Temperature's effect on the mowing season

Grass grows nearly anywhere

Grass grows nearly anywhere. Here in Ireland we see it mostly in our fabled green fields, but elsewhere there are vast plains entirely covered with it, and known by many different names.

In the United States and Canada, for example, they are called the "prairies" and in Russia "steppes"; the "savannas" are in Colombia and Brazil in South America and in northern Australia; southern Africa has its "veld", and the "pampas" stretch over vast areas of Uruguay and Argentina.

In Ireland some 24,000 square miles, or three-quarters of the entire surface of our island, is covered in grass. For most of us, however, our duty of care for this prolific plant stops with that little patch of green we call our lawn.

Three factors control the rate of growth of grass: temperature, the adequacy of the moisture supply, and sunshine.


In winter, temperature is the limiting factor, and the important temperature is that of the soil which holds the roots; the critical conditions, however, are known to coincide closely with an air temperature of about 5 Celsius.

Below this value there is no growth at all; above 5 Celsius, the warmer it is the faster the grass grows until the temperature reaches about 10; at 10 Celsius growth is at a maximum, and any further rise in temperature has little effect.

This direct relationship between growth and temperature has important consequences in Ireland, since it dictates the length of the growing season. Over most of the country it starts in early to mid-March, and provided there is enough moisture, the grass grows almost continuously until mid-December.

The longest growing season occurs in a narrow strip on the south and southwest coasts of Munster, where it is usually only in the month of January that the grass stops growing altogether.

In summer, lack of moisture in the soil, when all available moisture within root range has been exhausted, may cause grass growth to cease, or diminish the amount of grass produced. During the long dry periods which sometimes come along in midsummer, growth stops completely, and will resume only if a lawn is irrigated artificially.

And sunshine is of importance because direct radiation from the sun facilitates the process of photosynthesis by which the plants absorb carbon dioxide to generate new material.

Cloudy weather, therefore, means less growth, and exhausted toilers in suburban gardens trying to keep a lawn in check can at least take succour from the fact that it has been estimated that the volume of grass actually produced in Ireland is some 30 per cent less than would be the case if our skies were clear all the time.