Lythrum salicaria or purple-loosestrife is a flowering plant belonging to the family Lythraceae. It should not be confused with other plants sharing the name loosestrife that are members of the genus Lysimachia in the family Primulaceae. This herbaceous perennial plant is native to temperate regions of Europe, Asia, northern Africa, and eastern Australia. The specific epithet salicaria probably derives from the similarity of the leaves to those of willows or Salix species, though Linnaeus did not specify this in his choice of the name.

The English calque 'loosestrife' first appeared in written form in the 16th century. It was coined by English naturalist William Turner in his 1548 work A New Herball, in which he states: "Lycimachia purpurea, … may in englishe be called red loosstryfe or purple losestryfe".

Purple-loosestrife was referred to several times as 'long purples' by John Clare, such as in his 1821 poem The Wildflower Nosegay:

Description

Lythrum salicaria can grow tall, forming extensive clonal colonies, with numerous erect stems growing from a single woody root mass. The stems are reddish-purple and square in cross-section. The leaves are lanceolate, long and broad, downy and sessile, and arranged opposite or in whorls of three.

thumb|right|L. salicaria flowers

The flowers are reddish purple, in diameter, with six petals (occasionally five) and 12 stamens, and are clustered tightly in verticillasters in the axils of bracts or leaves. There are three different flower types, with the stamens and style of different lengths, short, medium or long; each flower type can only be pollinated by one of the other types, not the same type, thus ensuring cross-pollination between different plants. For instance, if the pistil is medium length, then the stamens will be long and short, but not medium. The flowers are visited by many types of insects, and can be characterised by a generalised pollination syndrome.

The fruit is a small capsule containing numerous minute seeds. Flowering lasts throughout the summer. When the seeds are mature, the leaves often turn bright red through dehydration in early autumn; the red autumn colour may last for almost two weeks. The dead stalks from previous growing seasons are brown. but is now treated as a separate species, from Central Asia, by the Plants of the World Online database.

Distribution

Lythrum salicaria is native to Europe, temperate Asia, northern Africa, and Australia. It is also naturalised in many temperate parts of the world, including parts of southern Africa, North America, and South America. The species was thought to be a recent introduction to Australia, but pre-colonial pollen samples have been found in New South Wales, indicating natural colonisation before modern human plant introductions; the question is considered unresolved by Australian authorities.

Ecology

Found in ditches, wet meadows and marshes and along sides of lakes. In North America, purple-loosestrife can invade sedge meadows.

Associated insects

The flowers are pollinated by long-tongued insects, including bees and butterflies. It is also cultivated as an ornamental plant in gardens, and is particularly associated with damp, poorly drained locations such as marshes, bogs and watersides. However, it will tolerate drier conditions. The flowers are showy and bright, and a number of cultivars have been selected for variation in flower colour, including:

  • 'Atropurpureum' with dark purple flowers
  • 'Brightness' with deep pink flowers
  • 'Happy' with red flowers on a short () stem
  • 'Purple Spires' with purple flowers on a tall stem
  • 'Roseum Superbum' with large pink flowers.

The cultivars 'Blush' with blush-pink flowers, and 'Feuerkerze' with rose-red flowers have gained the Royal Horticultural Society's Award of Garden Merit.

It has also been introduced in many areas of North America by bee keepers, due to its abundance of flowers which provide an important source of nectar.

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File:LythrumSalicariaBig.jpg|Flowers in Lower Saxony, Germany

File:Bombus sylvarum - Lythrum salicaria - Keila.jpg|Pollinating shrill carder bee, Keila, Northwestern Estonia

File:Блатия.jpg|Habit in Bulgaria

File:Lythrum salicaria 160269450.jpg|Bright crimson leaves at the onset of autumn, Velké Chvojno, Czech Republic

File:Cooper Marsh - Purple-loosestrife.jpg|Naturalised in the Cooper Marsh Conservation Area, near Cornwall, Ontario

File:Purple Loosestrife (Lythrum salicaria) naturalised in Pennsylvania.jpg|Naturalised in Pennsylvania

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As an invasive species

Purple-loosestrife has been introduced into temperate New Zealand and North America where it is now widely naturalised (spreading on its own) and officially listed in some controlling agents. Infestations may rarely result in dramatic disruption in water flow in rivers and canals, and the life cycles of organisms from waterfowl to amphibians to algae being affected.

A single plant may produce up to 2.7 million tiny seeds annually. Easily carried by wind and water, the seeds germinate in moist soils after overwintering. The plant can also sprout anew from pieces of root left in the soil or water. Once established, loosestrife stands are difficult and costly to remove by mechanical and chemical means.

However, research about how invasive plants are portrayed in media indicate that the threat from loosestrife and other infamous invasive plants is driven more by media attention than science. While the species does have negative impacts on the natural environment the scientific literature supports a much more modest assessment of how bad it is outside its native range and that resources would be better spent on preventing disturbance of wetlands than on the removal of purple-loosestrife.

Plants marketed under the name "European wand loosestrife" (L. virgatum) are the same species despite the different name. In some cases the plants sold are sterile.

In North America, purple-loosestrife may be distinguished from similar native plants (e.g., fireweed Chamerion angustifolium, blue vervain Verbena hastata, Liatris Liatris spp., and spiraea (Spiraea douglasii) by its angular stalks which are square in outline, as well as by its leaves, which are in pairs that alternate at right angle and are not serrated.

Biological control

Purple-loosestrife provides a model of successful biological pest control. Research began in 1985 and today the plant is managed well with a number of insects that feed on it. Five species of beetle use purple-loosestrife as their natural food source, and they can do significant damage to the plant. The beetles used as biological control agents include two species of leaf beetle: Galerucella calmariensis and Galerucella pusilla; and three species of weevil: Hylobius transversovittatus, Nanophyes breves, and Nanophyes marmoratus. Infestations of either of the Galerucella species is extremely effective in wiping out a stand of purple-loosestrife, defoliating up to 100% of the plants in an area. The moth Ectropis crepuscularia is polyphagous and a pest species itself, and unsuitable for biological control.

Notes

References

  • Invading Species.com Ontario Ministry of Natural Resources and Ontario Federation of Anglers and Hunters
  • Species Profile - Purple Loosestrife (Lythrum salicaria), National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for purple-loosestrife.