This topic has been covered ad nauseam in this and other GardenWeb forums. Any commonly available commercial bulb with a spectrum rating of 4100K or above will work. I use both 4100K and 5000K bulbs, myself. Much of the light produced by 3500K bulbs is not useable by plants, so these bulbs are inefficient. 6500K bulbs put out excellent quality light, but just not as much light as the lower frequency (lower K, more red) ones (depending on the specific lamps).

This post was edited by brandon7 on Thu, Jan 22, 15 at 22:49

I wish I could help you. Every tube listed at hd states t this or that but not the umpty k.

Must be a government secret.

CanadianLori, the "T" is the "size" of the bulb. The "K" indicates the peak wavelengh/frequency (color) of light. All fluorescent bulb packaging that I've seen gives some information about color. If there's not a numeric rating, there should at least be something like "Warm/Soft White" (2700K-3500K), "Natural/Cool White" (4100K-5000K), or "Bright White/Daylight" (6500K). Different manufacturers use different terminology, but, if the Kelvin rating is not given, these light color terms will at least give you an idea.

Thank you!