HPS bulbs are the most common HID to use, especially during flowering, giving off characteristic reddish light. The red in the light spectrum encourages good flower/ fruit growth. It is possible to use HPS during vegetative growth, but only for a week or two. Older plants start to look a little unnatural and 'lanky', with large internodal length and brittle stems. For this reason conversion bulbs, or enhanced spectrum bulbs are needed, or even better, a Metal Halide. HPS bulbs can be re-started after about 3 minutes if powered down.