In the UK, Rosebay Willowherb is considered to be a weed. The definition of a weed varies of course and while you could overly generalise and say that a weed is a ‘plant in the wrong place’ we prefer a more specific definition of a weed which requires it to have three main characteristics i.e. (1) a wild plant (2) growing in an unwanted place and (3) likely to hinder (or wipe out!) cultivated plants. Using that definition it’s easy to categorise Rosebay Willlowherb as a weed should you find it in your garden or on your allotment.
The Royal Horticultural Society describe Willowherb as a “… native perennial weed” so you might just want to take their word for it; they know a bit about Britain’s native flora.
Here in the UK, Rosebay Willowherb is a common native species but it is also an invasive species. At the time of writing (August 2019) Rosebay Willowherb is not on a UK government ‘most dangerous list’; unlike plants such as Japanese Knotweed, Giant Hogweed and Himalayan Balsam which are subject to several UK laws relating to notification, control and disposal. These are known as ‘invasive alien (non-native) species’ and are specifically listed in The Wildlife and Countryside Act 1981 which makes it an offence to plant them or “otherwise cause to grow in the wild”. There are also strict requirements on disposal to prevent the plant from literally being rehomed by moving it from one site to another.
So what do we know about the troublesome – yet striking – Chamaenerion angustifolium? That ‘angustifolium‘ part breaks down as narrow (angustus) and leaved (folium). We have John Gerard to thank for the ‘Rosebay’ part which often precedes ‘willowherb’; he was an English botanist spanning the 16th century and 17th century who logged the resemblance of the leaves to bay (Laurus nobilis) and the flowers to wild roses … ‘dog rose’ maybe? (Rosa canina). Regarding taxonomy it was denied the genus of Epilobium due to certain characteristics but nevertheless ending up as a sister to Epilobium.
Willowherb is often described as a ‘pioneer species’ because it’s often the first weed to arrive when there is disturbed ground. This trailblazer consequently thrived in London during the Blitz of World War 2 due to the extensive areas of rubble and ash, earning the nickname ‘Bombweed’. Despite our feelings about its weed status today those patches of pinky-purple flowers must have been a welcome visitor to the desolate and bleak bombsites; it has subsequently been chosen as London’s ‘County Flower’.