yellow flower with five petals

yellow flower with five petals

Observation - yellow flower with five petals - UK and Ireland. Description: Growing in quite dry ground but on the bank of a ditch. To my untutored eye the leaves looked a lot like Japanese knotweed, but when I look up that plant the flowers appear to be completely different.

Growing in quite dry ground but on the bank of a ditch. To my untutored eye the leaves looked a lot like Japanese knotweed, but when I look up that plant the flowers appear to be completely different.