Q. Why do leaves change color in the fall?
Leaves are in the business of making food. They take water from the ground (through their roots) and carbon dioxide from the air. Using energy from the sun, they create glucose (a sugar). Chlorophyll, a substance that makes the leaves green, facilitates this glucose-making process, called photosynthesis. The glucose created in photosynthesis is food for the tree.
As days get shorter, trees start preparing for winter. Many plants stop making food in the fall and begin to shut down for the winter. As chlorophyll production slows down, other colors in the leaves come to the fore. Chlorophyll now takes a back seat to yellow carotenoids and red anthocyanins. Yellow, orange, red and violet pigments that have always existed in the leaf become more prominent.
You will notice that yellow pigments tend to stay the same year after year, while reds and purples vary in their intensity and depend on seasonal factors. Why is this?
Red and violet pigments are formed when sugars become trapped in the leaves. On sunny days sugars are produced in the leaves. During cool fall nights, the veins in the leaves close up and prevent the sugars from moving into the tree. This excess of sugar encourages the production of anthocyanins, which makes the leaves red.
What then are the conditions that make the most vivid fall colors? In the spring there needs to be plenty of precipitation for the formation of big, healthy leaves. In the fall, bright, sunny days and cool nights promote dazzling autumn colors.
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- Courtesy of NYBG Plant Information Service
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