I had 15w and later 20w of light over my 29 gallon for a while and had slow but decent growth from java fern, crypts, and an Amazon sword. As soon as I got more light (I now have 72w) everything went nuts and the sword outgrew the tank, but with very limited light I had good but excruciatingly slow growth.
29 g planted community tank, 2.48 WPG, DIY CO2, Greg Watson ferts -- Tank Journal Here! -- 5 gallon planted Marineland 5 gallon hex with 15w 6500k CF, Excel, 100% flourite substrate -- RCS & male dwarf puffer -- Tank Journal Here! -- 2.5 gallon planted MiniBow w/10w 6500 k CF, Excel -- mystery snail & RCS -- Tank Journal Here!