The tall, mauve flower spikes of rosebay willow herb signal that midsummer is here. They can often be seen in thick stands in woodland clearings, roadside verges, grassland and waste ground. A very successful coloniser, it has increased from being a scarce woodland plant to its present abundance. This started with the railways as it readily colonised the burnt stretches of banking which resulted from the sparks flying from steam locomotives. A single plant produces up to 20,000 wind borne seeds which can land and germinate miles from the parent. It readily travels up railway lines, the seeds blown along by the turbulence from trains. Rosebay took hold in London just after World War 2 when it invaded waste ground, often ground that was bombed in German air-raids. It became the county flower of London. Its ability to colonise land that has been burnt led to the name Fireweed.
As it has a different flower structure to the other willow herbs it is now Chamerion angustifolium and not Epilobium angustifolium.