UK Pollen Calendar: When Hay Fever Season Starts and Ends

The UK pollen season runs from January to September: tree pollen (hazel, alder, then birch, oak and plane) from January to mid-May, grass pollen from mid-May to July, and weed pollen and mould spores through the summer into September. October to December is the quietest time of the year.

Hazel
Jan – Mar
Alder
Jan – Mar
Birch
Mar – May
Oak
Apr – May
Plane
Apr – May
Grass
May – Jul
Nettle
Jun – Sep
Mugwort
Jul – Aug
Mould spores
Jun – Oct

Axis: January to December. Typical months in England; Scotland is about two weeks later, and each year shifts with the weather.

The three seasons

A typical UK year: hazel and alder from January to March, birch from late March to mid-May (the main tree allergen, about one in four hay fever sufferers react to it), grass from mid-May to July, usually in two peaks (early to mid-June and early July), and weeds such as nettle, dock and mugwort from June to September. Scotland runs about two weeks behind the south of England. CAMS does not model hazel, nettle or plane tree.

Grass causes the most hay fever in the UK, and its peak is the reason June is the worst month for most sufferers. A dry, warm, breezy day after rain releases the most grass pollen; steady rain washes it out of the air. On hot days the count is usually highest in the early evening, when pollen that rose in the morning settles back down over towns. Mould spores (Alternaria, Cladosporium) peak in late summer and on humid days at harvest, and are why some people's hay fever carries on into September.

Thunderstorms

A thunderstorm on a high grass pollen day can break pollen grains into tiny fragments that reach deep into the lungs. London had one of the best-known episodes in June 1994, when hundreds of people needed hospital care for asthma in a single night. Thunderstorm asthma explained

Today's pollen in UK cities

Level scale (daily mean concentration, grains/m³; US National Allergy Bureau scale, AAAAI, as the UK has no open national scale): trees moderate from 15, high from 90, very high from 1500; grass from 5, 20 and 200; weeds (mugwort) from 10, 50 and 500. People with hay fever can have symptoms at low levels too.