White butterbur

White butterbur Petasites albus was introduced from central Europe, and has become established in many parts of UK.  It thrives in moist sites, especially along watercourses.  It forms colonies which outcompete most other vegetation.  These colonies steadily expand in extent and coalesce, effectively smothering all native ground flora.  The plant appears not to spread by seed, but is dispersed downstream when fragments of root are broken off during floods.