Sun Requirements and Growing Seasons for Plants

Light Quantity

• The intensity of light is the light quantity, sometimes measured in foot candles. Because of the earth's orbit and rotation, light quantity varies on an annual basis. In general, greater light quantity results in more photosynthesis; however, if the light quantity is greater than the plant's ability to use it, photosynthesis stops and the plant may burn. Gardeners decrease light quantity with shade cloth and increase it with supplemental lighting and reflective surfaces.

Light Quality

• The color of light is called light quality. Red and blue light are the most useful to plants, while green is the least useful. Blue light encourages vegetative growth. Since fluorescent lights produce a great deal of blue light, they are preferred for starting seedlings indoors. Plants are stimulated to flower when approximately equal amounts of red and blue light are present.

Photoperiod

• Light duration, the length of time the plant is exposed to light, is the photoperiod. Plants vary greatly in their response to photoperiod; short-day and long-day plants are highly sensitive and day-neutral plants are indifferent. Scientists once thought that plants flowered in response to light but now understand that it is actually the period of darkness that triggers flowering. Short-day plants flower when there are more than 12 hours of darkness, usually in the spring and fall. Long-day plants flower when there are fewer than 12 hours of darkness, usually in the summer. Gardeners can manipulate the photoperiod with supplemental lights or black covers.

Interaction of Factors

• Commercial growers use a variety of methods to control plant growth and can manipulate flowering out of season to meet buyer demand. Home gardeners can use some of the same methods to increase the growing season or to encourage flowering. The least laborious and most successful way to have plants that grow vigorously and flower and fruit profusely is to choose plants that are best fitted to the local environment.