How many hours of artificial light do plants require per day?

This varies with the plants and the season of the year. In general, plants require a total of twelve to eighteen hours of light a day to bloom. Therefore, you need electric light only long enough to make up the difference between the total light squired by a plant and the length of the day. For instance, if you are growing gloxinias, which require a total of about fourteen hours, and if in January daylight lasts only nine hours, keep the fluorescents on for five hours.