Utrecht sits exactly where the Netherlands stops being wet and turns to sand. East of the city the Utrechtse Heuvelrug begins: an ice-pushed ridge of poor, dry sand, and that is precisely the ground birch likes best. Birch is the strongest tree allergen in the country, and under an easterly its source lies less than ten kilometres away. Closer in, at Amelisweerd and along the Kromme Rijn, alder and willow stand in the wetter ground; alder does the early work, from January into March.
West of the city lies the Groene Hart, and with it the grass season. Unlike The Hague or Rotterdam, Utrecht has no sea to dilute its air: no wind direction reliably brings relief, because there is always land upwind. That makes it one of the places where the season is most evenly heavy — no sharp peaks from long-range transport, but no free clean days either. The months to plan around are March and April for birch, and May to mid-July for grass.