Spring’s vernal wonders

SPRING in Ireland is not like spring anywhere else in the world. Because of our position out in the Atlantic at a latitude of over 50 degrees north it arrives late and in a rush.

Four key factors control its advance — air temperature, soil temperature, day length and soil moisture content.

The relationship between air temperature and soil temperature is quite complicated. In general soil temperature lags behind mean daily air temperature by three or four weeks in spring. However, the difference is not constant. What holds back the soil temperature is cold nights, often dropping to below zero. However, as spring progresses the nights rapidly get shorter, their influence wanes and the soil begins to catch up with the air.

The arrival of spring is dictated by plants. All the other creatures, the flying insects, the swallows that feed on them and the hedgehogs emerging from hibernation, are directly or indirectly dependent on spring flowers and emerging leaves. Some plants are triggered into new growth by rising soil temperature, some by increasing day-length and some by a combination of the two.

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Birch trees will come into leaf on practically the same day every year whatever the soil temperature — they are reacting to the precise number of minutes of daylight on that day. Grasses, which are arguably the most important plants in this country, will start growing on any day in the spring once the soil temperature exceeds six degrees.

Soil moisture content is seldom a factor in Ireland because of all the rain that comes to us from the Atlantic. However, spring droughts do occur from time to time and they are the one card that can trump soil temperature when it comes to grass growth.

The countryman who greets you on a wet spring day and says approvingly ‘it’s a grand soft day’ knows this. He is dependent on spring grass growth for his livelihood and understands that moisture and mildness are both vital to making it happen.

Urban and suburban people may be less finely attuned to what is happening in nature at this time of year but they too will notice that it’s time to get out the lawnmower and see if it will start after the winter.

What makes an Irish spring so interesting is its variability. There is no dramatic day on which the ice breaks on the river or the first rain in six months arrives. However, this year we had high temperatures and little or no rainfall in mid-April followed by a fall in air temperatures and the return of wind and rain.

Observing the effect of all this is fascinating. The sun comes out and the countryside seems to have been taken over by dandelions. Two hours later it clouds over.

I sometimes feel a bit sorry for people who live in more predictable countries.

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