Home Depot! Seriously, the box stores carry efficient T8 tubes now. One thing that might be throwing you off is that a typical off-the-shelf 48" T8 for a 40W shoplight is only really using 32W. It is just as efficient as the 3500 lumen tubes you have been told to look for, but only produces 2900 lumens. Philips Alto seems to be the common one at Home Depot, but there are others.

If you want to grow your plants full-time under fluorescents, then you might want to read up a little on CRI, colour temperature, and spectrum. For seed-raising, any old cool white is great, but you might want your plants to flower and look nice under these lights, so maybe you don't want to see them through the greyish cast you get from a standard fluorescent tube.

Thanks I have done the reading already, lots and lots and lots of it. But I'm getting, of course, conflicting messages from different people. Some say only lumens matter, others say CRI matters (in terms of not having any gaping holes in the spectrum), some say buy cool white and warm white, others say forget it just buy the most lumens you can get. I've found that reading up on this has only made me more informed and confused :-)

I've said this already to someone (maybe you?): if CRI was the only thing that mattered then we'd be growing tomatoes under a 40W incandescent bulb :)

The amount of light is the most important thing. Lumens are the most widely used method of measuring light intensity, unfortunately not the best for plants. Even more unfortunately, just about every other method of measuring light intensity is so vaguely defined or so widely abused that it isn't of much use to you. So chasing lumens is a pretty good start, but note that some lamps with low lumen numbers actually produce just as much light, its just light that we don't see so well. The Gro-Lux lamps are a good example. But also look at some wide spectrum "daylight" lamps and you'll see slightly lower lumens, but these lamps are actually putting out just the same number of photons that can be used by plants. It is also worth calculating how many lumens per watt you are getting. An inefficient lamp may give you slightly more lumens but use a lot more electricity, and you'd be better off using a smaller highly efficient lamp for more hours or getting two of them. As an extreme example, a 40W fluorescent tube puts out about barely half as much light as a 400W halogen bulb but the halogen bulb uses ten times as much electricity. Using two 40W bulbs gets you the light and saves you big on your utility bill.

Spectrum is also crucial, but the lucky thing for you is that almost any fluorescent tube (excepting weird stuff like black lights, lizard UV lamps, or actinic coral reef lamps) provides a more or less suitable spectrum with at most a few percentage points difference for use lighting geeks to argue about. Get cool whites (4100K temperature), daylights, (5000K-6500K), or a mix with warm whites (3000K) and you'll be fine.

CRI is, frankly, close to irrelevant to a plant. The most efficient lighting sources available, in terms of getting plant growth for the least electricity, have such disjointed spectra that you can't meaningfully calculate a CRI. Gro-Lux fluorescents, which whether you think they're better than regular fluorescents or not certainly grow plants, have a CRI in single digits. So, plants clearly aren't that bothered about CRI. But, having said all that, there are some good reasons to choose a fluorescent lamp with a high CRI, assuming you are choosing amongst more or less "normal" fluorescents. CRI stands for colour rendition index and colours appear more realistic under a bulb with a CRI of 90 or more. The wider spectrum may also (or may not) provide more light in the best areas for photosynthesis, but the difference isn't huge. Much has been written and debated about which fluorescent spectra are best for plants, and by how much, but whatever the answer, CRI isn't the right way to measure it.

Thanks so much. I'll need to check more HDs, the one in my area only had one set of T8s.

i use T8 philips daylight deluxe (6500k). the local home depot has them for individual sale.

hey john, i have been growing my orchids under high output lights with great success.the unit i have is listed below. i buy mine locally. this is the complete unit, not just bulbs. i cant tell the footcandle output though. in fact i just posted a question to see if someone can help me with that. these are great lights. i have an 8 bulb and a 4 bulb unit. hope this helps

http://www.specialty-lights.com/960100.html