I have some red ludwigia in a 10 US gallon tank, 15 inches deep with 1.1 watt per gallon and Hagen (Nutrafin) CO2 kit. It is growing very fast, faster than my Egeria, which seems to have slowed down a lot - I must have run out of some nutrient it needs. Soon I will have to start throwing some of the Ludwigia away. The new growth is not as dark red as the original growth, but it gets redder as it grows towards the light, and the leaves keep their red color when I take cuttings from near the surface and plant them in the gravel. My ludwigia has not lost many leaves; the reddest leaves are still the ones near the base of the pot that originally developed under the brighter lighting it had before I bought it.
With regard to me only have 1.1 watts per gallon, it's worth mentioning this is not a straight tube, it's an 11 watt U-shaped tube. It is a normal tube available from any UK hardware store. The same tube is in my desk lamp and in the recessed down-lighters in some parts of the office where I work.
I believe that CO2 is making a huge difference to my plants. There is an article on the Tropica web-site (or was it Dennerle?) that contradicts the popular theory that CO2 is only useful on high light tanks - CO2 makes a difference no matter how little light you have. It seems the plants chemistry is clever enough to make use of extra light or CO2 when the other is held constant, though when there is an excess of both growth is really explosive.
It's also possible that the Tetra Easybalance I dose my small tank with once a week is helping the plants - I don't know what's in it, but they say you don't need to use any other plant fertiliser if you use it, so I guess it must at least contain all the micronutrients plants need.
I've actually run out of places my fingers can reach to plant new cuttings - I've resorted to just pushing the stems down behind existing growth where they grow quite happily despite the lowest part of the stem being left floating in the water a few inches above the gravel. The low parts develop huge root systems which just float in the water. (You can only really see them from the side/back of the tank, so it's not unsightly.)