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Purple Loosestrife

Lythrum salicaria

  • Written also: Purple-loosestrife
  • Family: Loosestrife Family – Lythraceae
  • Height: 40–120 cm (16–48 in.). Stem unbranched–sparsely branched, 4-edged, usually fine-haired.
  • Flower: Corolla regular (actinomorphic), purple, 8–10 mm (0.32–0.4 in.) broad; petals 6, approx. 10 mm (0.4 in.) long. Calyx tubular, 6-lobed. Outer calyx 6-lobed. Stamens 12. Gynoecium composed of 2 fused carpels. Inflorescence in axillary whorls forming a terminal raceme.
  • Leaves: Opposite, alternate or whorled, stalkless, stipulate. Stipules small, dropping early. Blade narrowly ovate, with round or cordate base, both sides short-haired.
  • Fruit: Egg-shaped, 3–4 mm (0.12–0.16 in.) long capsule.
  • Habitat: Meadows and rocky outcrops by lakes, rivers and sea shores, coastal rocky outcrops, waterside meadow which is prone to flooding, ditches, ponds, sometimes in water. Also ornamental.
  • Flowering time: July-August.

Purple loosestrife grows on almost all kinds of beaches, but nowhere else. The most impressive stands decorate bird islands and the edges of gull colonies at the end of the summer in places where ducks’ feet and abundant nitrogenous fertilisation create wonderful conditions for it to grow. Purple loosestrife is a wonderful domestic alternative to decorate garden ponds, and different varieties and hybrids are for sale in Finland. It is very surprising that the tiny and modest-flowered water purslane (L. portula) is its closest relative in Finland.

The prime function of purple loosestrife’s glowing purple inflorescence is not just to look beautiful for people but to help the plant propagate itself. A simple way to get pollen to the stigma is of course self-pollination, but most often plants try to cross-breed because this produces more vibrant descendants. Purple loosestrife has developed no fewer than three ways to safeguard against self-pollination. First of all it attracts insects to pollinate it: pollen-toting hymenopterans and flower flies, and perhaps butterflies too. Also, the flowers are protogynous, meaning that the pistils develop before the stamens and the anthers begin to release pollen only when the stigma withers. These precautions do not completely exclude the possibility of self-pollination because different flowers within the inflorescence are at different stages of development, and pollen falls downwards too. Purple loosestrife has three forms of flower, however, with regards to the lengths of the stamens and style. In one type the style is longer than the whorl of stamens, in which there are 6 short and 6 medium long stamens. In the second type the style is short and the stamen whorl long and medium long. In the third type the style is medium long, and the long whorl of stamens stretches beyond the style while the short whorl remains below it. It is easy to examine the flowers because the stamens are also different colours: long ones are greenish, medium long are dirty yellow and the short ones are bright yellow. Each plant has only one kind of flower, and the predominance of any given type varies regionally. The particles of pollen produced by each flower type are different and almost without exception can only pollinate another kind of flower.
invasive alien species

Somewhere else

According to the IUCN (International Union for Conservation of Nature) our beautiful and harmless purple loosestrife is on a global list of the hundred most harmful foreign species (including plants, animals, insects…).

“Purple loosestrife is a serious invader of many types of wetlands, including wet meadows, prairie potholes, river and stream banks, lake shores, tidal and non-tidal marshes, and ditches. It can quickly form dense stands that displace native vegetation. Purple loosestrife can spread very rapidly due to its prolific seed production; one plant can produce as many as 2 million seeds per year.”

Invasive Plant Atlas of the United States

Compare Japanese knotweed, leafy spurge and broad-leaved pepperweed.

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