The amount of bloom given by house plants depends in large part on the foot-candles of light they receive. A foot-candle is a measure of illumination. The accompanying table prepared at the United States Department of Agriculture gives an idea of the light output at the center of a pair of tubes. The light decreases somewhat as it nears the ends of the tubes.
African violets (more info with ios PlantSpot app) , for instance, need 300 to 600 foot-candles of light. Their requirements vary with age. Seedlings need more light than older plants. Pink- and white-flowered varieties require less light than blue-flowered types. For a guide, measure the distance from the center of the tube to the pot rim. Generally speaking, mature blue-flowered varieties grow into fine specimens when placed with eleven inches between the light tube and the pot rim. If window-grown plants show tall upright growth, they will become shorter and more compact in a fluorescent-lighted garden. If leaves of plants under fluorescents look yellow and bleached, they may be receiving too much light. Either cut down on the total hours of illumination per day, or move them farther from the lights. When petioles (leaf stalks) are too long and blooming is poor, plants are not receiving enough light. Move them closer to the tubes.
Set cuttings and seedlings as close as three to four inches from the tubes to promote rapid, compact growth. Use lights to hasten growth on newly planted gloxinias, other gesneriads, tuberous begonias and caladiums. Dwarf geraniums grow beautifully when given eighteen hours of light per day and spaced so there are six to eight inches between tubes and pot rims. Begonias of all types flourish under lights. And to help forced bulbs develop perfectly, place them under lights while flower buds and stems are lengthening.
My first light setup, made several years ago, was a pair of light tubes suspended by a chain over a collection of gesneriads. Since then I have had many different arrangements of light. My current favorite is a three-tiered portable cart manufactured for the purpose. It gives twenty-four square feet of ideal growing space. Another unit I like is a bookcase with lights above the top shelf, and books on the under shelf. This combination case is 51 inches long, 32 inches high, and 18 inches wide. It is equipped with sliding glass doors to maintain humidity. I use it as a display area for specimen African violets, rex begonias, gloxinias, and episcias.
An entire plant room can be made in a cellar or heated attic. Some enthusiasts grow hundreds of plants in areas such as these, and it is not unusual for fluorescent light-grown plants to win top prizes at flower shows.