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What determines when leaves change color in the fall?

Fall is a beautiful time of year in many parts of the world when deciduous trees put on a spectacular display of autumn colors. But what actually causes leaves to change from green to vivid shades of yellow, orange, red, and purple in the fall? Understanding the science behind these seasonal changes can help us appreciate the natural wonder of autumn foliage.

Why Leaves Are Green

During the spring and summer, leaves appear green because they contain high levels of chlorophyll, a pigment that absorbs sunlight and uses its energy to power photosynthesis. This process converts carbon dioxide and water into sugars that allow the tree to grow. In autumn, deciduous trees stop producing chlorophyll, allowing other pigments that have been present in the leaves all along to show through.

What Triggers the Change

Shortening daylight hours and cooler temperatures are the main triggers for leaf color change. As days grow shorter, trees produce less chlorophyll, causing the green color to fade. Cooler weather slows the flow of sap in trees, preventing the transfer of sugars from the leaves to the rest of the plant. This causes sugars and other compounds to accumulate in the leaves, further breaking down the green chlorophyll pigments.

The Role of Weather

Weather conditions during the fall play a major role in determining the intensity and duration of leaf colors. Warm sunny days and cool nights are ideal for vibrant autumn displays. Wind and rain can cause leaves to fall off trees before achieving peak color. Early frost or severe storms can also diminish foliage displays. The timing and intensity of fall colors varies from year to year depending on these weather variables.

Different Pigments Produce Different Colors

The specific colors that emerge in autumn foliage depend on the pigments present in each tree species:

  • Carotenoids – Produce yellow, orange, and brown colors in leaves. Maple, birch, aspen, oak, and beech trees are examples.
  • Anthocyanins – Result in red, purple, and crimson leaves. Found in some maple, dogwood, sumac, and sassafras trees.
  • Tannins – Give leaves a brown appearance. Oaks are especially high in tannins.

The Color Change Process

The transformation of summer green to autumn splendor follows these general steps:

  1. In late summer, production of chlorophyll slows and then stops as daylight hours decrease.
  2. Chlorophyll breaks down, allowing carotenoids and anthocyanins that have been present to show through.
  3. Cooler fall temperatures promote the development of more vibrant pigments.
  4. Bright sunny days increase sugars and anthocyanin production, further enhancing colors.
  5. Once the growing season ends, the Transport of nutrients stops and leaves eventually die and fall off.

Regional Differences

Peak fall foliage dates vary across North America, occurring later at more northern latitudes. Here is a table showing typical peak color times in selected regions:

Region Typical Peak Period
New England (Northeastern US) Late September to Mid October
Appalachian Mountains (Southeast US) Mid October
Midwest (Central US) Early to Mid October
Rocky Mountains (Western US) Late September
Pacific Northwest (Northwestern US) Mid October
Eastern Canada Mid to Late September
Western Canada Mid September

Higher elevations generally reach peak color earlier than lower elevations. Leaf changes usually start at northern latitudes and move southward over the course of the fall season.

Enjoying Autumn Foliage

Watching the multi-colored fall foliage transform landscapes provides an excellent reason to get outdoors and appreciate nature’s beauty. Some ways to enjoy peak autumn colors include:

  • Taking a scenic drive through areas known for vibrant displays
  • Hiking forest trails to witness the colors up close
  • Photographing the colorful seasonal change
  • Visiting parks or natural areas abundant in deciduous trees
  • Planting trees with outstanding fall color in your own yard


The splendor of autumn foliage results from a complex interplay of changing daylight, cooling temperatures, color pigments, weather patterns, and the natural life cycle of deciduous trees. Understanding the science behind fall colors allows us to fully appreciate the beauty of this seasonal phenomenon. Paying attention to leaf color changes reminds us of nature’s cycles and the passing of time. The fleeting fall foliage display is a reminder to get outside and enjoy the natural world around us.