Breda sits on the transition from Brabant sand to the marine clay of the west, and it has two woods on its southern side worth mentioning for hay fever. The Mastbos is one of the oldest pine plantations in the country, established from the sixteenth century onwards; the Liesbos to the west is deciduous, with old oak and beech. Of the two, the Liesbos matters more to allergy sufferers, because pine pollen from the Mastbos is abundant and highly visible but causes few symptoms in most people.
The birches are scattered through both areas and along the Mark and the Aa of Weerijs, which also carry alder and willow in their stream valleys. The birch season runs from late March to early May. Grass then takes over: Breda is surrounded by farmland, and the meadows towards Etten-Leur, Prinsenbeek and Ulvenhout deliver the year's highest readings from May to mid-July. The city lies some fifty kilometres from the sea — far enough that a westerly brings only part of the dilution Rotterdam gets.