Light intensity and quality

Light is made up of energy at different wavelengths. Sunlight consists of a wide range of wavelengths, some of which cannot be seen by the human eye. Artificial light can come close to duplicating the full range of wavelengths found in sunlight. One warm white and one cool fluorescent bulb used together will come close to producing the full spectrum of light wavelengths for indoor growing. Special grow lights also produce this quality of light. Incandescent lights produce an adequate quality of light, but their heat may damage plant tissues. The quantity of light decreases with distance and angle from the source. Light quantity as perceived by the human eye is measured in foot candles but plants "see" light differently so plants may not grow well when there appears to be adequate light to us. Plants receive maximum quantity of sunlight in summer and minimum in winter because of the angle at which the light hits the plant surface. Plants may need to be within a few inches from an artificial light source for the light to be effective. The time that a plant is exposed to light can be critical for 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. Long-day plants include those plants that bloom outdoors in summer. Spinach and lettuce are examples of long-day plants. Day-neutral plants do not depend on day length to form flowers, as long as there is enough light for normal growth. Everbearing strawberries are an example of day-neutral plants.