Purple Loosestrife
Lythrum salicaria
Loosestrife family (Lythraceae)

Description: This perennial plant is 2-5' tall, branching frequently below the inflorescence. The stems are variably hairy, becoming woody and glabrous below. The leaves are usually opposite, less often whorled in 3's; some of the upper leaves in the inflorescence may be alternate. These leaves are up to 4" long and " across, becoming smaller as they ascend the stems. They are lanceolate, smooth along the margins, slightly hairy (especially the upper leaves), and clasp the stems. The upper stems terminate in long spikes of flowers about –2' long.

Each flower is about –1" across, consisting of 6 purple petals, a green tubular calyx, 6 or more stamens, and a pistil with a stigma that is green and knobby. Each wrinkly petal has a dark purple line toward its base. The hairy calyx has 5 teeth at its apex and several veins along its length. Sometimes the flowers have fewer than 6 petals, and the relative length of their pistils and stamens is variable (in this regard, there are 3 different forms of flowers). The flowers are sessile against the flowering stalks, or they have very short pedicels. The blooming period occurs from mid-summer to early fall, and lasts about 2 months. Each flower is replaced by a small seed capsule that is surrounded by the tubular calyx. This capsule contains many tiny seeds that can float on water or be blown about by the wind. The root system is shallow and fibrous, frequently forming offsets by rhizomes. This plant often forms colonies, and can spread by its seeds, rhizomes, or segments of the roots and stems.

Cultivation: The preference is full or partial sun, wet to moist conditions, and a mucky soil containing organic matter. Ordinary garden soil containing loam is satisfactory as long as there is adequate moisture. This plant can spread aggressively in some situations and can be difficult to destroy.

Range & Habitat: Purple Loosestrife occurs occasionally in NE Illinois and scattered counties elsewhere (see Distribution Map). It was introduced into the United States from Europe as a horticultural plant because of the showy flowers. Habitats include fens, marshes, borders of ponds and rivers, and ditches. This is species is still grown in flower gardens, using hybrids that are supposedly sterile. However, research has revealed that many of these hybrids can form viable seeds when wild forms of Purple Loosestrife are present in a given locality as a result of cross-pollination between these two groups of plants. Purple Loosestrife often escapes from cultivation and invades wetlands, sometimes forming dense stands that exclude other plants. This plant has become a major problem in Wisconsin and some of the northeastern states.

Faunal Associations: The flowers attract long-tongued bees and butterflies, including Bombus spp. (Bumblebees) and the butterfly Pieris rapae (Cabbage White). The seeds are too small to be of any interest to birds, and it is unclear to what extent mammalian herbivores feed on the foliage. There have been attempts recently to release leaf beetles from Europe as a biocontrol measure. This species probably provides cover to some wetland species of birds because of its tall dense vegetation.

Photographic Location: Along a large drainage ditch near Kaufman Lake Park in Champaign, Illinois. This colony of plants has been destroyed.

Comments: The only other species that is similar to Purple Loosestrife is the native Lythrum alatum (Winged Loosestrife), which also occurs in wetlands. This latter species is a smaller and less aggressive plant with winged stems, while the stems of Purple Loosestrife are usually round (sometimes 4-angled). Their flowers are similar in appearance, although Winged Loosestrife has smaller flowers (" across or less).