THE windows in the schoolroom in which we endured maths were so high that the average child could not see out.

But when they were opened in summer, the most delicious scent wafted in. It was of freshly-mown grass - a sweet, exciting smell that allowed the mind to escape from the unfathomable drudgery of cosines and logarithms. On the scent drifted the tantalising prospect of an evening game of cricket, of spiked boots on the wooden pavilion floor and of indelible stains on the knees caused by an athletic slide on the boundary.

Nowadays, there is another smell that signifies the arrival of summer. Driving through the Dales last weekend in the pouring rain, car windows shut up tight, it sneaked somehow: an over-ripe smell, sickly-sweet like sugary honey.

Even before I'd noticed it fully, my nose was twitching and my eyes were itching. I looked around: high hedges screened the view but, just like a freshly-mown cricket pitch, I didn't need any visual clues. We rounded the corner and there it was rolling down the daleside: a bright yellow field of oilseed rape.

Oilseed rape has been grown for more than 4,000 years, but never in such concentrations as this year. Its name comes from the Latin 'rapum' which means 'turnip'. In pre-Christian days, the oils extracted from its seeds were used in soap. It spread to Europe in the 13th century when it was first grown for lamp oil.

In the middle of the 18th century it was discovered that growing rape one year made for stronger cereal crops in the same field the following year and from 1740 rape was used as a "break crop" to give the field a rest.

However, in this country those vivid yellow flowers were practically unknown - and eyes and noses were untroubled - until the mid-1970s. Then a chemist in Canada created a strain of rape that was palatable to the human stomach which meant that the seed could be squashed into vegetable oil. And the dreaded Common Agricultural Policy artificially raised the price so that oilseed rape became financially viable for British farmers.

Twenty-five years later and it is everywhere. It has escaped from its fields and is growing in unkempt corners of gardens and is sprouting on roadside verges as if it were cow parsley.

Last year the Department of the Environment, Farming and Rural Affairs estimated that 400,000 hectares of British farmland were planted with it; this year, the National Farm Research Unit believes it covers 498,000 hectares.

There are at least three reasons why. Having fewer animals following foot-and-mouth, farmers have to do something with their land; the profit from winter barley is declining so farmers are switching to well-subsidised rape; and bio-diesel.

If you mix the oil from rape seed with diesel you make bio-diesel which can be used in cars (and tractors).

Already in this country, £5m worth of bio-diesel is sold a year. By 2005, it is estimated that the industry will be worth £100m. In 2001, 200,000 tonnes of rape were processed into bio-diesel; by 2006, 700,000 tonnes will be required.

Bio-diesel may be kind to the environment, but on the red raw rims of the eyes it is very cruel.