My greenhouses are anything but dark, but I do supplement the light during the fall and winter months with fluorescent lights. I have 4 fixtures x 4 tubes of T-8 x 4' in each GH and they are all on timers. The shorter the day-light, the longer they are on. I use really full-spectrum tubes not the ordinary "soft" light tubes.
You can purchase longer length tube fixtures for larger areas.
