Fireweed

Fireweed gets its name from its appearance after grass and heath fires. In the UK it is a non-native species having been imported (accidentally) from North America. It is also know as Rosebay Willowherb.

Here in the UK it tends to be found on railway embankments and alongside rivers and other watercourses. A single plant will, in about 6 weeks, produce something in the order of 50k seeds. Presumably their wafting by passing trains ensures their dispersal along almost all the nations railway lines. These were found not unsurprisingly in a field ditch.