Why Leaves Change Color

Many people believe that frost is responsible for the change in colors, but Jack Frost has little to do with it. In fact, many times leaves change color before the first hint of frost.

Indian legend has it that celestial hunters slew the Great Bear Autumn and the spilled blood turned the leaves red. The yellow of fall came from the fat splattering out of the kettle as the hunters cooked their prize. Other legends persist as well, but we know today that the changes are the result of chemical processes taking place in the tree as the growing season ends.

From the time the leaves emerged from the green buds in spring, they have served as factories, creating the food a tree needs to grow. The food-making process takes place in millions of leaf cells which contain a pigment known as chlorophyll.  Chlorophyll is green and there is so much of it in a grown leaf that it gives the leaf its green color.

But, in addition to the green chlorophyll, leaves also contain some yellow or orange carotenoids which, by the way, give carrots their familiar color.  For most of the year, the little bit of yellow/orange carotenoid color is hidden by the huge amounts of green chlorophyll. But, in the fall, the food factories shut down for the winter. The chlorophyll breaks down and the green fades away, letting the yellow/orange carotenoids blaze forth, giving autumn its splash, dash and panache. 

At the same time, other chemical changes occur, giving rise to more pigments which vary from yellow to red to blue. It is to these changes we owe the reds and purples of sumac, the brilliant orange or fiery red and yellow of sugar maple, and the golden bronze of beech.

The fall weather reaches a point where the days are warm enough for the food factories to operate, but the nights are too cold for the sugars which are produced to move downward in the tree. In the presence of bright light, the sugars trapped in the leaves form the red pigments, anthocyanins.  The brighter the light, the greater the production of anthocyanins, and the more brilliant the colors we see. When the days of autumn are bright and cool, and the nights chilly, but not freezing, the brightest foliage colors will develop. Familiar trees with red or scarlet leaves are red maple, dogwood, red oak, scarlet oak, and sassafras.

Only a few regions of the world have seasonal displays of color like Connecticut's. The eastern United States and southeastern Canada have large areas of deciduous forests, ample rainfall, and favorable weather conditions for vivid fall colors.  However, eastern Asia, southwestern Europe, and some areas of the western United States (notably the mountains) have bright fall colors.

CT Fall Foliage  |  Current Foliage Report  |  Where to View Fall Foliage in Connecticut  | The Colors of Fall