Why do leaves turn red and yellow in autumn?
I have summed up how the colors change in 30 seconds.Leaves are green in summer because they are full of green pigment. When autumn comes and the nights get longer, the leaf stops making green pigment and breaks it down, so the yellow pigment that was in the leaf all along can be seen. Red is the color of a pigment that the leaf newly makes after autumn arrives.
Three pigments inside a leaf
Leaves have 3 main kinds of pigment: green chlorophyll, which is used for photosynthesis; yellow and orange carotenoids; and red and purple anthocyanins. During summer there is so much chlorophyll that the other colors are hidden, and the leaves look green.
The signal is the length of the night
The main trigger for the color change is the nights getting longer. Unlike temperature or rain, the length of the night changes the same way every year, so it is a reliable signal for trees. As nights get longer, chlorophyll stops being made and slowly breaks down.
The yellow was there all along
The yellow carotenoids were in the leaf the whole summer. As the green chlorophyll decreases, the hidden yellow comes into view. Before dropping its leaves, a tree pulls nutrients out of them and back into its branches and roots, and the color change shows up near the end of that work.
Red is made in autumn
Red anthocyanins are, in many cases, newly made after autumn begins. They form more easily when sugar builds up in the leaf and strong light shines on it. The red pigment is thought to act like a sunshade, protecting the weakening leaf from strong sunlight. Autumn colors are said to be brightest in autumns with a run of sunny, warm days and cool nights that do not reach freezing.
Want to know more?
- In a forest owned by Harvard University in the United States, about 70% of shrubs and trees were reported to make red pigment before dropping their leaves.
- Leaves in the sun tend to turn red more easily, while shaded leaves on the inside of a tree sometimes do not turn very red.
- The yellow does not change much from year to year, but how bright the red is depends on the weather that autumn.
Quick quiz: test yourself
What mainly tells leaves to start changing color in fall?
Show the answer
- Longer nights
- Colder temperatures
- Less rain
Longer nights Night length changes the same way every year, so it is a reliable signal for trees. As nights get longer, the green pigment stops being made.
Night length changes the same way every year, so it is a reliable signal for trees. As nights get longer, the green pigment stops being made.
Next quiz ›Sources
- Science of Fall ColorsUSDA Forest Service（米国農務省 森林局）
- The Process of Leaf Color ChangeHarvard Forest（ハーバード大学）
- Leaf Color Change in AutumnUniversity of Wisconsin-Madison Extension（ウィスコンシン大学）
This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.
