Frankfurt - The sudden onset of spring in north-western Europe has sent the plant world into overdrive, triggering a wave of hay-fever from the cloud of pollen over the continent.

"It's very, very unusual," said Gerhard Lux of the state-funded DWD German Weather Service at its office in Offenbach near Frankfurt.

The development has surprised phenologists, the experts who track the impact of weather on seasonal plants.

At the start of April, the progress of spring in Germany, which spans much of Europe's north-west area from the Alps to the coast, was about two weeks behind average.

But now spring's progress, as measured by blossoming, is as much as two weeks ahead of average at some locations.

With one week of April still to go, the average April temperature so far in Germany has been four degrees Celsius above the mean for the period 1961-1990, DWD data showed Thursday.

In parks and gardens, apple trees have been coming into bloom before the early cherry blossoms have fallen. Normally the different genera blossom weeks apart.

In the Rheingau, a wine-growing region west of Frankfurt on the bank of the Rhine, the difference from the mean has been 5 degrees, said Bianca Plueckhahn of DWD's farm weather unit at Geisenheim.

That has prompted plants to race out of their winter dormancy. Rheingau apple trees normally blossom 75 days after the flowers of hazel, or Corylus, trees appear, but this year the delay has been only 50 days, she said.

Is the sudden onset of spring a sign of global warming or just proof that nature can be unpredictable?

"It certainly fits what one would expect from global warming," said Gerhard Mueller-Westermeier, a DWD climatologist.

So far, the average for April 2009 is the second warmest for any April since reliable records began in Germany. The warmest was April two years ago, 4.2 degrees hotter than the long-term mean.

Two extremely warm Aprils in quick succession definitely give food for thought, the climatologist said.

It began with an upsurge in temperatures in week 15, said Lux. A week later, many parts of Germany were experiencing daytime highs of 25 degrees, which is summer warmth in German terms, and the weather has stayed warm this week too.

There has of course been isolated April heat in the past in Europe, such as a German record 32 degrees measured in April 1968 near Berlin.

The fine weather has not been welcome to everyone, especially because there has been two thirds less rain than usual in April, Lux said. The drought has been pronounced in the north and east of the country. Farmers and golf-course operators have been worrying.

Hay-fever sufferers there have been getting an added dose of pollen. In some regions, a thick layer of pollen and dust has settled on cars, houses and streets with no rain to wash it away.

Some locations have even been put under forest-fire alert because of the mass of dry leaves on the ground left over from last autumn. (dpa)