what lighting is best to use for plant growth.

source "http://www.plantedtank.net/forums/lighting/37501-best-kelvin-rating-plant-growth.html"
In regards to the original question, I don't think there is a definitive Kelvin rating that is best for plant growth. The measurement itself is only meaningful to humans, as it generally indicates what kind of hue the light gives off as perceived through the human eye. Lower values are more red-ish in appearance, while higher values appear more and more blue.

That said, shorter wavelength blue light penetrates much farther into the water than the longer wavelength red. This may be why 6700k+ Kelvin bulbs seem to be better for plant growth, since the blue light is more readily available for the plant to use compared to the lower Kelvin lights.

However, plants actually use a broad range of the light spectrum for photosynthesis... not just the blues. This is known as PAR, or Photosynthetically Active Radiation. Basically, it is the output of light needed by plants for the process of photosynthesis. Photosynthesis actually uses the very much of the 650 to 670 nm range of the light spectrum which appears to humans as deep orange to red color. It also uses much of the 430 to 475 nm range of the spectrum (varying shades of blue), but not quite as much as the red range. The range of colors which human eyes are most sensitive to are the yellow/green range (550-620nm) is almost completely unused in photosynthesis, which is why most plants appear green in color (reflected light). It also explains why many warm/cool white bulbs are so terrible for growing plants--they mostly have the yellow/green tint that humans are so sensitive to but plants don't use at all.

So what should you look for in a bulb if the Kelvin rating isn't the best measurement to use for growing plants? I would recommend trying to find the spectral power distribution graphs for the light bulbs you plan to use. Most manufacturers of aquarium-specific light bulbs will put this graph right on the cardboard case of the light bulb for you to read. You want the graph to look like the letter "M"... good bulbs for growing plants will have a higher concentration of light being output in the 430-475nm range, almost no power in the 500-600nm range, and another large curve or spike around 650-675nm.

so if you can get hold of the spectrum chart of the light that you want to buy and you see that there is high output of blue and red light then that is a good light for plant growth even if the kelvin rating is below 6500/k