Why Leaves Change Color

Look at this forest. In summer it's just a wall of green — and then almost overnight, the same trees explode into reds and oranges and yellows. So here's the question. Where did all that colour come from? The leaves were just GREEN yesterday.

Let's zoom in on a single leaf cell. Surprise — there isn't just ONE pigment in there. There are three. Green chlorophyll. Yellow and orange carotenoids. And in some trees, red anthocyanins too. All summer long they're all hiding in there together. But there's just SO much chlorophyll that the green completely masks the others. You can't see them — but they're already there.

But wait — why does chlorophyll look green to begin with? Sunlight is actually a mix of every colour. When light hits a leaf, chlorophyll grabs the red and the blue and uses that energy to make food. But it can't do anything with the green light, so it just bounces back. That bouncing green is what hits your eyes. The leaf looks green because that's the colour the plant DOESN'T use.

Now here's the trigger. As fall comes, the days get shorter and the nights get colder. The tree senses it — winter is on the way. So it stops making new chlorophyll, to save energy. The green that's already in the leaves just breaks down and fades away. And suddenly — the yellows and oranges that were hiding the whole time become VISIBLE. Some trees go one step further. They take the sugars stuck in the leaf and turn them into anthocyanin. That's where the bright reds come from.

Okay, your turn. Drag the month slider all the way through a year. Try the maple, then the aspen, then the oak — and finally the evergreen. See how the SAME triggers — shorter days, cooler nights — give totally DIFFERENT colors for different species. That's the chemistry of fall.

Once the colour show's over, the tree does something kind of brutal. It grows a tiny layer of cells at the base of every leaf — called the abscission layer — and that layer literally cuts the leaf off from the rest of the tree. Water and nutrients stop flowing in, the leaf dies, and the first good gust of wind sends it tumbling down. The tree seals the wound shut and goes to sleep for winter. Come spring — new buds, fresh chlorophyll, and the whole cycle starts again.

So here's the whole trick. The yellows and oranges of autumn weren't painted on. They were inside the leaves the whole summer — just buried under all that chlorophyll green. Cold nights and short days tell the tree to shut chlorophyll down. The green fades, and the hidden colors finally get their turn. A few trees go further and brew red on top. Fall colours were there all along. They were just hidden under green.