Why Do Leaves Change Colour in Autumn?

Every autumn, whole landscapes shift from green to gold, orange, and red. It looks like the leaves are simply 'turning', but what's really happening is subtraction plus a little chemistry: the tree is dismantling its leaves before winter, and the colours that emerge were partly hidden all along. Here's the science, minus the myths.

Green comes from chlorophyll

Through spring and summer, leaves are packed with chlorophyll — the pigment that captures sunlight to make sugars in photosynthesis. Chlorophyll is intensely green and constantly replenished, so it masks every other pigment in the leaf. As long as the tree keeps topping it up, the leaf stays green.

What triggers the change

The main trigger isn't temperature — it's day length. As nights grow longer in late summer and early autumn, trees sense the coming winter and begin to shut down their leaves, which are too fragile to survive freezing and would lose precious water. They stop producing chlorophyll, and the existing supply breaks down. Temperature and light then fine-tune the display: a run of warm, sunny days and cool (but not freezing) nights produces the most vivid colours.

Yellows and oranges were there all along

As the green chlorophyll disappears, it unmasks carotenoids — the same family of pigments that make carrots orange and egg yolks yellow. These have been in the leaf the whole time, quietly helping with photosynthesis; you just couldn't see them under the green. That's why some trees, like birches and ginkgos, turn reliably yellow every year.

Reds and purples are made fresh

Reds and purples work differently. They come from anthocyanins, which — unlike the carotenoids — are not present all summer but are actively manufactured in the leaf during autumn. As the tree seals off the leaf, sugars get trapped inside, and in bright light these are converted into red anthocyanin pigments. Because they depend on sunlight and trapped sugar, red colour is strongest after crisp, sunny autumn days. Maples are the classic example.

Why the leaf finally falls

While the colours develop, the tree builds a special band of cells — the abscission layer — across the base of each leaf stalk. This gradually seals the leaf off from the branch, cutting its supply of water and nutrients and eventually severing it, so the leaf drops. Sealing the wound protects the tree from water loss and infection over winter. It's a controlled release, not decay.

Why evergreens stay green

Conifers and other evergreens keep their foliage through winter because their needles are built to cope: a thick waxy coating and a compact shape reduce water loss and resist freezing, so there's no need to shed them all at once. They do drop old needles — just gradually and year-round rather than in a single autumn flush.

Why some autumns are more spectacular than others

• Warm, sunny days followed by cool nights (above freezing) produce the brightest reds.
• A sunny autumn beats a grey, overcast one for colour, because reds need light to form.
• Drought, early frosts, or storms can cut a display short by browning or stripping leaves early.
• The mix of tree species sets the palette: birches and poplars for yellow, maples and dogwoods for red, oaks for russet-brown.

Tip: the browns that eventually take over are tannins — waste compounds left behind once the more colourful pigments have broken down. That's the leaf's final chapter before it decomposes and feeds next year's growth.

Curious what's blazing red across the park? Point PlantMe at it — the app identifies trees and shrubs from a library of 35,000+ plant classes, so you can put a name to the colour.