Plant 134

Geranium robertianum L. (Geraniaceae)


Herb Robert

Herb Robert is a common, distinctive, variable, pink-flowered, ruderal plant of woodlands, hedges and open habitats. Herb Robert stems are frequently reddish, which some people think gives the plant its common name. Besides its colour, herb Robert is usually covered in glandular hairs and has a characteristic, unpleasant scent. It is native throughout the British Isles and continental Europe, and extends east into European Russia, Caucasus, western Siberia and Central Asia. It has been introduced in other parts of the world and is naturalised in North America and China.

Herb Robert typically flowers from early spring to early autumn. Each flower has five, free petals, ten stamens and five stigmas. Between the bases of each petal there are nectaries but to get at the nectar reward, insects must have long mouthparts. Herb Robert flowers have an elaborate opening routine. When the flower opens, the stigmas are close together. The five anthers closest to the centre of the flower split open releasing their pollen; the other five stamens bend towards the petals. The stigmas then separate to expose their receptive surfaces, the inner stamens wither and the outer ones move towards the stigmas and open. Anthers are typically pink or purple, the pollen is bright orange. Despite herb Robert flowers being visited by numerous types of insects, seed production appears to be by self pollination.

The seed dispersal mechanism of the herb Robert, as with other members of the genus, is rather unusual. The fruit is called a schizocarp, which is a type of dry fruit that splits into separate single-seeded pieces (mericarps) at maturity. In the case of herb Robert, the seed-containing mericarps may be hurled more than five metres by a catapult mechanism. Once released, seed-dispersal may also be aided by animals such as ants and snails.

Populations of Herb Robert tend to be uniform in their characteristics but there are frequently marked differences among populations, for example, in hairiness. In north-eastern Poland there is a form of herb Robert that is specifically adapted to growing near railways lines. Railway soils are typically well drained, nutrient deficient and polluted with heavy metals. Herb Robert growing in these conditions is shorter, with smaller, differently shaped leaves than the typical form of the plant. Furthermore, physiological experiments show the railway form is well adapted to the water and insolation stresses of railway life, whilst genetic experiments show these characteristics are heritable.

Further reading

Bertin RI 2001. Life cycle, demography, and reproductive biology of Herb Robert (Geranium robertianum). Rhodora 103: 96-116.

Tofts RJ 2004. Biological Flora of the British Isles 234. Geranium robertianum L. Journal of Ecology 92: 537-555.

Wierzbicka M et al. 2014. Geranium robertianum L., plant form adapted to the specific conditions along railway: "railway-wandering plant". Plant Systematics and Evolution 300: 973-985.

Stephen Harris