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Latin name: Betula spp.

Plant description: Medium sized trees with small, triangular, serrated leaves and white bark. Yellow catkins produce the pollen. Fruit: composed of layers of small seeds in a catkin shape, green at first, then brown.

Distribution: UK: countrywide. Also across much of Europe, Asia and Northern USA, restricted to cooler climates or found at altitude.

Allergenicity and other health impacts: Birch pollen affects about 25% of the UKs hay fever sufferers and has high allergenicity. It can cross-react with hazel and alder pollen and some tree-pollen sufferers can experience symptoms starting in Winter and lasting until late May or early June.

Season: Mid-March to early June, although the start of the season can vary by up to one month and often occurs about two weeks later in Scotland compared to southern England.

The peak period occurs in April. Birch trees each produce millions of wind-dispersed pollen grains so in the peak of the season the count can be very high.

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