The 4100K simply refers to the overall "color temperature" and is fairly meaningless in describing plant light applications, since it's just an overall description of the color blend, rather than a specification of wavelength outputs. I grow all my plants under close fluorescent light and they do great.

I think that GE is pretty reputable. The only other things I would check are the LUMEN OUTPUT and LAMP LIFE. I think you're pretty safe buying these lamps.

The lamps will work but you will be disappointed. They are at the surplus store for a reason. They have low light output, the light gets even lower after a thousand hours or two, and the tubes don't last very long. Go back to Home Depot and get their extended performance Sylvania tubes, or something with a similar long lifetime, high output, and good lumen maintenance. You don't say what you are planning to grow but these will be good for most purposes.

4100K is not meaningless, it describes the colour of a fluorescent tube. It is possible to have two tubes both rated 4100K with a very different combination of wavelengths, but in practice the wavelengths emitted by fluorescent tubes are all much the same (exactly the same with the newer triphosphor tubes), just some put out more blue and less red, or vice versa. 4100K is probably what you want. 2700K/3000K tubes are common for domestic use, they have a warmer orange tint (more red, less blue), but plants don't grow so well. 6500K tubes are increasingly available, I use them on cacti and succulents and like the results.

Puzzlefan,

Our Home Depot has always had Philips cool white T8's in stock in boxes of 10 for $19.95. They aren't sold individually. They have worked fine for me. I have no trouble using a box of 10, because I have several plant stands. In my opinion, paying more than $2 for a bulb is a waste of money.

MM

I was in HD but didn't see any large boxes of bulbs. I'll have to check again the next time in. I did see some Sylvania which matched the specs of those listed in Gardeners Supply; 6500K I have hesitated on those because they had a warning on the wrapper about mercury. Do the the ones that come in the box have mercury in them also? It wouldn't be problem (I am assuming this) unless they were dropped but there is always that possibility.

puzzlefan,

"Do the the ones that come in the box have mercury in them also?"

As far as I know, all fluorescent bulbs have some mercury in them, and should be disposed of according to local regulations. Tossing them in the garbage is not a good idea, and is illegal in many areas. Here in Maine we have special places to take hazardous items like fluorescent bulbs, computers, computer monitors, TVs, etc. The Philips T8 bulbs do have less mercury than many fluorescent bulbs.

Do ask an employee at Home Depot the next time you are there. Our local HD has never been without the 10-paks of Philips cool white T8s. They are the "sweet spot" for gardeners starting seeds.

Early on, I bought some warm white T8 fluorescent bulbs, because I had heard suggestions to mix warm white and cool white bulbs for the best results. I had a fixture loaded with warm whites (which cost over $3 each) beside a fixture with $2 cool whites and, to my surprise, my seedlings all leaned strongly in the direction of the cool whites. That was all the convincing I needed. If my seedlings actually preferred the cheaper cool whites, why should I pay extra for something they didn't like?

MM

paying more than $2 for a bulb is a waste of money.

Actually, getting anything less than the most efficient tube is a waste of money. Over 90% of the cost of fluorescent lighting is the electricity. Getting a tube even 10% less efficient to save yourself a dollar or two up front is not only worse for the plants, it is worse for your pocket. The hardest thing to judge is the lumen maintenance. Fluorescent tubes used to lose so much light after a few thousand hours that growers would throw them away after six months. The best modern fluorescent tubes lose so little light after even 20,000 hours or more that it is hardly measurable. There are still enough old spec tubes around that the cheapest tube in most stores is not the one to buy, the best performance comes for a dollar or two more, then with the very expensive tubes you are paying for things like wide spectrum, specialist colours, or very high colour rendition that you probably don't care about for raising seedlings.

The Philips 741 cool white tubes are pretty good but the high spec Philips 841 tubes (or the Sylvania that I mentioned) are certainly brighter, last better, and look better, for human eyes at least. The 741 tubes use halophosphates and so the spectrum is a little different, you might want to argue that they are equally effective for plants. Unless you have strong feelings about the spectrum, I suggest getting the slightly more expensive 841 tubes. But then if you have strong feelings about the spectrum you shine on your plants, you will probably wish to purchase a much more expensive high-CRI light or special grow light ;)

The "leaning" experiment is not proof that the seedlings actually grow better under cool whites, but it is a good demonstration that blue wavelengths control the shape and orientation of plants. Fluorescents with more blue wavelengths such as the cool whites will generally produce more compact seedlings although experiments suggest that plants grow just as much (total yield by mass) under the warm whites. A mix of 6500K and 3000K fluorescents produces almost exactly the same spectrum as 4100K fluorescents, which may be handy to know if you are using compact fluorescents that may be hard to find in cool white.

SnB,

"Unless you have strong feelings about the spectrum, I suggest getting the slightly more expensive 841 tubes."

I'll look for them the next time I buy bulbs.

"The "leaning" experiment is not proof that the seedlings actually grow better under cool whites, but it is a good demonstration that blue wavelengths control the shape and orientation of plants. Fluorescents with more blue wavelengths such as the cool whites will generally produce more compact seedlings..."

Keeping the seedlings as compact as possible is important to me because I grow them to a fairly mature state before setting them out here in Maine's short growing season. If they get too tall, I have room for only two shelves per plant stand. I much prefer to have three shelves per stand.

My pots are 6 inches tall and if my seedlings reach 18 inches above the pot their effective height is two feet and I start to run out of space for my fluorescent fixtures. Unfortunately, that has happened to me several times. I have even considered using a plant growth regulator to keep my plants more compact.

MM

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This post was edited by moochinka on Wed, Sep 11, 13 at 12:58

The cheapest bulb/tubes for lighting @ HD cost from $3.50 to $5.00 for T8, if you buy them in packs of two. The so-called GROW lights cost even more. Those are Phillips brands. No other alternative I could find.

I buy my T8's in packs of 10, because they are cheaper that way.