Leiden lies between the dunes and the Groene Hart and takes something from both. To the west are the Coepelduynen and the dunes at Katwijk and Noordwijk, with marram grass and dune reed; to the east the peat meadows begin, grass all the way to Alphen and Woerden. In the city itself stand the Leidse Hout with its old beeches and the Hortus botanicus, the oldest botanical garden in the Netherlands.
The bulb region just north of the city is often blamed unfairly. Tulips, hyacinths and daffodils are insect-pollinated: their pollen is heavy, sticks to insects and barely becomes airborne. The symptoms people develop among the bulb fields in April almost always come from the birches and alders standing along the roads and farmyards. The sea is ten kilometres away, close enough that a westerly gives a noticeably lower level. The grass season from May to July is the hardest period here, with the Groene Hart as one large continuous source directly east of the city.