Leaves are nature's food factories. Plants take water from the ground through their roots and carbon dioxide from the air. They use sunlight to turn water and carbon dioxide into glucose which they use as food for energy and as a building block for growing and this process is called photosynthesis. A chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is what gives plants their green colour. As summer ends and autumn arrives, the days get shorter and shorter and this is a signal for the trees to begin getting ready for winter.

During winter, there is not enough light or water for photosynthesis. The trees will rest and live off the food they stored during the summer. They begin to shut down their foodmaking factories. The green chlorophyll disappears from the leaves and, as the bright green fades away, we begin to see yellow and orange colours. Small amounts of these colours have been in the leaves all along but they aren’t visible in the summer because they are covered up by the green chlorophyll.

The bright reds and purples we see in leaves are made mostly in the autumn. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves to turn this glucose into a red colour. The brown colour of trees like oaks is made from wastes left in the leaves.