Plant 298

Lythrum salicaria L. (Lythraceae)

Purple loosestrife

In lowland riverine and marshy habitats across much of northern temperate Europe and Asia, the tall, dense, magenta-coloured flowering spikes of native Lythrum salicaria are familiar sights during the latter half of summer. In areas where the species has been introduced, for example, North America and New Zealand, vast expanses of Lythrum salicaria flowers also reveals the species to be a major, problematic coloniser of wetlands.

The flowers of Lythrum salicaria have attracted the attention of evolution biologists since Charles Darwin drew attention to them. Populations of Lythrum salicaria may contain individuals that have flowers of three different sorts (tristyly). These sorts, called short-, medium- and long-styled, are defined according to the length of female style at the centre of the flower, and the positions of the male anthers. For example, short-styled flowers have anthers on long and medium stalks, whilst long-styles flowers have anthers on short and medium stalks. Biologically, this means that plants with the same flower type cannot mate with each other. That is, the mechanism reduces inbreeding and promotes outcrossing. In the mid-twentieth century, as the importance of genetic data for understanding evolutionary phenomena was becoming established, a series of classic papers began to unravel the complexities of tristyly in Lythrum, and that the phenomenon is controlled by two genes.

Lythrum salicaria flowers are insect pollinated and reproduction is primarily from seed. It has been estimated that individual Lythrum plants can produce more than 2.5 million seeds each year. The tiny seeds are readily distributed by both wind and water, and a long-lived seed bank may form. Such features can make Lythrum salicaria a difficult species to control. However, biological control using different leaf- and seed-eating beetle species is being used successfully to control purple loosestrife, especially in North America.

Despite its potential as a weed species, the conspicuous, showy flowers have attracted gardeners to select and develop numerous, large-flowered cultivars. Lythrum salicaria has provded particularly popular in situations with poorly drained soils, although the value of the plant for bee populations has not been established.

The generic name Lythrum (meaning 'blood') refers to the colour of purple loosestrife flowers, whilst the specific name references the supposed similarity between the leaves of loosestrife and willow. In contrast, the common name is a translation of the Greek name used by the Graeco-Roman doctor Dioscorides in the first century CE; the plant was believed to calm quarrelsome oxen.

Further reading

Costa J et al. 2017. Experimental insights on Darwin's cross?promotion hypothesis in tristylous purple loosestrife (Lythrum salicaria). American Journal of Botany 104: 616-626.

Mal TK et al. 1992. The biology of Canadian weeds. 100. Lythrum salicaria. Canadian Journal of Plant Sciences 72: 1305-1330.

Wilson LM et al. 2004. Biology and biological control of purple loosestrife. USDA Forest Health Technology Enterprise Team.

Stephen Harris