Why did the leaves change color?

The timing of color change is primarily regulated by the calendar and the increasing length of night. Leaf pigments and weather directly influence leaf color.

Chlorophyll gives leaves their basic green color and is necessary for photosynthesis, the chemical reaction that uses sunlight to manufacture food sugars for plants. Carotenoids produce yellow, orange and brown colors in such things as corn, carrots, bananas and daffodils. Anthocyanins give color to such things as cranberries, red apples, blueberries, cherries and plums.

Both chlorophyll and carotenoids are present in leaf cells throughout the growing season, while anthocyanins are produced in the autumn, in response to bright light and excess plant sugars within leaf cells. During the growing season, chlorophyll is continually being produced and broken down and leaves appear green. As night length increases in the autumn, chlorophyll production slows down, stops and eventually all the chlorophyll is destroyed. The carotenoids and anthocyanins that are present in the leaf are then unmasked and show their colors.

Certain colors and the timing of the color change vary by species, while the amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions, mainly temperature and moisture. A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays.

Photo of Crepe Myrtle in fall color,
courtesy of the National Arboretum

- Steve Koehn, State Forester
Maryland Department of Natural Resources

For More Information
Why Leaves Change Color