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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved
WHY AUTUMN COLORS APPEAR
The magical colors of autumn appear as part of the natural life cycle of the plant. Plant tissues start hardening off as fall comes. Leaves stop growing and a corky layer of cells forms at the base of the petiole. These cells form the abscission layer that separates the leaf from the stem. The layer holds the leaf in place until broken by frost or wind.
The abscission layer clogs the conducting tissue so food production in the leaf slows down. This means chlorophyll production ceases and the green color disappears. This allows other colors to show through.
All the pigments that color leaves in the fall are present throughout the year. None of them magically appear with cold weather. Yellow and orange pigments are carotenoids and form in the protoplasm of the leaf cells. Purple and red pigments form in sap in cells that are rich in sugar, such as maples, sumac, some oaks and pears.
Colors develop in response to many different environmental factors. Cold temperatures usually get credit for the changes, but light is very important. Most plants never develop good fall color unless they have plenty of light. Cloudy or rainy weather or hot dry conditions prevent pigments from developing brilliant colors.
The most spectacular colors develop in the landscape when days are bright and sunny and nights are cool. These same conditions produce the most vibrant fall colors in our Utah mountains. Similar conditions are responsible for the showy colors in New England in the fall.
Although the same trees grow in other areas of the country, they never develop the striking colors that develop with sunny days and cool nights. Warmer areas never have the same show of autumn color, so enjoy and capitalize on this unique phenomenon of our temperate zone climate.
Frost is primarily responsible for leaf drop. Ice crystals form in the abscission layer of the petiole of the leaf where it attaches to the stem. This breaks the woody fibers that hold the leaf in place, and as the ice melts the following morning, the leaves flutter to the ground in a shower of color.
Some oaks and several other plants never form strong abscission layers, and their leaves remain throughout the winter. Leaves that die prematurely from diseases or other causes are also never able to form an abscission layer and remain on the tree. Dead branches are easily recognized because they hold their leaves after others have fallen.
The scientific reasons for fall color are interesting and show the wondrous environmental changes that take place in the world around us. Learn to take advantage of these to create beauty by selecting woody plants to create and enhance spectacular colors in your landscape.