Why do Leaves Change Colour?

A chemical gives the plants their green colour. This chemical is called chlorophyll. 

During winter, there is not enough sunlight for the trees to make food through photosynthesis.  When the ground is frozen, they can’t access water either.  So trees rest and live off the food they stored during the summer. The change in light and temperature during winter causes deciduous trees to shut down their food making factories and the green colour - the chemical chlorophyll, disappears from the leaves.

As the bright green fades away, we see yellow and orange colours.  Small amounts of these colours have been in the leaves all along. All leaf colour comes from pigments. These are natural substances produced by leaf cells to help them obtain food. As well as the chlorophyll that makes leaves green, there are carotenes which make them yellow and anthocyanins which create reds and pinks. We just can’t see these other colours in the summer, because they are covered up by the green.  The brown shades we see in leaves come from waste left in the leaves.

The colours of the leaves can be brighter or fainter depending on how cold the temperatures drop, how dry the weather is and how much sunlight the leaves get in the autumn.