Why do Leaves Change Color in the Fall?

To humans, a deciduous Tree’s leaves mean shade in summer and pretty colors in the fall as the tree leaves change color. To the Tree, they mean survival.

Leaves are food factories. By the wonder of photosynthesis, leaves convert sun, water, and carbon dioxide into sugars, or food. The Tree uses this food to grow, flower, set seed, and carry out all the functions a Tree needs to perform.

The Science Behind How Leaves Go From Green To Autumn Colors

Chlorophyll, the green pigment in leaves that makes photosynthesis possible, is repeatedly manufactured and broken down throughout the summer. Food production, too, marches on all season, but it can’t go on indefinitely, because winter cold will bring photosynthesis to a standstill. So, in the fall something must be done. The cafeteria closes down. The leaves must be shed.

As fall approaches, the Tree shuffles all the leftover food it can into the roots, branches, and trunk for storage over the winter (it will help the Tree push out new leaves in spring). Chlorophyll renewal slows and then stops.

With that green pigment gone, carotenoids are revealed. Carotenoids are the orange and yellow pigments that were present in the leaf all summer but were masked by chlorophyll. They’re what make carrots orange (as in “carrot-enoids”) and bananas yellow. They appear to have a protective role in the leaves, guarding them against overexposure to the sun’s damaging rays—kind of like sunscreen.

Red pigments, called anthocyanins (which also make strawberries red), are not manufactured until fall, when new chlorophyll production has already begun to peter out. Scientists don’t agree on why anthocyanins are produced, but Leaf-Peepers take note: it is clear that reds are more intense in years when fall days are warm and sunny, and nights are cool but not freezing-cold (isn’t that the best kind of fall anyway?).

Carotenoids are not much influenced by weather, so the intensity of yellows and oranges is pretty consistent year-to-year.

What Determines The Color Of A Fall Leaf?

What color a particular Tree’s leaves turn is wired into its DNA. Most Birches, Honeylocusts, Willows, Ginkgos, and Lindens turn yellow in fall. Most Flowering Dogwoods turn purplish-red, Sugar Maples are usually blaze-orange or scarlet, and Red Maples are normally sealing-wax red. However, there are exceptions. You may be surprised to know that some Red Maples turn yellow. Buying a named selection with a verifiable track record, like Red Sunset® Red Maple, will ensure that your tree will color up to your expectations.

The timing of fall color is also programmed into a tree’s genes. Flowering Dogwoods, for example, are some of the first trees to begin their transformation every year; Oaks and Ginkgos are some of the last. This is something to consider if a long season of color is your desire.

At Bower & Branch, we like to encourage you to think of your fall landscape as an Autumn Canvas. Even though most flowers are spent, you can still paint your canvas with brushstrokes of crimson, orange, and yellow in the fall. Choose to plant a diversity of trees, and you might just have a masterpiece on your hands!