Autumn bonfires – but not of the kind filling the air with smoke and, if not carefully checked before lighting, may roast hedgehogs and other small animals seeking shelter from the cold weather ahead.

Gardening column Autumn
Gardening column Autumn
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Manea School 
of Gardening

I am talking about the wonderful shades of autumn provided by the trees and shrubs as they too prepare for the winter. Leaves of so many shades; often more stunning than the blossom that may have adorned their branches only a few months ago.

So why do the leaves turn different colours in the autumn?

Photosynthesis, ‘putting together with light’, is the process by which plants make the food they need to grow and reproduce. I am sure many of you remember this from school days! Unlike us, they can manufacture their own food, turning carbon dioxide and water into sugar (carbohydrates) using the energy in sunlight to power the reaction. Obviously it is a little more complex than this, as scientists are still unable to actually replicate this process in the laboratory.

The sunlight is captured by chlorophyll, the green pigment found in leaves. However, chlorophyll requires warmth and light to produce it, which is fine in summer, as it is normally being made continuously. If you leave a tent, for example, on your lawn, you can see the yellowing of the grass where the chlorophyll has broken down.

So as the days get shorter and cooler in the autumn, chlorophyll production reduces and so the leaves will be less green.

Leaves contain other chemicals as well as chlorophyll. These all have their individual pigments, but the green of most leaves tends to be stronger, and masks these.

Carotene – an orange-yellow pigment. Yes, the colour of carrots! It is also found in feathers, scales, and eyes of many animals. Carotene is not affected by sunlight or temperature, so remains in the leaf after chlorophyll has broken down – giving rise to oranges, golds and russets.

As the plants prepare to drop their leaves for winter, a layer of corky cells form across the base of the leaf in preparation for shedding. This restricts the movement of sugars (produced by photosynthesis) into the main part of the plant.

Anthocyanin is another chemical found in plants. This shows itself as a red/purple pigment when the concentration of sugars in the leaf increases. It is enhanced by sunlight, drought and temperature. It is responsible for the red skin of apples and the colour of ‘red’ or ‘black’ grapes. An apple may be green on the side that has been in shade and red on the more exposed side. We may summer prune our apples to expose them to more sunlight to get better colour.

So the green of chlorophyll, the oranges of carotene and the reds of anthocyanin are the main culprits for our wonderful shades of autumn. The weather conditions can make an autumn particularly wonderful. Bright, dry, sunny days – with cold but not freezing nights – will lead to the greatest variety and intensity of colours. Let’s keep our fingers crossed!

• Manea School of Gardening is a Royal Horticultural Society Approved Centre with RHS courses taught at all levels. Visit: www.maneaschoolofgardening.com or email: msog@btinternet.com