Use metal halide lamps for the vegatative stage of growth in lieu of high pressure sodium. MH puts off a lot of light in the blue spectrum, which promotes what you seek. Many folks use a combination or the two types or one of the newer HPS bulbs desigbed to produce the blue spectrum. Typically HPS lamps put off high intensities of the red spectrum which promots the flowering cycle. This assumes you switch to HPS when you flower the plants.
Nutrients are an entirely separate arena and also have a profound affect.