If possiable run both lights. For some reason i have found mixing hps and mh makes for better growing conditions. Example,I will flowering under 400 hps, along with at least 100-250watt mh. Guess i do this cause my granpappy taught to grow this way.Kinda just a tradition. But to answer your question i add hps when i see pre-flowering.
bobjob4u Reviewed by bobjob4u on . When to switch from MH to HPS How many weeks into flower should it be before I switch from 600w MH to 600wHPS? Rating: 5