Ragweed (Ambrosia artemisiifolia) has generated a number of hay fever headlines in recent years. A native to North America, where it is a significant allergen source, it has become naturalised in central and south-eastern Europe. Studies suggest that climate change could help contribute to it spreading further into north-western Europe. The outcome could be a four-fold increase in airborne ragweed pollen by 2050, with the number of people in Europe suffering ragweed pollen allergy doubling to 70 million in just 35 years.17,18 The Botanical Society of the British Isles says that the plant is currently scarce in the UK, but is €increasing in range and probably in abundance, possibly because there are more people putting out bird seed€. However, €plants rarely persist€, it notes.

Giant ragweed (A. triffida) is also present in the UK but at even fewer sites.19 It can release up to 8,000 million pollen grains in five hours, illustrating why ragweed is considered such a concern.12 Ragweed should not be confused with ragwort (Senecio jacobaea), which is very common in the UK and contains hepato-toxic pyrrolizidine alkaolids.