Florescent Lights/Seed Starting

Helena, MT

I have seen comments on florescent lighting and seed starting in various threads recently. It has been my understanding that the more expensive grow lamps sold in most hardware stores are actually less beneficial to seed starting than the combination of a soft white and 'warm white' light. I have read that one of each is the best combination to use when using the two bulb shop lights. The problem I have is defining what is 'warm white'. I don't see any labels as such and there is a wide spectrum of lights with increasing "K" values. Assuming the higher the "K" value, the closer you come to actual sun light. I think we could use some clarification on this subject.

Phoenix, AZ(Zone 9b)

Morgan, what I've been using, with good success is one 'red' spectrum and one 'blue' spectrum, 48" long tubes. I don't know about all those K values, lumens and such. My eyes begin to glaze over when attempting to figure it all out. I have not bought new tubes in a while but I do remember reading all the info on the flourescent display section at HD and getting totally confused. In the end I'm pretty sure I found the red spectrum/blue spectrum labels and just went with that. I can easily over-engineer everything I do!

Hopefully somoene with more informative details will arrive and help. It would be good to get this in plain English.

Chaska, MN(Zone 4a)

I'll start my reply by first saying that I am NOT an expert. But I do remember having the same issue with finding the right combo of K values when I was researching my set up. I can only say that I have used my set up now for 3 years and the plants seem to like it. They grow great. So- I use the 2 bulb 48 inch "shop" lights- We got ours at HD. And then I have a box of 4100k bulbs and a box of 6500k bulbs. both are 40 watts. I bought the box of 10 - I think it's 10- just cuz the price was better per bulb. I have them all on a timer and I have it set for like 15 hours or 16 hours a day. Plus I use a heating mat for the seeds that like that. The heating mat is not on the timer.

Durhamville, NY(Zone 5b)

Let's see if I can make this as simple as I can.
Lumens is a measure of light out. It basically adjusts the pure power output of a light to account for how the sensitivity of our eyes are different for different colors. As an example 1 watt of blue light won't look as bright as 1 watt of greenish yellow light.

K stands for Kelvin a temperature unit that was named after Lord Kelvin. It is the same size as a Celsius degree but the scale starts at absolute zero which is -273 degrees Celsius.

If you take a solid object and heat it up it will first glow red, then orange, then yellow, then greenish, and finally blueish. The color of the light from a hot solid is determined by its temperature.

Take the above two ideas and you have the color temperature of a light source. A florescent light of 5500k will appear to be the same color of the Sun. One that has a higher K value will appear to be bluer and one with less yellower or redder than the sun depending on how far you went. The rest of the information has to do with the fact that the way we perceive color is highly complex.

Now any gas discharge light ,which a florescent is one type, only looks like a hot solid body. Where a hot solid body will have some light at every color from the deepest red to the bluest blue florescent light doesn't. They make white light the same way that you television makes various colors. That is by making red, green, and blue light at the proper levels so it looks white.

The idea behind grow lights is that since plants don't use green light why use energy to make it.

In practice it doesn't seem to matter. You can do what Mary did or if only the K numbers are available pick one that is less that 5500 and one that is more than 5500. Or you can go with what the store has available at a reasonable cost.

In reality the design of the ballast and how well it does at actually lighting the tubes to the brightness they are supposed to be will have as big if not a bigger influence that which color temp tube you pick. What most people don't realize is how much brighter Sunlight is than artificial light

SE Houston (Hobby), TX(Zone 9a)

I used a case of "cool white" cause they were on sale. And I picked up regular shop lights for $1/per at the Habitat for Humanities ReStore. Regular $10 shop light kits at HD/Lowes...

I put two light kits side by side under each shelf, for a total of 4 bulbs per shelf. I had four shelves for a total of 16 fluorescents. When the seedlings were still in trays, I ran the lights for 12 hrs/day. After potting into so many cups, instead of building more shelf units (VERY easy), I rotated the trays out every 12 hours, so the lights were running 24/7 for three weeks. Actually, my light bill was do-able, too.

My plants didn't suffer any...208 seedlings, and no damping off...

This message was edited Feb 23, 2011 10:18 AM

This message was edited Feb 23, 2011 10:21 AM

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Charlotte, NC(Zone 7b)

I've had my box of ten "cool white" for several years, so assume they are still available according to Gymgirl's post.

These are Philips 48" X T12 X 4100K. They are on a timer for 16 hours every day. When I replace a tube, I write the date on the end. They can last anywhere from a few months to about a year or so.

Dayton, WA

I keep everything as simple as I can. Four 25 watt ordinary shop lights. My plants hum along nicely under these. I think the trick is to keep the seedlings within an inch or two of the lights.

Helena, MT

I share all your views on the use of florescent light fixtures and would add one other piece of equipment to the mix a small fan. I use this fan for a few hours on my seedlings to prevent mold and dampening off as well as getting them ready for our extreme winds. For some reason the wind velocity here in the center of the valley is amplified four or more time that which exists in town. Plants also seem to have more vigor as well when using a fan.

Thank you Doug for your scientific response to the question. The Kelvin explanation is quite helpful. I recently purchased two each of the Cool White, 4100 K, 34 w, 2650 lumens; and Ecolux Technology 'Sunshine F40", 5000 K, 40w, 2250 lumens. I have read in several places that you should use one of each in your light setups, but I don't truly understand the reason for the Cool White bulbs.

Due to expense I don't replace all my bulbs at once, but I have heard that bulbs for raising seeds need to be replaced yearly. I have some onion plants which were started using the best of last seasons bulbs and they seem to be doing fine. I don't think there is an easy way to quantify the need to replace bulbls yearly without devoting the space and time which I really don't have, so I take it for granite that last years bulbs need to be rotated out as soon as possible. Any bulbs which show black on the ends of the bulbs are placed in a stack in the shed to be used in the house as necessary. I have not disposed of any bulbs still usable, but due to mercury content many states require they be disposed of properly.

Durhamville, NY(Zone 5b)

The idea behind using a cool tube and a warm tube is that the warm tube should have more red light and less blue light and the other way around for the cool one.

The idea behind replacing tubes once a year is the fact that florescent tubes get dimmer with use. The problem is that besides just time lit there is also the number of starts and the design of the ballast. A poorly designed ballast -read cheap- will beat a tube up much faster than a properly designed one. Starting a florescent also is hard on them. Some methods are harder than others. Someone desided a good rule of thumb was replace them each year. One a year in a good fixture sounds like a lot.

Let's say you leave them on for 16 hours each day for 12 weeks.
That's a total of 16 x 7 x12 = 1344 hours. and only 84 starts.

The life of a florescent tube is quoted as 20,000 hours based on being started once every 3 hours. Unless your fixture is particularly bad, I don't think that a tube should need replacing at about 7% of it's rated life and 1.25% of its starts.

By the way those cheap shop lamps are noted for short life.

Durhamville, NY(Zone 5b)

I just Googled this site up. It seems to give a good overview of the factors. It happens to be comparing T12 -the old standard 1 1/2 inch tubes with the newer T8 - 1 inch Tubes.

Canyon Lake, TX(Zone 8b)

I use T-12 48" Full Spectrum 6,500K bulbs. I prefer these to shop lite bulbs or standard cool white or daylight bulbs because I have fewer problems with plants getting leggy. I've used them all and have settled on the full spectrum bulbs.

Irving, TX(Zone 8a)

where did you buy your bulbs: T-12 48" Full Spectrum 6,500K?

Canyon Lake, TX(Zone 8b)

drthor, it was one of the big box stores in Dec 09 right before starting tomato plants for Spring 2010.

I replaced some 15 - 20 year old VeriLux bulbs and a mixture of cool white, shop lights and daylight bulbs that were in my triple shelf grow light cart from when I grew African Violets. The cart is good for starting seedlings so I'm glad I didn't get rid of it when I gave up the violets.

Helena, MT

I'm not surprised at the short ballast life on the cheap shop lights. Mine were made in China which means junk to me. I have replaced ballasts in aquarium light fixtures which have lasted for many years, so it's possible I will need to replace the ballast in the shop lights within a couple of years of operation. Would be helpful to add some information on the subject of ballast replacement here as well. Several have commented on leaving the lights on 24/7 and simply rotating their seedling flats after 12 hours. I would presume this practice could extend the life expectancy of the shop light fixtures for a number of years. At least ballast failure in recognizable by difficulty in starting, so assuming the bulbs are new the ballast failure could be recognized in time to make a change of ballasts. I purchased my replacements at an electrical supply shop, but you need to know exactly which type of ballast to use, and whether it will fit the shop light in question.

For some reason I could not click on the link you posted Doug and get to the source of the article. I have found this true of several such links here lately. Anyone else having this problem?

Phoenix, AZ(Zone 9b)

Morgan, the link works fine for me. Try pressing and holding the CTRL key when clicking. See if that helps. There's a Windows setting somewhere that controls that behavior.

Helena, MT

I guess the problem is mine Mary, I'm just not getting anything, but thanks for helping.

Helena, MT

I have Earthlink dialup which refuses me to down load videos. My kids are continually sending me videos of the grandkids which will not work on my service as well.

Helena, MT

I found a solution to my problem. Left click on link, add to favorites, and access the link from there.

Nice information again Doug.

Springfield, IL(Zone 6a)

Thanks for this thread...

I have to say I'm a bit confused. I've been learning a lot on a seed starting thread and I've been told that I shouldn't be using the lights I'm using. I bought those clip-on single bulb fixtures and put grow light bulbs in them for my seeds. Someone who seems to know a lot said that I need to be using the florescent fixtures. I posted that I did learn that I should use one bright white and one cool white in order to get the full spectrum (instead of purchasing expensive grow bulbs). She replied that I should just be using the cool white because seedlings need the blue range only, not the full spectrum. What is correct????

SE Houston (Hobby), TX(Zone 9a)

Or, just copy and paste the link directly into your browser...

Helena, MT

Susie, I'm a bit surprised at the magnitude of response here as well, and well pleased with interest. I'm going to be spending some time researching the recent link Doug provided. But to answer your question, I have never heard that blue light alone was all that was necessary for starting seedlings.

As for the use of incandescent lighting I have a clip on shop light which I use periodically on an indoor tomato planting which gets minimal sunlight from a south facing window. I clamped it to a stake in the center of the planter and put a high intensity halogen bulb in the light fixture. I read that the sodium vapor bulbs were actually slightly better for this purpose. It was just a recent experiment so I don't have much to add on the subject, however the three determinate tomato plants (Prairie Fire from saved seed) in this container are actually producing fruits. The only ripe one is small, but so far it seems to be working. I have twenty or more green tomatoes which are still small, but should not be long now before I can harvest a few.

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Helena, MT

Doug, I read though the summary article you posted above several times and it an excellent article on this subject. The article mentions how simply the shop lights can be converted to the T8 (1-inch diam.) bulbs which will be the only bulbs available in another year along with incandescent light bulbs. I would encourage anyone with the inexpensive shop light fixtures to start looking into the replacement ballasts for your fixtures ASAP. Even if you don't change them out right away, they could become scarce or pricy after the transition is made.

Laingsburg, MI

I have just started this adventure in raising flowers/veggies from seed. Since I have learned the need for the lighting source to be inches above the flats, I have raised them up on books with an Ott light (for crafting) several inches above them and have just added a halogen desk light. At present I cannot create an area with flourescent lighting on a stand. Would all of this info on long flourescent lights apply to using desk lamp bulbs?

Charlotte, NC(Zone 7b)

Here's a link to a pocket guide from Phillips that tells you everything you need to know about fluorescent lamps


(Pam) Warren, CT(Zone 5b)

I've been using T5's since last year. This is my setup in a city window on a courtyard that gets very little light. On the top two shelves the lights are from Home Depot, not the high-output, with daylight bulbs I bought separately. On the bottom, new recently from ACF, is a 2 bulb high-output fixture with daylight bulbs. I had a Hydrofarm single bulb fixture there before, but thought there was too much fall-off on the edges for the deeper tray. The plants down the middle were significantly bigger.

There's no mention of the T5's above. Does anyone have experience/knowledge/opinions? I started out with the Hydrofarm and assumed they would be better, but they're not cheap. Does it make that much of a difference?


(Pam) Warren, CT(Zone 5b)

Sorry, forgot to add the pic

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Grosse Pointe Shores, MI(Zone 6a)

I've been growing orchids and starting tomatoes under T5s for several years. I had been confused about the "warm" and "cool" bulb issue. An acquaintance of mine helped me get a better handle on it through pictures...and in this case, at least for me, they really are worth a thousand words...most of which I didn't understand!

The key is that you don't really need to replicate what sunlight looks like. You only have to provide the color spectrums that plants use. In the first chart, the white lines show the colors that chlorophyll absorbs. Notice it is heavy on the blue and red ends, and not so much in the middle.

The second chart compares the colors that a 6500K (top) versus a 3000K bulb emit. The color areas between the two black dots on the top chart is the chlorophyll range. Notice that the 3000K bulb emits very little of the "chlorophyll colors" by comparison. What is not easy to see on these charts is the scale on the left side that measures the intensity. The 3000K bulb has a max energy density of 3.0, while the 6500K bulb has a max of well over 100. So not only does the 6500K bulb give you more of the colors your plants need, they give you a LOT more.

I just use all 6500K bulbs in my fixtures now, and I can bloom high light orchids, so it must be working!

Thanks to Ray Barkalow for this information!


Thumbnail by koshki Thumbnail by koshki
(Pam) Warren, CT(Zone 5b)

Thanks, I'm glad I spent the extra $$ on daylight bulbs! And I have seen a difference since I went to 2 bulbs. Petunias, Marigolds, Bacopa are blooming, and Datura is huuuuuge!


Liberty Hill, TX(Zone 8a)

I've found that when I put 2 or 4 bulbs in a fixture the plants lean towards the "cool" bulbs and the plants grow well but don't bloom, unless they are exposed to sunlight too. They will bloom if they are exposed to light in the "hot" spectrum.

Phoenix, AZ(Zone 9b)

Katherine, I Googled 6500k and found this: "T5 fluorescent grow lights are available with 3000K lamps for flowering & fruiting growth, 6500K for vegetative growth, and a 50% mix of 3000K and 6500K for all purpose growth."

Considering your comments above, I'm surprised that the 6500k is for vegetative growth, I had expected it was the other way around. Just when I think I might be starting to understand this whole lighting thing, something confuses me again. sigh.... :-

Grosse Pointe Shores, MI(Zone 6a)

Mary, on technical things like this (which tend to make my eyes roll in the back of my head), once I grasp an explanation, it's best if I stop there! ;)

All I can tell you is my experience. With my new light setup, I can grow and flower high light orchids like cattlyas. And I was never able to grow basil indoors until I put them under these lights.

Good luck!


Phoenix, AZ(Zone 9b)

I hear you - if it works, just go with it. I'll try your 6500k's when I start growing again. Thanks!

Brooksville, FL(Zone 9a)

Great information here as I'm about to purchase lights again, (couldn't take mine when we moved south so have to start all over again). This is what is so unique to Dave's, folks from all over sharing.

Thanks all.


Charlotte, NC(Zone 7b)

Jan, when I lived in Palm Beach County, I started all my seeds outdoors in bright open shade. I was home all day with young children, so was able to keep an eye on the seedlings. With all that bright sunlight and warm weather, you could skip having to purchase a light set up.

Brooksville, FL(Zone 9a)

thanks honeybeenc I might just give that a try, that sure would be nice. I'll post picture of how it goes.


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