While it's clear that plants need light to grow, attempts to grow plants in too much sun, too much shade or using artificial light make it clear that the intensity of light is also important. Shade-loving plants grown in too much sun burn, while sun-loving plants in the shade seem spindly and sick. Matching plants with their preferred light intensity helps them grow successfully.
Shade-loving plants need lower light intensity, while sun-loving plants will use all the light they can get. Some plants can even grow under intense lights for 24 hours a day, but will need periods of darkness to produce blooms and fruit. If a plant is adapted to shade conditions, it doesn't possess the proper mechanisms to protect itself from too much light, especially ultraviolet light. This is why the cast iron plant, for example, gets leaf burn in full-sun areas. Conversely, plants that need full sun won't receive enough light energy to grow in shady conditions, and will suffer. Sun-loving tropical plants, such as hibiscus, might put out a smattering of leaves in heavy shade, but they will have few defenses and will fall prey to insects or mold.
Plants grown with insufficient light intensity may grow tall quickly in an attempt to outgrow neighboring plants and obstacles to reach sunlight. Prolonged time in this state will stress the plant, because all of the plant energy has gone into trying to reach light that never became intense enough to support the plant. Eventually, the plant will run out of energy to survive and die. Saplings trying to grow under a dense forest canopy, for example, will shoot up tall and weak, and then bend and die.
Plants grown in areas with sufficient light are often more compact, with shorter stems and larger leaves than their counterparts, because they don't need to reach for sufficient light intensity. The leaves and stems tend to be darker green because they can produce sufficient chlorophyll to convert light energy into storable energy. A bed of well-spaced annuals in full sun, for example, may not get very tall, but their leaves are wider, thicker and stronger, and they have excess energy that they can commit to plentiful blooms.
Bloom and Fruit Production
Plants turn light energy into sugars, proteins and starches to use as energy to produce seeds and fruit to reproduce. If the plant is grown in insufficient light, it will struggle just to survive, and will likely not have enough energy to reproduce. This is why flowering plants grown in spots that are too shady won't bloom. The few blooms that may appear often drop before they are fertilized, because there is not enough stored energy to support them. Many vegetable gardeners make the mistake of planting flowering crops, such as tomatoes, indoors or under an overhang, and then become disappointed when the blooms either never set, or drop before they can set fruit.
It is not only the intensity of light, but also the quality of light that determines plant growth. Light rays most affecting plants are the red, blue and ultraviolet wavelengths. While blue rays help with vegetative growth, red wavelengths are conducive to blooms and fruit production. This is important for plants grown indoors, because artificial light rarely has the proper balance or intensity of each wavelength. Supplementing artificial light with a few hours of sunlight or with broad spectrum fluorescent bulbs will help correct this problem.