The Met Office has a pollen calendar on its website indicating when the plants most commonly associated with hay fever are likely to be producing pollen. Around 15 plant species are behind most SAR symptoms, with another five regularly seen on UK pollen traps but causing fewer problems.

Grass pollen, especially from timothy or rye grasses, is the most common trigger for hay fever, peaking from early June to late July. Lawn, meadow and ornamental grasses Pollen count Low Moderate High Very high Grains of pollen/ m3 of air <30 30-49 50-149 150+ elicit a response in up to 95 per cent of SAR sufferers, but cereal crops such as wheat are not usually a problem for most of the population away from fields.

Around one in four people with hay fever are allergic to one or more tree pollens. In the first few months of the year, trees and shrubs which produce catkins are a significant source, especially birch. Trees commonly associated with hay fever and their peak period of pollen activity include:
€¢ Alder: February-April
€¢ Hazel: February-March
€¢ Yew: February-March
€¢ Elm: March-April
€¢ Willow: March-April
€¢ Poplar: March-April
€¢ Birch: April-May
€¢ Ash: April-May
€¢ Plane: April-May
€¢ Oak: April-June
€¢ Pine: May-June
€¢ Lime: June-July.

Problematic weeds and wild flowers include:
€¢ Plantain: June-July
€¢ Nettle: June-August
€¢ Dock: June-July
€¢ Mugwort: July-August.

Oilseed rape flowers from May to June. While its pollen affects some people, it is the volatile organic compounds the crop gives off that can irritate the upper respiratory tract and eyes, if close by. Fungal spores can be present year round, but tend to build from late spring, peaking over the summer before dropping off again as the year ends. Alternaria, Cladosporium, Aspergillus/Penicillium and Didymella are most commonly implicated.