Significance

• Light is the fuel or source of energy by which water and air are combined in the cells of a plant's leaf. Without light, no sugar or starch can be made. According to The Wise Garden Encyclopedia, when the stored supply of sugar or starch is used up, the plant dies. Some plants require direct sunlight; others need indirect or diffused light. There are also plants that prefer some combination of light and shade.
  
  

Influence

• Scientists once thought flowering was triggered by the length of light periods, not its color. Scientists then discovered it was the length of uninterrupted dark periods rather than light periods that is crucial for floral development. However, the color of light that reaches the plant also affects its flowering. Phytochromes are proteins in plants that allow it to sense light and its colors, according to the Structural Genomics Knowledgebase website.
  
  

Effects

• Sunlight is a combination of red, orange, yellow, green, blue, indigo and violet light. It also contains infrared and ultraviolet light, which influence plant growth. Red and blue light affect plant growth the most, notes the Montana Master Gardener Handbook. Blue light is mostly responsible for vegetative growth. Red light, when combined with blue light, encourages flowering. Green light is least effective because plants reflect it. Our eyes perceive this reflected light and that's why plants look green.
  
  

Types

• Cool white fluorescent light contains much blue light, which encourages leafy growth and is excellent for starting seedlings. Incandescent light is high in red or orange, but produces too much heat to be a valuable light source. Fluorescent "grow lights" have a mixture of red and blue that mimics sunlight closely. They are expensive and usually not of any greater value than a combination of cool and warm fluorescent lights, The Montana Master Gardener Handbook points out.
  
  

Considerations

• Light is a major consideration when growing flowers indoors where natural sunlight is limited. Some greenhouses use high sodium grow lights for their large amounts of red light to induce flowering, but they must be used in conjunction with natural sunlight or other lighting. Full spectrum fluorescent lights encourage both growth and flowering, as flowers cannot live just on red light, which can make for a leggy plant. They need the blue light for healthy vegetative growth.