Arnhem lies on the transition from the Veluwezoom to the Rhine, which means woodland above the city and flood meadows below it. The Veluwezoom national park begins at Rheden, a few kilometres out of town, while Sonsbeek, Zypendaal and Gulden Bodem run right into the centre. This is old deciduous forest on ridge sand: beech, oak and a great deal of birch. That birch is why spring is noticeable here, peaking from late March to early May.
Below the city lies the river region. The flood meadows along the Rhine and the pastures of the Betuwe are grassland, and from May to mid-July that is the dominant source. In the wetter ground along the water stand alder and willow, doing the early work from January. The height difference between ridge and river is considerable by Dutch standards and gives the city its own microclimate: flowering on the slopes above town can run a few days out of step with the valley. An easterly also brings pollen from the German hinterland, which is considerably more wooded than the Netherlands.