The time frame that determines when autumn leaves start to fall is somewhat dependent on the geographic area in which they grow and weather conditions that occurred throughout the growing season. A combination of dry weather and longer periods of sunshine make all the difference in when the biological process will occur.
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Regions that receive more direct sunlight for a longer period of time in late summer -- or have undergone drought conditions during summer -- will see a later changing of color. As days shorten and the host tree begins to notice that it is receiving less sunlight, it begins to slow down the production of the chemical known as chlorophyll, the element that allows leaves to adopt a green color during spring and summer.
Photosynthesis is the chemical process that provides nutrition to the tree through the absorption of sunshine and the production of sugars. As photosynthesis declines, unused sugars begin to develop within the leaf, contributing to a change in color. The leaf also begins to develop a clump of cells at the base of the leaf; this membrane blocks the leaf from absorbing energy and nutrition from the tree -- part of the hosts' efforts to protect itself as it moves toward dormancy.
Soil moisture varies annually, ensuring that the onset of color and falling leaves will never occur at precisely the same time each year. The U.S. Forest Service indicates that a late spring, or an intense summer drought, may delay the onset of fall color -- and thus the falling of leaves -- by as much as several weeks. The fall foliage season normally begins in late September in New England and moves southward into the Smoky Mountains by early November. Regions in the west also typically experience the changing colors in early November.
The dropping of leaves begins to slowly occur within a few weeks of the onset of coloration and takes place at higher elevations before reaching the valleys. The fallen leaves eventually decompose and stock the soil with nutrients and hold water within the earth for absorption by surrounding plants and organisms. The process is vital not only to the survival of the host tree but also to the entire forest ecosystem.