Expert reveals why autumn leaves more colourful than usual

While New England in the US is lauded annually for its autumn colour, bright weather and low winds in Ireland this year have combined to produce one of the most spectacular autumns here in years.

In September, the weather was drier than normal and temperatures reached a high of almost 24 degrees at one Met Éireann weather station. In October, temperatures were above average everywhere in the country, with some stations recording their highest temperatures in 42 years. Hours of sunshine were also above average in many places.

Expert horticulturist Eileen Murphy, from Teagasc Horticultural College at the National Botanic Gardens in Dublin, said that when autumn begins, the shortening days and lengthening nights trigger a reaction in deciduous trees. A protective barrier forms between the tree and each of its leaves, so that when the leaves fall the tree is not exposed to disease. "That happens every autumn and it has happened this autumn," Ms Murphy said.

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Killiney, Co Dublin, this week. Photograph: Eric Luke/The Irish Times

Inside the leaf during the day, the process of photosynthesis using water, carbon dioxide, sunlight and green chlorophyll creates sugars, and those sugars are normally transported to other parts of the tree. But when the tree has already created its protective barrier, the sugar tends to be trapped in the leaf. Daytime sunlight and cool nights cause the leaf to turn the sugars into a red pigment, anthocyanin.

“Normally in Ireland we get dark autumn days so there isn’t an awful lot of sugar produced and there isn’t a lot available to make the red pigment,” Ms Murphy said. “But this year we’ve got a very good, prolonged period when it is sunny enough for sugar to be produced and it’s trapped in the leaves and it is producing the red pigment.”

The visibility of autumn colour can also be shortened by high winds and stormy weather, but conditions this year mean trees have managed to hold on to their leaves much longer.

“The yellow pigment is there all the time, but you don’t see it because there is so much chlorophyll; the green masks the yellow, but as winter approaches the chlorophyll production is slowed down, triggered by the shortening days,” Ms Murphy said. In Ireland, autumn leaves most often still contain a lot more green than yellow when they fall.