Blue spectrum light is most effective for vegetative growing, whilst red spectrum light triggers hormonal changes in a plant stimulating fruit and flower production. High Pressure Sodium (HPS) is best for flowering,whilst Metal Halide and to a lesser extent Grolux are best for vegetative growing. However, you can use an HPS Nav T-super or Son T-plus for vegging and flowering with good results. Use a contactor with all HPS systems if working with a timer! HPS lamps should be replaced every 12-14 months and Grolux lamps should be replaced every 6 months.