Plants receive maximum quantity of sunlight in summer and minimum in winter because of the angle at which the wavelengths of light hit the plant surface. Plants must be positioned 2 to 6 inches from an artificial light source for the light to be effective. Duration Duration is the time that a plant is exposed to light, critical to growth and flowering. In indoor growing situations, lights must be on for 12 to 16 hours in most instances. Outdoors the duration of light varies with the season.
Summer days have a longer duration of light than winter days.
Some plants flower in response to the length of darkness they experience. This response to light and dark periods is called photoperiodism. Some plants are short-day types that form flowers when day length is 12 hours or less. Poinsettia and chrysanthemum are popular plants that require short day lengths in order to flower. Long-day plants form flowers during day lengths of more than 12 hours.