Identification

illustration

Purple Loosestrife
Lythrum salicaria L.
Lythraceae (the Loosestrife Family)

General appearance: A showy, stout, erect perennial. Mature plants often form multistemmed clumps to 1.5 m (5 ft) or more tall.

Stems: 4-sided, often becoming woody towards the base. Older plants form a large woody crown from which the stems grow each year. The old, dead stalks often persist through the winter and into the next spring.

Leaves: Lance-shaped, simple (not lobed) and entire (not toothed), sessile (without a petiole), and 3-10 cm (1.2-3.9 in) long. Leaves are opposite or occasionally ternate (in 3s) and decussate (with consecutive leaf pairs at 90 degrees to each other) on the stem. The lower leaves are glabrous (hairless) or nearly so, become progressively smaller and more pubescent upward on the stems. The larger, lower leaves are often slightly clasping or heart-shaped at the base.

Flowers: Pinkish-purple, with 5-7 sepals and 5-7 petals (usually 6 of each) and usually 12 stamens of varying lengths. Flowers are arranged in dense spikes 1-4 dm (4-16 in) long. Plants flower from mid July through August and into September.

The flower parts of purple loosestrife and other members of the loosestrife family (which are dicots, or so-called "broad-leaf" plants) usually appear in multiples of sixes. While flowers having parts in multiples of 3 is the rule among monocots (grasses, sedges, lilies, irises, etc.), it is relatively unusual among the dicots. Thus purple loosestrife and its relatives can often be told from look-alikes simply on the basis of numbers of flower parts (most look-alikes will have flower parts in multiples of 4 or 5.)

Seeds: Large numbers of tiny, nearly black seeds ripen in fall. The seeds can live for as long as 10 years or more in the soil, allowing the build-up of a huge "seed bank".

Habitat: Moist to wet habitats including wetlands, shorelines, riverbanks and sedge meadows, as well as roadside ditches and borders of agricultural fields. Does best in full sun but can tolerate partial shade. Its long-lived seed bank allows it to quickly overtake wet areas after disturbance.

Sources: Voss (1985), Gleason and Cronquist (1991).

Updated February 2006.


Visitor since March 17, 2006