Unknown plant - anything invasive?

Unknown plant - anything invasive?

Observation - Unknown plant - anything invasive? - UK and Ireland. Description: Growing in a large stand - stand roughly 15 metres long, 4 metres deep, 2 metres high, along the bank of a small woodland stream on Dartmoor, altitude about 1000m. On the edge of a large nature reserve. Doesn't look quite like knotweed. Is this something

Growing in a large stand - stand roughly 15 metres long, 4 metres deep, 2 metres high, along the bank of a small woodland stream on Dartmoor, altitude about 1000m. On the edge of a large nature reserve. Doesn't look quite like knotweed. Is this something invasive that the reserve owner should be concerned about?