De zomerbitterling [Blackstonia perfoliata (L.) Huds. subsp. perfoliata] bij Amsterdam

Blackstonia perfoliate (L.) Huds. subsp. perfoliate! which is not considered to be indigenous in the Netherlands, was found near Amsterdam for the first time in 1974, on manmade sandfields for the extension of an industrial area. The subspecies has since increased tremendously in numbers (part of this increase being caused by human dissemination to similar surrounding habitats). The subspecies survived the rather severe winter 1978/79, possibly because of protection by snow, which had fallen before the lowest temperatures occurred. Though now present in tens of thousands the main threats for the future are increasing competition by many other species in these still rather unstable habitats, possible severe winters (particularly those without snow) and industrial development. The figures 1 and 2 show seasonal progress of flowering in 1981 and the daily process of shutting of the flowers on several dates in 1981 respectively. Rosettes are formed in July-August, but few, if any survive, even in mild winters, so that (practically) all plants develop from seed in May-June.