Hemp Agrimony (Eupatorium cannabinum). The pink flower heads turn two-tone as the white haired seeds form. Oughtonhead Common in Hertfordshire is particularly abundant in Hemp Agrimony which is a plant of damp grassland, stream sides, riverbanks, marshes and ditches. 

 A ditch that runs across the common is full of it as well as other moisture loving species like Greater Willowherb and Purple Loosestrife. John Shipton -the proprietor of Shipton Bulbs- once published a blog called 'A guide to five star ditches'. The one at Oughtonhead is absolutely first rate, top marks.