Het Weidehavikskruid (Hieracium caespitosum Dumortier) in de IJsselmeerpolders en aangrenzende gebieden

Hieracium caespitosum appeared very soon (1948) after the reclamation (1942) of the IJsselmeerpolders. The species mainly flowers in June and it takes approximately thirteen days between fertilization and riping of the achenes; therefore, to enable proper dispersal, mowing of the populations should take place in the second half of June. The species is facultative apogamous and is frequently visited by insects, mainly Muscidae en Syrphidae. From the ecological point of view Hieracium caespitosum has been regarded to be a characteristic for Molinion-vegetations. However, on soils which are humous and lutum-rich, the species shows a preference for Arrhenatherion-vegetations; on poor soils it is part of vegetations that are related to the Thero-Airion. In fact only two locations should be looked upon as Molion-vegetations. The increase of 57 hour-squares since 1980 is, at least partly, due to new inventories.