When is hay fever season for tree, grass and weed pollen?

Three pollen seasons, one after another

Hay fever season is really three seasons that overlap. The Met Office describes them in order. Tree pollen comes first, typically from late March to mid-May, and affects around 25 percent of people with hay fever. Grass follows from mid-May until July. Weed pollen can be released at any time, but its season typically covers the end of June to September.

Grass deserves the most attention, because the Met Office says most people are allergic to it. Its season has two peaks. Across England and Wales the main peak usually starts in the first two weeks of June, and a second, lower peak follows in the first two weeks of July. Someone who feels better in late June has not necessarily seen the end of it.

The NHS gives the same overall window from the symptom side: hay fever is usually worse between late March and September, especially when it is warm, humid and windy.

How the timing changes with where you live

Within the UK, the Met Office notes a later start and a shorter season in the north, where there is generally less pollen. Urban areas have lower counts than the countryside, and inland places have higher counts than the coast. Two people with the same allergy can have quite different summers depending on whether they live in a northern city or an inland farming area.

In the United States the order is the same but the tail is longer. The American College of Allergy, Asthma and Immunology (ACAAI) says tree pollination begins earliest in the year, grass follows later in spring and summer, and ragweed takes over in late summer and fall. Ragweed pollen runs from August to November and usually peaks in mid-September. The ACAAI counts 17 species of ragweed in the US and says ragweed allergy affects as many as 25 million people there.

In Germany, the Stiftung Deutscher Polleninformationsdienst publishes regional pollen calendars from its own measurements; the current version is based on daily values from 2016 to 2021. When the foundation compared its 2007 to 2011 calendars with data from 2000 to 2007, several pollen types, including ash, birch, grasses and mugwort, had started flying earlier, and mugwort and nettle were also flying for longer. During the season, the German Weather Service issues its pollen hazard index for eight pollen types in 27 areas of the country.

Weather that pushes the count up or down

The calendar sets the window; the weather decides each day. According to the Met Office, early, heavy and prolonged rain is likely to keep counts low all day, while rain in the afternoon has less effect. For grass, a maximum temperature between 18 and 28 degrees Celsius can give a high count on a dry day with low humidity and a gentle breeze. Above 28 degrees, all pollen levels decrease.

Wind cuts both ways. The Met Office says that if the wind is too light, pollen barely gets off the ground, but if it is too windy, pollen is blown further afield and thins out significantly. Daylight matters because of photosynthesis: in a particularly cloudy spell, plants and trees produce less pollen. The ACAAI adds that rain washes pollen away, yet counts can soar after rainfall, and that on a day with no wind airborne allergens stay grounded, while windy, warm days make counts surge.

Winters matter months later. The ACAAI says mild winter temperatures can cause plants to pollinate early, so a soft winter can pull the start of tree season forward.

What time of day pollen is highest

The ACAAI gives a seasonal rule of thumb. In spring and summer, during tree and grass pollen season, levels are highest in the evening. In late summer and early fall, during ragweed season, levels peak in the morning.

That makes the season part of the answer. By the ACAAI's description, an evening in June falls in the higher hours for grass, while in September it is the morning that carries the most ragweed. A daily forecast gives one figure for the whole day, so the time of day is something you add yourself.

Common mistakes about hay fever season

Thinking it is only a spring problem. Weed pollen runs to September in the UK, and in the US ragweed lasts into November.

Planning by the calendar alone. The Met Office's dates are typical, not fixed. A dry, warm week in the right window can push counts high, and a wet morning can hold them down.

Missing the second grass peak. A quieter spell in late June can be followed by the lower July peak the Met Office describes.

Not knowing which season is yours. Since tree, grass and weed seasons differ, knowing which pollen you react to tells you which months to watch. Pollergo's Week tab, for instance, shows the days its source forecasts for the pollen types you have chosen, and leaves the rest of the week blank rather than guessing.

On days with high counts, the NHS suggests keeping windows and doors shut as much as possible, showering and changing clothes after being outside, wearing wraparound sunglasses, and not drying clothes outside.

This guide describes pollen seasons and weather. It is not medical advice. For symptoms or treatment, talk to a pharmacist, doctor or allergist.

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