Friday, 17 January 2014

Week 34: Rosebay Willowherb (‘Chamerion angustifolium’)

Rosebay Willowherb was @SpeciesofUK from 9st to 15th September, 2013.

Rosebay willowherb is easily recognisable from its tall, pink flower spikes crowding in thick stands in open spaces like verges and waste ground.[1]

Rosebay Willowherb
[Source: Flickr Creative Commons © Anita363]

It is a member of the Onagraceae family, which includes the willowherbs, evening primroses and fuchsias. 

Rosebay willowherb is a common sight.

Rosebay willowherb used to be a scarce woodland plant. In the eighteenth century it was considered rare and confined to only a few locations on damp, gravelly soils.[2]

However, as human activity intensified and natural habitats were cleared, rosebay willowherb proved itself to be a great coloniser of open ground. It is now extremely common.[3]

Rosebay Willowherb Colonisation
[Source: Flickr Creative Commons © joysaphine]

This rapid expansion of its range occurred in part as a result of two World Wars which resulted in clearing huge areas of forest and burnt and damaged ground in both town and countryside - just the right conditions for rosebay willowherb to thrive.[4] Learn more in this fabulous short BBC video.

Unfortunately rosebay willowherb has become a victim of its own success and is now commonly regarded as a weed.

A Weed?
[Source: Flickr Creative Commons © wallygrom]

Rosebay willowherb’s common names allude to its history.

Rosebay willowherb has a number of other names. It has become commonly known as ‘fireweed’ (especially in the USA) or ‘bombweed’ in no small part due to its success on burnt ground.[5]

[Source: Flickr Creative Commons © Dave ®]

Its rapid spread across urban London has also led to it becoming known as 'London pride' and 'London’s ruin.' It is London's official county flower.[6]

In Clydebank during the war, rosebay willowherb sprung up in the ruins of the Singer Sewing Machine factory, leading to the local name there of 'singerweed.'[7]

In folklore, rosebay willowherb should not be picked, otherwise a thunderstorm will ensue. This led to another dramatic name, 'thunder-flower.'[8]

[Source: Flickr Creative Commons © Steve_C]

Finally, the likeness of rosebay willowherb leaves to some willows has resulted in names such as ‘blooming sally’ (Cumberland), or ‘flowering withy’ (Berkshire).[9]

Rosebay willowherb has some interesting adaptations that explain its success as a coloniser.

It has fine shallow rhizome root systems that can take hold in new areas quickly.[10] A 'rhizome' is a modified stem of a plant that grows underground and sends out roots.[11]

Rosebay Willowherb Rhizomes
[Source: Wikimedia Commons © Rasbak]

Rosebay willowherb flowers appear from June to September. The flowers have four magenta to pink petals, 2 to 3 cm in diameter. The styles have four stigmas.[12]

Rosebay Willowherb Flowers
[Source: Flickr Creative Commons © squamatologist]

Once pollinated a rosebay willowherb flower produces red-brown, linear seed pods that can hold between 300-600 seeds. [13]

Rosebay Willowherb Seed Pods
[Source: Flickr Creative Commons © joysaphine]

The seeds produce a tuft of hair. These ‘parachutes’ allow rosebay willowherb seeds to disperse easily even on a light breeze, another reason for its success.[14]

Bursting Seed Pod
[Source: Flickr Creative Commons © helen.2006]

Burst Seed Pods
[Source: Flickr Creative Commons © Steve_C]

The leaves of rosebay willowherb are quite unique.

The veins on rosebay willowherb leaves do not terminate at the edge of the leaf like most plants. They circle back in and join together.[15]

This leaf structure is the surest way of identifying rosebay willowherb in its earlier stages of growth, before it has developed other distinctive features.[16]

Rosebay Willowherb Leaves
[Source: Flickr Creative Commons © aposematic herpetologist]

The leaves are lance-like in shape and arranged spirally up the stem.[17] The Latin species name angustifolium is a composite of angustus and folium meaning 'narrow-leaved.'[18]

Rosebay Willowherb Leaves in Autumn
[Source: Flickr Creative Commons © anneke1998]

Strange but true…

The leaves of rosebay willowherb are used in tea in Russia, under the name of Kaporie Tea.[19]


  1. Excellent article. I know what 'angustifolium' means but what does 'chamerion' mean?

    1. Great question. I've tried looking this up but so far without success. Answers on a postcard!