The tall, robust spikes of purple loosestrife commonly grow in wet ditches and pond margins. However, away from its native Europe and Asia, the plant has become a pest, spreading rapidly along waterways.
The name lose-strife is said to be a literal translation of the Greek name for the plant, which was believed to be so powerful that it could be placed on the yoke of quarrelling oxen to calm them.
The scientific name Lythrum salicaria comes from the Greek for blood or gore, and probably from salicaceous meaning willow-like. Certainly this plant provides a red colouring, it grows like willow trees along the water’s edge. It also has leaves similar in shape to those of willow.
Purple loosestrife is a perennial herb, growing up to 1.5m (5 feet) tall in its native lands, but achieving as much as 2.4m (8 feet) in other countries. It has a rigid square or hexagonal stem and narrow leaves up to 10cm (4 inches) long. The plant flowers from June to September, with whorls of purple blossoms around the stems – each plant may have 30 or more stems. Each blossom has four to six crinkled purple petals.
Charles Darwin, among others, discovered that this is a “most interesting plant”. There are three forms of the flower, with the stamens and stile of different lengths. Each flower type can only be pollinated by one of the others, thus ensuring cross-pollination.
A single plant can produce over a million tiny seeds. The seeds fall into water, where they may germinate and be carried some distance, either by the water itself, or by mud adhering to people’s clothes or to livestock, before lodging in the soil at the water’s edge. The plant also spreads by underground roots or rhizomes which become thick and woody in mature plants. The leaves and stem die in the autumn, allowing space for now shoots to arise from buds at the top of the root crown in spring. The root crown grows to a maximum 50cm (20 inches) diameter.
Purple loosestrife thrives in a variety of wetland habitats, especially in marshes and along the margins of rivers and streams, to altitudes of 1,000m (3,300 ft). Open moist soils and high temperatures promote growth, and seedling densities can reach up to 20,000 plants per square metre (2,000 per square foot).
With the name Lythrum coming from the Greek word for gore, it is not surprising that the ancients used the plant to staunch bleeding. It was also used to treat heavy menstrual bleeding in women, and in a poultice or lotion applied to leg ulcers and eczema. Add to that its uses as a hair dye, an eyewash, a remedy for diarrhoea and as an insect repellent, it is easy to see why people took it with them when they colonised new lands.
Purple loosestrife as an invasive species.
In the early 1800s the plant was found growing in North America. It may have arrived in ships’ ballast, but was also taken there as a medicinal herb. By the 1830s it had spread along the east coast, and by 1996 was in all the lower US states except Florida, and across all the southern Canadian provinces. It is also a problem in the North Island of New Zealand, but surprisingly is considered a native plant in south-east Australia. Until recently, cultivars of purple loosestrife were available in garden centres in these countries. It was believed that the cultivars were sterile, but some have proved to be fertile.
In Europe and Asia, the plant is kept in check by the various invertebrates that feed on it, including insects and nematodes. These are often species-specific, and are found only in the native areas. Elsewhere the plant is able to grow unhindered, and so out-competes native species in the same habitat. Not only are these plant species now in decline, but birds that breed on open braided river systems, such as the black tern (Chilodonius nigra) in north America and the black stilt (Himantopus novaezelandiae) in New Zealand, are losing their nesting sites.
Control of purple loosestrife is proving very difficult, as chemicals that could kill it would also kill all the other species around it. Biological control is being studied, with four insects showing good potential for the purpose.
More detail about the biology, spread and control of this species is given in this study by the USGS.