Botany

Chamerion Angustifolium



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The bright pink spikes of rosebay willowherb are a common sight on roadsides and waste ground from June to September. 

Its alternative name of Fireweed derives from its ability to very quickly colonise ground that has been burnt.  It quickly colonised parts of European cities devastated by World War Two bombing, giving it the name of “bombweed” and is now the county flower of London, England.  It was one of the most abundant colonisers on Mount St Helens after that volcano erupted.  It is also the floral emblem of the Yukon Territory

The second part of its scientific name Chamerion (formerly Epilobium) angustifolium refers to its long narrow leaves.  The name willowherb also suggests leaves like those found on willow trees.

Description

The tall spike of pink flowers is easy to recognise.  The lower part is covered with the long (up to 15cm or 6 inches) lance-shaped leaves, occurring alternately on opposite sides of the stem. The buds develop on the top part of the stem which can elongate to up to 3 metres (9ft) by the end of the season. 

The flower parts are in fours: four petals, four sepals, 8 stamens and a four-chambered ovary.  The lower buds flower first, and are setting seed by the time the mid-level buds are opening. Pollination is by insects.  The seed capsules are long green or pink pods.  These ripen, often before the last buds open, and split open to reveal hundreds of tiny seeds, each with a tuft of long hairs on one end. It is these hairs that enable the seed to be carried long distances by the wind.  A single plant has been estimated to produce 80,000 seeds a year.  Seeds will germinate within ten days if they land in suitable habitat, and the plant will overwinter as a rosette of leaves.  The seeds can remain dormant for up to two years, but germination success is then considerably reduced.

Reproduction also occurs by underground roots, or rhizomes, which allow the plant to form large colonies.  The rhizomes sprout quickly after disturbance, producing shoots which can bloom within five weeks.  The rhizomes are part of the adaption to fire.  They are found in the top few inches of soil, below the level of heat penetration.  Rosebay is therefore able to survive relatively intense fires, and regrow quickly in areas that are repeatedly burnt.

Habitat

Rosebay occurs around the northern hemisphere – throughout North America (but excluding the south-east and Texas) and throughout Europe and Asia.  It thrives on disturbed ground, from recently felled or burnt forests, to swamps and seasonal rivers, from building sites to railway cuttings and roadsides.  In the 19th century it spread along railway lines as the seed was carried along in the draughts created by trains, and it was able to colonise ground that had been burnt .

While it is an important first coloniser of disturbed ground, it soon declines as it is out-competed by other vegetation.  It is found in open forest, but declines as the canopy closes.

It does not like being trampled, but will grow right next to a well-used footpath.

Associations with other species

Rosebay is a popular plant with insects, being a nectar source for bees and butterflies, and for hummingbirds in some parts of its range.  Small mammals will eat the seeds, and larger ones, particularly deer, will eat the leaves.  

It is the main food-plant of the elephant hawk-moth caterpillar.

Uses

Unlike many other wild plants, Rosebay has been tested for its cosmetic and therapeutic uses, and has been proven to have anti-irritant, anti-oxidant, and calming properties.  It is rich in bioactive compounds, as are other members of the same (evening primrose) family.  Both Native American tribes and Europeans have used it in cooking as well as in medicine.

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