U can, and I think u should use both HPS AND MH throughout the entire grow... I use a lil more MH during veg than HPS, and I use a lil more HPS then MH during flowering, but, I always have both at all times... I have it timed so my HPS's come on first, like sunrise, and then a few hours later, the MH comes on and also goes off, about three hours before the off period begins, like sunset... I think it is most beneficial to the plants to receive both HID spectrums for the duration of a grow... Also, if u have a HPS, u dont NEED "MH conversion" lamps, as HPS ballasts will safely fire and run MH lamps of the same rated wattage... The conversion lamps only emit a fraction of each spectrum also... But, it doesn't work in reverse- like if u have a MH ballast, it will NOT fire or run a HPS lamp of the same rated wattage, because HPS's require a starter to ignite, and MH's do not... If u ONLY use HPS, even with conversion, or dual-arc lamps, u will be lacking much of the white and blue ends of the visable spectrum, which r great for keeping internodal spacing closer together, more rapid vegetative growth, and the UV-B that MH's emit also causes more THC-production, in that it is the plant's natural response to UV-B to produce more trichomes... I have always had very good luck in germinating my seeds (and getting above 65% of them to be females), and growing them out under MH-rich lighting, and have also heard people who do this sometimes report gettting more females from their seeds than the textbook 50/50 ratio-although, I'm on the fence with that one... But, it certainly doesn't hurt to have the added canopy penetration of MH's to reach and "bring out" lower growth, as well... If u r using HID's of any type, 1-you probably aren't as concerned in the added costs of using these instead of flourescents of any type, and 2- As long as it's all under 2,000W or so, the increase in power bills is usually, and in my case, not very noticable, at all... So, why not?
