Marsh marigolds
Sometimes in parts of the country on May Eve (tomorrow) Marsh marigolds were placed at the entrance or on window ledges of houses to ward off evil  and were called May Flowers.They are commonly found in damp places in spring and early summer and often known as Kingcups (Calathos Palustris) with the latin name derived from the Greek word ‘kalathos’ for ‘cup’ such as kings might have used and the latin word ‘palustris’ for marsh.  During medieval times the flowers were used in churches at Easter as a tribute to the Virgin Mary, as in "Mary gold" 
Here it is growing among the water horsetail plants