Alphen aan den Rijn lies in the heart of the Groene Hart, and it is exactly as green as that sounds — but it is grass, not forest. The peat meadow country between Leiden, Gouda and Woerden is open pasture cut through by ditches and canals, with farm ribbons and pollarded willows as the only vertical elements. For hay fever that means one dominant source and one dominant period: grass, from May to mid-July.
Tree pollen is secondary here and mostly comes from elsewhere. Alder and willow line the Oude Rijn and the lakes, flowering from January and March respectively, and birches stand along the streets in the village centres and the Zegersloot area. But the large birch sources lie on the sandy soils of the Heuvelrug and the Veluwe, tens of kilometres east; an easterly brings that pollen over, a westerly keeps it away. The sea is roughly twenty kilometres off, close enough for a firm westerly to press the level down — except during the grass season, when the source is the meadow next door.