long post

    Bookmark   December 8, 2013 at 5:48PM
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I don't grow African violets, but I do know that Tillandsia grow and flower under 6500k.

T. ionantha (under 6500K T5)

T. ionantha (under 6500K PCF)

T. ionantha (under 6500K LED)

T. aeranthos (under 6500K T5HO)

Each manufacturer's bulb is different. The manufacturer's Kelvin number (example: 6500K) means there is a strong emission line at that frequency. Different spectral emission patterns (nanometer wavelengths) can produce bulbs of similar color - e.g., 6500K - reaching the same Kelvin temperature.

Phosphors are used inside the light tube to peak associated spectral light wavelengths. Photons are released from the phosphors (when your light is turned-on/energized) and have specific frequencies... often times not producing the spectrum curve expected with a Kelvin number.

Basically, not all 6500K bulbs are the same. Color temperature has little to do with spectrum. Most bulb manufacturers will have spectral data for their bulbs, although I have not (to date) seen this information for common screw-in compact fluorescent bulbs. As slimak mentioned, plants prefer a specific spectrum for chlorophyll-A, chlorophyll-B, and carotenoids. Most 6500K bulbs seem to provide the desired spectrum needed by my Tillandsia. Would a different color temperature provide the same results? Probably, especially if spectrum was favorable. 6500K is pleasing to 'my' eye, and my plants thrive under it, so that's why I use it.

    Bookmark   December 8, 2013 at 9:19PM
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I do understand what you're saying about different brands using different emission patterns. However, overall doesn't a 3000K bulb have to peak more in the red spectrum in order to produce this color?
As seen in the link below, the GEs 3000K bulb peak more in the orange range more than blue spectrum and the 6500K is opposite of that:

Either way I believe that there are still discoveries to be made about how plants grow and what causes certain behaviors. I just read this and totally confused my self :)

I think it's impossible to replicate what the sun is doing and the plants do grow differently without experiencing all the wavelengths of light that the sun produces. Simple example is lettuce. If you grow them outside they lettuce leaf is thick and it takes a while for it to wilt after harvest. Lettuce grown under the lights is flimsy and it takes only few minutes for it to wilt. My guess is that the UV and IR rays do cause the plants to grow tougher somehow and overall all spectrum of light is beneficial to a certain extent. The evolution of plants took who knows how long and they adapted for sure to what the sun throws at them further make best use of it. Isolating certain wavelengths and saying that plants need only them to grow , in my opinion is nonsense. I'm not a scientist but I don't believe that we are so advanced and knowledgeable on the subject yet to call these theories facts. That's why I'm a proponent of try and see what happens approach because who knows maybe one of us will some day stumble onto a discovery :) LOL and will revolutionize the horticulture industry! Indoor growing is relatively new so I'm sure there will be new and better ways in the future.

    Bookmark   December 9, 2013 at 12:23AM
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Wanted to reply while I had the time. I will 'try' to stay on subject and not get long winded. So on that note... but first and foremost: I am not a lighting "expert" by any means. The best definition I've heard for 'expert' is from a friend who is a water professional, introduced as an 'expert' witness at a court hearing. He stated to the judge that there is no such thing as an expert because an expert realizes they don't know everything about their so-called expertise and that there is something new to learn everyday. After the hearing concluded, he uttered the following to me: "Expert, huh? An expert is someone who knows how to perform sex a thousand different ways but has no one to perform it with."

I agree with you slimak... there is nothing in the artificial lighting world that matches sunlight. Most indoor gardening involves replicating the sunlight as best we can, using lights with values that are believed to be (or have proven to be) the most beneficial for growing plants. Regardless of the type of light and/or technology used, it all boils down to how our plants respond to the light we provide. Stating what the best light bulb, or light fixture, to use is like opening up a can of worms ~ full of differing opinions, debate, agreement and contention.

You are on the right track regarding 3000k and more of the red spectrum wavelengths associated with that Kelvin temperature. From what I understand (for what it is worth), there is a visible human light region and a useable plant light region. Our eyes cannot detect many light wavelengths in the electromagnetic spectrum ('ultraviolet' bluer than blue with wavelengths too short to see, and 'infrared' redder than red with wavelengths too long to see). Kelvin is simply how the color of light looks to our eyes, human vision.

It's pretty much accepted that the baseline spectral curve needed by most (but not all) plants covers the 400-700nm range [400-520nm vegetative, 610-700nm flowering, 520-610 cartenoid]. Can anyone say with certainty that a light emitting 610-700nm would be the best for flowering, when adding a higher percentage of 400-520nm & 520-610nm to the mix could develop better flowering? Or that a light emitting 400-520 nm is best for vegetation, when adding a higher percentage of 610-700nm & 520-610nm to the mix could produce better vegetation? I also agree with you about plants adapting to light (heck yes, plants will adapt and grow without isolating/adding/subtracting certain wavelengths). Bottom line: what ever light works favorably for you and your plants is the "right" light to use. Personally (through experience, and from doing my own tests) spectrum, not Kelvin, has proved to be more favorable for my plants, but hey: that's just my opinion.

    Bookmark   December 9, 2013 at 5:04AM
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Everyone here is more 'expert' than me. Thanks to you all for your input, giving me the scoop on the 'how' as well as the 'why'.

When I first started out with African violets, a light bulb was just a light bulb was just a light bulb to me. And then I adopted one warm and one cool b/c that seemed like the thing everyone was doing. And recently I stumbled upon color temperature. And now I'm in deep: CRI, spectrum, nm, lumens, foot candles, T5 vs T8 vs T12.... I'm printing out this thread to read and digest in bits as reading stats and technical info on screen is making my eyes cross :)

I think I will try using 6500K exclusively for a while since I already have some and like you all suggested see what happens. And then I will try mixing daylight with warm white to compare.

Hitherto my knowledge of the love affair b/w light and chloroplasts was limited to The Magic School Bus television show that the kids watch. Thanks again, I think my African violet hobby is going to a new level.

@o2tiller: Your Tillandsia are very neat, good to know you've found the right lightening for your growing conditions!

This post was edited by ultra_violet on Mon, Dec 9, 13 at 18:44

    Bookmark   December 9, 2013 at 9:38AM
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Thanks for the compliment, ultra_violet. Let us know your conclusions and how it turns out.

Enjoyed seeing your reference to The Magic School Bus, it put a smile on my face. Although a completely different scenario in the late 60's & early 70's, I started thinking about my 'magic school bus' days... it was definitely an unusual experience involving light!

    Bookmark   December 9, 2013 at 12:41PM
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here is a nice article that explains how Fluorescent lamps work

now I get the whole picture :)

    Bookmark   December 9, 2013 at 4:02PM
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@seysonn: May I ask what brand of shop light fixture you use? It sounds very economical and functional.

@o2tiller: I didn't realize that Tillandsia is commonly called the Air Plant b/c all the ones I've ever encountered in-passing have never been in bloom and looked rather sad and neglected in the store. But wow! Your Tillandsia are stunning.

@slimak: Just a preliminary skim through that article has left my eyes permanently crossed :) It takes me time to digest technical info.

    Bookmark   December 9, 2013 at 7:28PM
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seysonn(8a WA/HZ 1)

I bought my shop lights from HD.
The brand Name is: COMMERCIAL ELECTRIC., just made for HD.
part number : 201-462; cost : $12 +?. bulbs not included.
comes wired, with cord and hanging chains. just install your bulbs.

    Bookmark   December 10, 2013 at 12:50PM
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For those of you whom may be interested, the University of Colorado has produced many simulations for educational purposes. Those related to physics can be found at

Go there and click on the âÂÂBlackbody Spectrumâ icon; then click on âÂÂRun Nowâ to run the simulation. The sim allows you to set any kelvin temperature you want (use the slider or just type it in) and observe the wavelength and intensity of the spectrum curve. As you change the temperature, youâÂÂll also see (top of the sim) the resulting color that our eyes would see at that particular temperature. Use the plus or minus buttons on the left to keep your curve on the visible part of the spectrum.

The simulation is only accurate for blackbody radiators, such as incandescent bulbs. Fluorescent tubes donâÂÂt create light by heating, so theyâÂÂre not blackbody radiators. However, the higher the Color Rendering Index (CRI) of a fluorescent tube, the closer itâÂÂs color spectrum would be to that shown by the simulation. So it can still be interesting and helpful.

Take a look at it, I think youâÂÂll find it interesting. You can even download it to your computer if you can read .jar files.


This post was edited by art33 on Tue, Dec 10, 13 at 13:49

    Bookmark   December 10, 2013 at 1:35PM
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