Question: Why do leaves change color in the fall?

Answer: As we transition into autumn, we enjoy watching the leaves change from green into vibrant colors such as red, orange, and yellow. As we watch the leaves change colors, one often wonders or asks, “What causes the leaves to change colors in the fall?”

A change in leaf color that occurs as the seasons change is due to a chemical process taking place throughout tree. The leaf is where the food is made for the rest of the tree, through a process called photosynthesis where sunlight, carbon dioxide, and water are combined to form glucose or sugar. The glucose is used as food that supplies a source of energy for the tree. A chemical called chlorophyll helps the photosynthesis process take place. Chlorophyll is what gives the leaves its green color. Also within the leaf there are yellow or orange carotenoids pigments, however throughout the spring and summer, the green pigment is in larger amounts, which enables it to mask the other color pigments.

As fall arrives, trees begin to shut down their “food factories” because during the winter there is not enough light and water to continue making food through the winter. As the tree begins to prepare for the winter, chlorophyll breaks down causing the green color to disappear and the yellow and orange pigments that were there all along become visible. The process of the yellow and orange colors becoming visible is what gives the leaves their beautiful fall color.

You may have realized that I mentioned how the leaves are green and then turn to yellow or orange in the fall but what about red? There are trees whose leaves turn red in the fall. Just like there is chlorophyll that is related to green and carotenoids related to orange and yellow foliage, there are other pigments that lead to red and purple fall colors. These additional pigments and combinations of different pigments can lead to red, yellow, purple, and brown leaf colors in the fall.

Another question that commonly comes up about leaf color in the fall is why it varies from year to year. You often hear of people saying that the weather will influence how brilliant the fall color is from year to year. They are right in that weather will affect the fall color. The ideal weather for brilliant red autumn colors is warm sunny days followed by cool nights with temperatures below 45°F. The reason this weather combination is a recipe for brilliant red autumn color is because the warm, sunny days encourage sugar to be made in the leaves, and then the cool nights prevent the sugars that were made from moving out of the leaves. From this sugar trapped in the leaves, the red pigment called anthocyanin is formed.

How the overall climate is in the summer and into autumn will have an influence on how good the fall leaf color is in a given year. A year with vivid fall color occurs when there were warm, dry summers and early autumn rains, which prevent the leaves from falling too early. If there is a long period of wet weather in late fall, the fall leaf color tends to be less vivid.

Whether the tree is in the sun or shade can have an influence on variation in color from tree to tree. For example, leaves on a tree that gets direct sunlight may turn red while leaves on the same tree that are in shade could be yellow.

Now as we get into the autumn season and you are enjoying the beautiful fall color on the trees, you can now say that you know why leaves change colors in the fall.

Reminder: A growing program this year is the “Sampson County Friends of Horticulture.” This program offers monthly “How To” Horticultural Seminars. Please call (910) 592-7161 for more information. Please call the Sampson County Cooperative Extension Center at (910) 592-7161 with your horticultural questions and to register for any upcoming events. Be sure to check out the Ask An Expert Widget at sampson.ces.ncsu.edu for any questions you may have.