Zwolle lies in the IJssel delta, where the river widens towards the Zwarte Water and the Ketelmeer. This is river clay and flood meadow: wet, open grassland with willow and alder along the water. For hay fever sufferers that means grass above all, and the season from May to mid-July is the part of the year that counts. The flood meadows along the IJssel start at the city's edge.
Tree pollen comes from further away. The Sallandse Heuvelrug lies some twenty kilometres east, with woodland and birch on sandy ground, and the Vecht valley towards Dalfsen supplies birch and alder as well. Closer to home, Park de Wezenlanden and the estates towards Wijhe and Heino contribute, but modestly. Alder flowers here from January along the water, birch from late March to early May. Zwolle is a good eighty kilometres from the North Sea, so maritime dilution is limited, although the open delta does help carry pollen away on windy days.