By Julia · weer-meteo.be editorial team
The Belgian pollen calendar, season by season
In Belgium, the pollen season generally opens as early as late winter with hazel and alder, two trees whose pollen can fly in still-cold weather, sometimes with the first mild spells in February. This first wave, often underestimated because it is barely associated with spring, affects people sensitive to early tree pollens and can cause discomfort while gardens still look dormant.
Birch then takes over, typically in April, with a concentration that can climb very quickly over just a few days and which ranks among the most allergenic pollens on the Belgian calendar. Next comes the long grass season, from May to July, which concerns meadows, roadsides and unmown lawns, and which remains the broadest and most frequently reported period among allergy sufferers. In late summer, mugwort and then ragweed take over in some areas, extending the season into September depending on the year and the region.
Noticeable differences between the north and south of the country
The pollen calendar does not unfold the same way in Flanders and Wallonia. The slightly milder, earlier climate of the Flemish plains and the coast often favours an earlier start to the tree season compared with the more continental, higher-lying areas of Wallonia. Conversely, the more pronounced relief and altitude in the south of the country, particularly towards the Ardennes, slightly delay the arrival of tree pollens and can also shorten the grass season at higher elevation.
The farming landscapes also differ: Wallonia has more meadows and large arable fields that locally influence grass pollen density, while the urbanised, densely populated areas of Flanders concentrate a larger exposed population close to pollen sources. These regional nuances are why it is always worth checking the pollen index specific to one's own municipality rather than a national average, since differences from one region to another can be noticeable within the same week.
When weather makes symptoms worse or better
Dry, windy weather favours the spread of pollen through the air: grains detach more easily from plants in warm, dry conditions and are then carried over long distances by the wind, which explains the exposure peaks on sunny, windy spring days. Thunderstorms can paradoxically make things worse in the short term: moisture causes pollen grains to burst into smaller fragments, which can penetrate deeper into the airways, a phenomenon sometimes linked to thunderstorm asthma episodes.
Conversely, steady, sustained rain tends to pin pollen to the ground and clear the air, often giving sensitive people temporary relief. Calm, windless days also limit pollen being stirred back up. To reduce exposure, it is advisable to air out the home at times when concentration is lowest, generally late in the day or after rain, and to avoid prolonged outdoor activity during announced peaks, though this is no substitute for a healthcare professional's advice in cases of marked symptoms.
Official measurements and the model: two complementary sources
In Belgium, pollen is measured by Sciensano, the Belgian institute for health, using traps in several cities across the country. Counts and forecasts are published on the AirAllergy website, which also covers species missing from our map, such as hazel and ash.
Our map is based on the European CAMS model and shows the species it computes: alder, birch, grasses, mugwort and ragweed. For hazel in February, or ash, which flowers in April at the same time as birch, Sciensano's measurements remain the reference. Many people allergic to birch also react to certain raw foods such as apples or hazelnuts: cooked, these are usually better tolerated, and a doctor can confirm the link.
For a day out or a holiday, comparing two municipalities on our pages often says more than an average: with a sea breeze, the air reaching the coast between Ostend and De Panne is usually poorer in pollen than inland, while grasses in the Ardennes are still flowering when the season in Flanders is over.
Reading the site's pollen index and its limits
The pollen index shown on weer-meteo.be is based on data from the European Copernicus programme, through its Atmosphere Monitoring Service (CAMS), which models the spread of several pollen families across the continent. For each municipality, the index combines an estimated concentration for the currently dominant pollen type with a trend for the coming days, making it easier to plan an outing or adjust an allergy-sensitive activity.
It is worth keeping in mind, however, that this data rests on large-scale modelling rather than measurements taken in every garden: local reality, shaped by nearby vegetation, a flowering garden next door or a nearby park, can differ from the general estimate. The index should therefore be read as a useful trend to help prepare, not as an exact, individual measurement. In cases of severe or unusual symptoms, seeing a doctor remains the right step, since the site's index is not meant to replace medical advice.