loosestrife, common name for the Lythraceae, a widely distributed family of plants most abundant as woody shrubs in the American tropics but including also herbaceous species (chiefly of temperate zones) and some trees. Several shrubs of this family have been introduced in the United States as ornamentals and are now naturalized, e.g., the crape (or crepe) myrtle of China (Lagerstroemia indica) and the henna shrub, or mignonette tree (Lawsonia inermis). The latter, cultivated especially in Muslim countries, is the source of henna dye (from the leaves), oil and pomade scents (from the flowers), and a medicament (from the bark). The wild marsh plants called loosestrifes (genus Lythrum) include several native American species with pink or lavender flowers, but the tall, showy species that blankets moist meadows and swamps with magenta to purple flowers in late summer and autumn is the spiked loosestrife (L. salicaria), introduced from Europe and now so widespread as to be a weed. Several species of the unrelated family Primulaceae (primrose family) are also called loosestrife. True loosestrife is classified in the division Magnoliophyta, class Magnoliopsida, order Myrtales.

Any ornamental plant of the family Lythraceae, especially in the genera Lythrum and Decodon, and two genera of the primrose family (Lysimachia and Steironema). Purple loosestrife (Lythrum salicaria), native to Eurasia, grows 2–6 ft (0.6–1.8 m) high on riverbanks and in ditches. Its branched stem bears whorls of narrow, pointed, stalkless leaves and ends in tall, tapering spikes of red-purple flowers. Introduced into North America early in the 19th century, it has become a noxious weed in many parts of the U.S. and Canada because its dense growth outcompetes native wetland vegetation that provides food and habitat for wildlife.

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Loosestrife is the common name of a number of different flowering plants. The species belong to two taxonomic genera, Lythrum, within the family Lythraceae, and Lysimachia, within the family Primulaceae.

Lythrum species known as loosestrife include:

• Lythrum californicum (California loosestrife)
• Lythrum curtissii (Curtiss' loosestrife)
• Lythrum flagellare (Florida loosestrife)
• Lythrum hyssopifolia (hyssop loosestrife)
• Lythrum ovalifolium (low loosestrife)
• Lythrum portula (spatulaleaf loosestrife)
• Lythrum salicaria (purple loosestrife)
• Lythrum thymifolia (thymeleaf loosestrife)
• Lythrum tribracteatum (threebract loosestrife)
• Lythrum virgatum (European wand loosestrife)

Lysimachia species known as loosestrife include:

• Lysimachia ciliatum (fringed loosestrife)
• Lysimachia clethroides (gooseneck loosestrife)
• Lysimachia quadrifolia (whorled loosestrife)
• Lysimachia vulgaris (garden or yellow loosestrife)