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Pollen

As the British climate becomes warmer and wetter, pollen levels will increase due to enhanced plant growth, whilst frost damage, which reduces pollen levels, will decrease.

The birch pollen season in spring may simply start and end earlier, but the grass pollen season will commence earlier as every 1°C rise in temperature will bring forward the pollen season by 15 days.

This will soon bring peak hay fever season into the middle of exams. It is likely that a damper summer will also increase the length of the grass pollen season.

More worrying still is that the worst hay fever plant in the world could become established in Britain; not only could a new and highly irritant plant be present, but the hay fever season would extend into the autumn.

Ragweed, Ambrosia artemisiifolia (not to be confused with Ragwort Senecio jacobaea) and its relatives are currently considered as the worst hay fever plants in the world.

Ragweed pollen carries the major allergen Amb a I (antigen E) which is far more irritant than grass or birch pollen.

Ragweed causes misery to hundreds of thousands of sensitised people in late summer, long after the grass pollen season has finished. Surely if ever a plant was misnamed - Ambrosia being food of the gods - this is it.

Ragweed is an annual member of the daisy family, but unlike most members of the family has small green flowers which are pollinated by the wind.

To maximise the chances of pollination, each plant produces millions of pollen grains, which fertilise the flowers allowing each plant to produce up to 62,000 seeds.

Seeds mainly germinate in the spring, and the plants flower in late summer and early autumn.

Ragweed

Ragweed originates from North America but is now widespread around the world where the climate is suitable, and is already well established in the warmer parts of Europe.

For example, around Lyon and Dijon in eastern France, Ragweed occurs on fertile, sandy ground on waste ground, fallow agricultural land, roadsides and building sites - similar habitats to where it occurs in North America.

These are similar habitats to where it occurs in North America. Ragweed seems to be repeatedly introduced to Britain accidentally as a contaminant of products such as bird seed or chicken food, and it can be found on rubbish tips, waste ground, docks, oil seed mills, farm land, and increasingly in gardens and allotments.

Although Ragweed is frequently recorded in Britain, it is currently rare for populations to persist from one year to the next, and it is not yet a problem plant as the climate is too cool and damp for it to establish and spread.

Temperature changes

As the climate warms, this will change. Seed germination is mainly controlled by temperature, with maximum germination occurring in conditions of alternating 10°C and 30°C, as seen in night/day cycles in spring in areas where it is currently abundant.

The sensitivity of Ragweed germination requirement for fluctuating spring temperatures may currently result in low germination rates in Britain, and our cool, damp summers may result in low survival and reproductive rates.

If our climate warms by 0.5°C per decade as predicted, by 2050 southern England will have a climate similar to SE France and Ragweed will persist.

One saving grace may be that one of its fungal pathogens Sclerotinia sclerotiorum is favoured by damp climates, which might limit its growth.

Other species

But it could also get even worse as A. artemisiifolia is not the only Ambrosia species introduced to Britain.

Giant Ragweed Ambrosia trifida is been found regularly on river sides and in docks, accidentally introduced with waste from oil-seed and soya and with chicken food.

It is also an annual but does not persist again probably also because the climate is currently too cool.

Perennial Ragweed A. psilostachya does persist, sometimes for decades, but is quite rare and seems only to have established on sand dunes and in sandy places in a very few places.

Another six Ambrosia species have been recorded once or twice only. The winds of climate change can blow a little pollen a long way.

Dr Tim Rich - Welsh National Herbarium, National Museum Wales.