Tuesday 24 April 2018

Vintage year for colour spectacular in autumn

Gerry Daly

Autumn this year has seen a spectacular show of leaf colour on trees and shrubs. It has been exceptionally good, the best for many years, and lots of people have remarked on it. To get a show as good as this is like a vintage year for wine: everything has to happen just so.

There are several factors that can affect the autumn show of colour. The summer must be sunny and warm, and it should be followed by autumn weather that is warm and sunny by day and cool at night. There must be no significant night-time frost and the soil must be reasonably moist, certainly not dry.

Each of these factors was favourable this year. The only blip was a short period of very strong winds but that came early enough in October when the leaves were still well attached. The autumn weather was so good that the leaves this year have lasted about a week or more longer than usual, and are still in place on some trees.

Parts of eastern North America are famous for the autumn colour show each year. The reason that the colour is so good there is that the weather is more likely to be conducive to the formation of colour, and much more reliably so than it is here.

The process of leaf loss is triggered by shortening days. Trees lose their leaves to avoid moisture loss in winter and loss of food that would be needed to keep large leaves alive in a period of little or no growth. But in losing their leaves, there could be a loss of minerals and nutrients that the leaves contain.

To avoid the loss of important minerals in the leaves, trees have the ability to withdraw minerals into the trunk, branches and roots. When the green chlorophyll is withdrawn, the yellow pigments, present all the time but masked by green, are revealed.

After a sunny summer and autumn, sugar levels in the tree are higher and this favours autumn colour, the leaves lasting longer in colour. High levels of sugar in the leaves are converted to red pigments, especially the leaves on which sunshine falls.

Autumn colour in the garden is often not much more than an afterthought, or a bonus. But it can be as spectacular as spring blossom. Cherries, maples, juneberry, staghorn sumach, sweet gum, cornus and parrotia produce brilliant colours. Deciduous azaleas, witch hazel and fothergilla are very good too, at their best in acidic soil that is well-drained.

Sunday Independent

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