The Colours of Fall

The green colour of leaves in summer is caused by a chemical called chlorophyll, which uses energy from the sun to transform air and water to energy (sugar) for the tree. As the days grow shorter and cooler towards the end of the summer, deciduous trees begin to develop a corky layer of cells across the base of the leaf stem. Unable to bring water into the leaf, the chlorophyll starts to break down. As the green colour fades, yellow and orange pigments show through. At the same time, crimson red and purple colours form because of a chemical reaction triggered by the sunshine, short days and cool weather. Browns come from the presence of complex tannins in the leaf. When the corky layer is complete, the leaf dies and falls off the tree.

The variety of leaf colour can be attributed to combinations of these pigments, to genetic variations of the tree species and to variations in the local environment. Inheritance, soil conditions, disease and weather may affect seasonal colour. Contrary to popular belief, frost does not cause colour change, in fact it may kill the leaf prematurely, leaving them crumpled and brown.

Leaf litter (leaves on the ground) if allowed to rot, provide nutrients to the soil. A 30 year old mixed forest will have taken 160 kg of nutrients from the soil; about 110 kg is returned to the soil as leaves and twigs. Instead of raking leaves and disposing of them consider the following options:

• If they are not in the way leave them alone.
• If they are in the eavestrough remove them.
• If they are on your lawn mulch them.
• If they are on the street gather free fertilizer.
• If your neighbour blows them your way thank them.
• If your municipality landfills them suggest they compost.
• If you have lots of leaves build a composter.