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Changes in the petioles block nutrients to to the leaves as the days grow shorter, causing leaves to turn color.
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Paul James, host of HGTV's Gardening by the Yard, discusses the dazzling display of color provided by leaves during North America's fall season. Leaves offer cool shade, pump tons of oxygen into the atmosphere and enrich our gardens when they fall and turn into compost. But they also offer an astonishing show of color as they pass from life to death. Oddly enough, the North American continent is one of the few places on the planet where deciduous trees produce gorgeous fall colors. In Germany a sugar maple turns a dull yellow at best; the same variety in Vermont turns a yellow so bright that it's almost fluorescent. Several factors influence this phenomenon, but the most important is the climate of much of the United States -- particularly in the central, eastern and northern regions. The combination of crisp, cold nights and clear, sunny days is needed to produce great fall color. Sunlight is especially important, as trees planted in full sun almost always yield bolder colors than those growing in shade. If the weeks before the color change are warmer or cloudier than usual, chances are the trees won't color as vividly. Climate conditions trigger various chemical reactions that affect leaf color. During the growing season water and nutrients are transported from the roots of the tree through the trunk and branches, the petioles (leaf stems attached to the branches) and ultimately the leaves. As the days get shorter, changes begin to occur where the petioles meet the branches. At their junction a sort of dam is formed, preventing nutrients from traveling through the pipeline. At this point the leaves change color and fall off. Why do some leaves turn brown, whereas others create a kaleidoscope of hues? Green leaves contain chlorophyll, a substance essential for plant growth. While plants grow, chlorophyll is present in large quantities, and the leaves remain green. Once the process of abscission starts -- the creation of the "dam" described above -- leaves stop producing the chlorophyll, so the green color disappears and the pigments responsible for creating vibrant fall colors begin to appear. (The pigments responsible for yellow and orange are xanthophyll and carotene. Anthocyanin pigment creates red leaves, and purple leaves are produced by a combination of all these pigments plus tannin.) Not all leaves contain sufficient amounts of pigment to produce vibrant color, which is why the leaves of some trees turn from green to brown. Here's a list of some of the best fall-foliage trees: Yellow: sugar maple, birch, redbud, American yellowwood, ash, ginkgo, lacebark elm, larch, thornless honey locust, tulip tree Orange to rust: sugar maple, bald cypress Red: red maple, black tupelo, sourwood, Chinese pistachio, scarlet oak, northern red oak Mixed colors: dogwood, sumac, smoke tree, crape myrtle, sweet gum, callery pear, sassafras. If you're looking for trees with excellent fall foliage, visit local nurseries when the colors are at their peak. Fall is also the best time to plant trees, and often they're on sale at the end of the growing season. The best trees for providing yellow fall foliage are ashes, birches, redbuds, Tulip Trees, and American Yellowwoods. For orange to rust foliage, seek out the Sugar Maple or Bald Cypress. Red foliage is provided by Red Maples, Chinese Pistaches, Black Tupelos, Scarlet Oaks, and Sourwoods, while Dogwood, Crape Myrtle, Sumac, Sweet Gum, and Smoke Tree all provide mixed color.
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