Horticulture/Lythrum salicaria

Purple loosestrife (Lythrum salicaria) is a semi-aquatic herbaceous plant belonging to the loosestrife family, ../Lythraceae/, native to the wetlands of Eurasia, but growing in other parts of the world as an invasive plant. Other names include spiked loosestrife, purple lythrum, or salicaire. It should not be confused with other plants sharing the name loosestrife that are members of the family Primulaceae.

Description
It is a herbaceous perennial plant, growing 1-2 m tall, forming clonal colonies 1.5 m or more in width with numerous erect stems growing from a single woody root mass. The stems are reddish-purple and somewhat square in cross-section. The leaves are lanceolate, 3-10 cm long and 1-2 cm broad, downy and sessile, and arranged opposite or in whorls of three. The flowers are reddish purple, 10-15 mm diameter, with six petals (occasionally five), and are clustered tightly in the axils of bracts or leaves. The fruit is a small 3-4 mm 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 colour may last for almost two weeks. The dead stalks from previous growing seasons are brown, generally remaining erect through the winter months.

Ecology
Purple loosestrife has become an invasive species since its introduction into temperate New Zealand and North America and is now considered a noxious weed in many regions. The seeds may have arrived in the plant's non-native areas in muddy ballast water emptied from ships, but has also been used as a medicinal herb and cultivated as a garden plant. The flowers are quite showy and bright, and monotypic fields of purple loosestrife are attractive, however purple loosestrife has had a very destructive impact on North American wetland ecology since its arrival in the early 19th century.

The plants grow vigorously and spread very fast when removed from their natural controlling agents. Infestations result in dramatic disruption in water flow in rivers and canals, and a sharp decline in biological diversity. Native food and cover plant species, notably cattails, are completely crowded out, which may be disruptive to the life cycles of other organisms such as waterfowl, amphibians, or algae.

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

A single plant may produce up to three 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, infestations are extremely difficult and costly to remove by mechanical and chemical means.

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. Four 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 and two species of weevil.

Uses
Outside of wetland areas, Purple Loosestrife is not particularly aggressive, and its long bloom period makes it an excellent garden plant. It should be handled with care however, and not planted where seeds might be washed into watersheds (including storm sewers). Composts made from materials including this plant should likewise be kept well away from wetland areas.

"European wand loosestrife" (L. virgatum) is the same species, and the plant often appears under this name in nurseries. The sale of purple loosestrife is illegal in many areas due to the risk of introducing seed into uninvaded wetlands.

The flowers are long lived in the vase, and can be grown in cutting gardens for this purpose.

Control
When growing in a wetland, Purple Loostrife is a serious issue, and very hard to control due to the delicate nature of wetland environments. Heavy equipment and most herbicides should never be used in wetland areas. For infestations too serious to control using mowing or pulling, gardeners and horticulturists alike should consider seeking help from a wetland specialist.