Rosebay Willowherb which I think of as Epilobium angustifolium but which has been reclassified as Chamerion angustifolium. If you see a pink haze by a roadside or railway line at this time of year it will be Rosebay Willowherb.

 Readily forms large drifts as it spreads by both self-seeding and rhizomes. Rosebay Willowherb has a wide distribution across the northern hemisphere. Possibly native to the UK, old records suggest it was present but not abundant. E. angustifolium seems to have become prolific during the course of the twentieth century.

 My father tells me it covered bomb sites around St. Paul's in London after the war. An American name for the plant is 'Fireweed.' Seeds dormant in the soil will germinate after an area has been scorched by fire.

 A good plant for pollinators in the latter part of the summer. This swathe was attracting masses of honeybees as well as bumblebees and butterflies.