Purple loosestrife's flowers are a bright, clear shade of purple that tempts many into inviting it into their gardens - but don't do it (unless you live in Europe, its native range, in which case, enjoy!)
Beauty from Hell. Probably no other flower is as beautiful and at the same time as hated as purple loosestrife. The plant is a clump forming perennial with square, semi-woody stems and opposite or whorled leaves. A single plant can have an extensive root system and dozens of erect stems, each 4-10 ft (1.2-3 m) tall. The lance shaped leaves are covered in a downy fuzz, and around 4 in (10 cm) long. The inflorescence, which is remarkably showy, is a terminal spike of small magenta or purple flowers. The spike can be 18 in (45 cm) long, and each star shaped flower about 3/4 in (2 cm) across. A large stand of purple loosestrife in full bloom is almost breath-taking in its beauty. There are numerous cultivars in the trade, selected for differing mature sizes and flower colors.
Native to Europe and temperate Asia, Lythrum salicaria has escaped cultivation and established itself over much of North America, Australia, New Zealand, and other parts of the world wherever it can tolerate the climate. In the United States, purple loosestrife now occurs in every state except Florida, Hawaii and Alaska. It grows in all manner of wetlands, even brackish marshes and tidal wetlands. Purple loosestrife was intentionally imported into the U.S. as an ornamental. Today, state and federal agencies and conservation organizations try to control its spread.
Culture Light: Purple loosestrife thrives in full sun and survives in partial shade. Moisture: Purple loosestrife is a wetland plant that tolerates poorly drained sites and does not like to dry out. Hardiness: USDA Zones 3-9. Propagation: Don't. The tiny seeds germinate easily. The roots can be divided any time. Cultivars are propagated vegetatively.
A single purple loosestrife plant in the flower bed can beget huge populations in nearby wetlands and other open areas (Jack observed it invading a steep sunny hillside in Northern Kentucky)
Purple loosestrife is highly invasive. It has an extraordinary ability to colonize disturbed and even natural wetland sites, take over them, and out-compete and eventually replace native wetland species. Purple loosestrife often replaces native species that are important to ducks and other wildlife. A single plant can produce 3 million or more seeds in a season. The seeds are tiny and are carried on the wind. In addition, purple loosestrife reproduces vegetatively by expanding its root system and sending up new shoots. Purple loosestrife should not be cultivated outside of its native range in temperate Eurasia.
Herbicides and hand picking have been used to control infestations of purple loosestrife, but recently biological control has been employed with some success. Three species of Eurasian beetles that are known to eat various parts of purple loosestrife have been released in North American wetlands dominated by the invasive loosestrife, and these beetles have significantly reduced the loosestrife infestations. Several other insect species also are being studied for possible use in biological control.
The flowers are beautiful, of course, and that is why the plant was brought to North America in the first place. If it isn't illegal in your state, you can sure make a beautiful bouquet of the cut flower spikes from wild plants you find growing in a local marsh. Just don't let the mature seeds get away.
You may see "guaranteed sterile" cultivars for sale in the horticultural trade, but the National Park Service reports on their alien plant web site that these cultivars have proven in fact to be highly fertile and to spread rapidly if released into the environment.
Purple loosestrife is considered a noxious weed and its sale, possession and cultivation are banned by many (most?) U.S. states. If you really must have this prolific invader, check your own state rules first.