Among their distinctive traits, plants have the ability to create food for themselves using the energy from the sun. This ability roots them at the base of the food chain and makes much of life on Earth possible. Because light is essential to plant growth, then, plants have also evolved different mechanisms for maximizing their exposure to light, resulting in behaviors that you can observe in your garden or among your houseplants.
The most basic function of light is that it provides the energy plants need to grow. Plant growth involves rapid division of cells, primarily at the tips of shoots and roots. Cell division, a process called mitosis, requires energy, which plants produce from light. If you've ever tried to grow seeds in the dark, you know that plants grow rapidly, then die. Rapid growth uses energy stored inside of the seed. Once that energy is gone, in the absence of light, plants lack the energy needed to fuel further growth.
Light occurs in wavelengths, which we perceive as different colors. Plants can use any color of light to make energy except for green light. Plants appear green because they reflect back green light, rejecting it for photosynthesis while absorbing all other colors. Red and blue wavelengths of light, in particular, exert special effects on plant growth.
You're probably familiar with how flowers in your garden blossom at the same time each year. Forsythias bloom in early spring, for example, while black-eyed Susans wait for late summer, no matter the temperatures or climate conditions. Plants know when to bloom because of the interaction between red light, periods of darkness and different types of a chemical called phytochrome found in their cells. As the Missouri Botanical Garden explains, plants use phytochrome to measure the length of each night's darkness, then use this information to know when to set flowers and consistently blooming at specific times during the season.
Blue light triggers a different behavior in plants called phototropism. If you've ever grown houseplants in a sunny window, you've probably noticed that they begin to lean in the direction of the light. This behavior has the obvious advantage of exposing more of the plant to the light, causing higher energy production. As explained by biology professor John W. Kimball, lack of light on the shaded side of the plant stimulates a hormone called auxin that causes cells to grow faster. Uneven elongation of one side of the plant's stem tilts the plant toward the light.
Understanding the effects of light and different types of light on plant growth helps you to promote vigorous growth in your garden and houseplants. The Oregon State University Extension recommends exposure to red light, provided by incandescent lighting, to encourage flowering, while blue light, available from fluorescent lights, encourages leafy growth and the maturation of seedlings.