Purple Loosestrife (EwA Guide to Common Invasive Species in Middlesex County (MA, US) )

Purple Loosestrife

Lythrum salicaria

Summary ⁶

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 family Primulaceae. Other names include spiked loosestrife and purple Lythrum. This herbaceous perennial is native to Europe and Asia, and possibly Australia.

Short Description ⁷

Wetland plant with lance-shaped leaves. The leaves have smooth edges, usually arranged in opposite pairs along the stem. The stem is square-shaped (4 to 6 sides). The flowers are long purple spikes that grow up to a foot long from July to Sept.

Source: EwA Invasive Pocket Fieldguide | © Earthwise Aware

Description ⁶

Lythrum salicaria can grow 1–2 m (3 ft 3 in – 6 ft 7 in) tall, forming clonal colonies 1.5 m (4 ft 11 in) or more in width, 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, 3–10 centimetres (1–4 in) long and .mw-parser-output .frac{white-space:nowrap}.mw-parser-output .frac .num,.mw-parser-output .frac .den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output .frac .den{vertical-align:sub}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}5–15 millimetres (3⁄16–9⁄16 in) broad, downy and sessile, and arranged opposite or in whorls of three.

The flowers are reddish purple, 10–20 millimetres (1⁄2–3⁄4 in) in diameter, with six petals (occasionally five) and 12 stamens, and are clustered tightly 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. The flowers are visited by many types of insects, and can be characterized by a generalized pollination syndrome.

The fruit is a small 3–4 millimetres (1⁄8–5⁄32 in) 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.

L. salicaria is very variable in leaf shape and degree of hairiness, and a number of subspecies and varieties have been described, but it is now generally regarded as monotypic with none of these variants being considered of botanical significance. The species Lythrum intermediumLedeb. ex Colla is also now considered synonymous.

Invasiveness ⁸

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 result in dramatic disruption in water flow in rivers and canals, and a sharp decline in biological diversity as native food and cover plant species, notably cattails, are completely crowded out, and the life cycles of organisms from waterfowl to amphibians to algae are 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.

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, which is preferable.

In North America, purple loosestrife may be distinguished from similar native plants (e.g., fireweedChamerion angustifolium, blue vervainVerbena hastata, LiatrisLiatris 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.

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.

🚧 Control Methods (EwA Content Development in Progress) ⁷

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Biological control

Sources and Credits

(c) Bill MacIndewar, some rights reserved (CC BY-NC), uploaded by Bill MacIndewar, https://www.inaturalist.org/observations/133686507
(c) Claire O'Neill, some rights reserved (CC BY-NC), uploaded by Claire O'Neill, https://www.inaturalist.org/observations/136431274
(c) Claire O'Neill, some rights reserved (CC BY-NC), uploaded by Claire O'Neill, https://www.inaturalist.org/observations/100263329
(c) ssen, some rights reserved (CC BY-NC), uploaded by ssen, https://www.inaturalist.org/observations/28841990
(c) Claire O'Neill, some rights reserved (CC BY-NC), uploaded by Claire O'Neill, https://www.inaturalist.org/observations/123832424
(c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Lythrum_salicaria
(c) Claire O'Neill, some rights reserved (CC BY-SA)
Adapted by Claire O'Neill from a work by (c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Lythrum_salicaria

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