Amersfoort lies at the foot of the Utrechtse Heuvelrug, and that is the decisive factor. Immediately south and west of the city lies Birkhoven-Bokkeduinen, with the woods of Leusden and Soest behind it and the whole ridge beyond. This is pine and oak forest on poor cover sand, and on exactly that ground birch is the tree that comes up fastest. The birch season from late March to early May is therefore the most conspicuous part of the year in Amersfoort.
On the northern side the picture is reversed. There lies the Eem valley: low, wet pasture along the Eem towards the Eemmeer, with alder and willow along the water and grass as far as you can see. That is the source from May to mid-July. The city therefore sits between two different seasons, sand and birch on one side, peat and grass on the other. Amersfoort is roughly sixty kilometres from the coast, too far for a westerly to bring much relief.