Hybridizes with : Nothing! Unlike most other members of the Willowherb family which happily cross-breed with one another, Rosebay Willowherb is the black sheep of the family, it hybridizes with not a single one of them. This may also explain its scientific name, which does not start with Epilobium like all the other willowherbs, but with Chamerion instead, almost as if it wasn't really part of the same family. Clearly, there are great physical similarities, but also great genetic differences.

Uniquely identifiable characteristics : there is no other flower quite like this one.

From a distance a rift of Rosebay could be confused with one of Purple Loosestrife.

A single plant produces up to 20,000 parachuted seeds ready to take flight in the wind to land and germinate miles from its parent. Rosebay Willowherb took a hold of Britain just after WWII when it invaded waste ground, usually ground that was bombed flat in German air-raids. In London it was known as 'Bombweed'. Its propensity to colonise land that has burnt gave rise to another nick-name, 'Fireweed', especially when great swathes are in flower or when leaves are in autumn colours and it begins to resemble a grass-fire. From the 1950's onwards it spread rapidly, borne on the wind. It readily travels up railway lines, the seeds wafted further along by the eddies from passing trains.

Each plant produces about 20,000 seeds.

All Willowherbs apart from Rosebay Willowherb have four petals, which has five.