It's best to use an MH bulb for veg and an HPS for flowereing,however a HPS ballast can only light a HPS bulb so if you have an HPS ballast you need a conversion bulb thats a MH/HPS bulb for a HPS ballast you can also get a HPS/MH conversion if your ballast is for a MH. HPS high pressure sodium, MH metal halide. Hope I didn't confuse you but I bought a 400Watt HPS light so now I need to get a MH/HPSconversion bulb so I can use my HPS light fixture and ballast to veg with an MH bulb.Hope that clears this up. But to make it easier you can use an HPS for veg and flower , just that the MH is a little better for veg ........