Why Do Leaves Change Color in the Fall? The Science Behind Autumn's Big Show

Green comes from chlorophyll

All spring and summer, leaves are full of chlorophyll. It's the pigment plants use to turn sunlight, water and carbon dioxide into food, and it's so green that it covers up every other color in the leaf.

Basically, chlorophyll is the loudest person in the room. Nobody else gets noticed until it leaves.

Shorter days tell trees to shut down

As fall arrives, the days get shorter and the nights get cooler. Trees that drop their leaves take that as a sign to stop making food for the winter. They stop making new chlorophyll, and the green slowly breaks down.

Yellow and orange were there all along

Once the green fades, other pigments called carotenoids finally show through. They're the same family of pigments that make carrots orange, and they were hiding in the leaves all summer.

So yellow leaves aren't changing color, exactly. They're finally showing their true colors. That's very inspiring for a leaf.

Red and purple are made fresh

Reds and purples come from pigments called anthocyanins. Unlike yellows, many trees make most of these new in the fall. Trees like red maples and sweetgums are famous for them.

Sunny days and cool (but not freezing) nights help trees make more of these pigments, which is why some years are much more colorful than others.

Then the leaves let go

Finally, the tree forms a special layer of cells where each leaf stem meets the branch. It seals off the leaf, and eventually the leaf falls. Brown leaves are what's left when all the colorful pigments are gone, leaving mostly tannins behind.

And that's when you go outside and rake. And rake again. Science is beautiful.