12-12 bulb switch

HPS doesn't tell the plant to flower.Light color won't do that. You decide when it starts flowering,your light schedule.you can run a whole grow with a hps,no problem.only your plant will stretch more than what it could be under MH. HPS causes longer internodal distance, MH keeps it under control.that's all. and HPS usually have more lumens.

Depending on the stretch you needs , the MH is ble so the plant will stretch less under a blue light even during pre flowering , the red colours (orange) of the hps will makes them stretch so generally you switch when you go to 12/12 , if your space is limited in height then prefer to change the bulb after 1 week or 2 of 12/12 to keep the plants shorter

Hey I would let the MH run about 7 - 12 days after switching to 12/12, and only then switch to the Hps :-) regards