Intensity also affects the growth of plants. The brighter the light, the more energy the plant receives. To determine how much light a plant will require, consider where and how it grows best in its natural environment. Most vegetable and many mature flowering plants that grow in full sunlight need a high light intensity of 25 to 30 lamp watts per square foot. Mature leafy house plants and growing seedlings will do well with 15 to 20 watts per square foot.
Many plants are photoperiodic.
That is, they actually measure the length of each night and then produce either flowers or vegetative growth on the basis of this information. This response places them in groups as long night, short night or indeterminate plants. Long night plants need 11 to 14 hours or more of darkness per night to flower. Chrysanthemums and poinsettias are good examples of this. Short night plants generally require less than 10 hours of darkness to flower.