The main part of the Bollenstreek – bulb region – stretches behind the sand dunes west of Leiden to just south of Haarlem and is particularly suitable for growing flowers because of its sandy soil and mild climate. The bulb growing industry has been concentrated here since the 16th century when rich families from Amsterdam and The Hague began to cultivate tulips – with all the madness that entailed. Dutch growers export some four billion bulbs every year.