Identification

Purple loosestrife is a stout, erect, perennial herb with up to 50 stems growing from a well developed root system. Populations are variable, and stems can be four- to eight-sided, glabrous (hairless) on the lower stems, or pubescent on the upper stems and inflorescence. The tough stems normally grow to about six feet (2 m) tall but can reach heights up to 10 feet (3.5 m). The leaves are lanceolate, similar in shape to willow (Salix spp.) leaves, from which the species name is derived (salicaria, see Figure 2). The leaves are one to four inches (2.5 to 10.2 cm) long, with entire margins. Like the stems, the leaves can be glabrous or pubescent. The lower stem leaves are heart-shaped (cordate) at the base, and they are attached directly to the stem (sessile) in an opposite or whorled arrangement (lower leaves). The flowers have a tubular calyx with five to seven lobes (sepals) and linear ridges between the lobes. There are five to seven separate petals attached to the top of a hypanthium (see the flower cross section in Figure 2).

Figure 2. The leaves, inflorescence, and flower of purple loosestrife.

Ten to 14 stamens of varying length are fused to the petals (two per petal). The petals are typically reddish-purple, but can be white, pink, purple, or red (the name Lythrum is from lythron, the Greek word for blood). The ovary has one style that can be short, medium, or long; the length of the style being different from the lengths of the stamen filaments. The spatial separation of stigmas and anthers, studied by Darwin in 1877, encourages self incompatibility and cross pollination. The flowers range from one-third to one-half inch (10 to 13 mm) long and are more or less densely arranged in numerous whorl-like cymes on a terminal spike (see Figure 2). The fruit is a brown capsule 1.25 to 1.9 inches (32 to 48 mm) long. The seeds are minute, light weight (<0.06 mg each), flat, angular, light tan, and thin-walled. There are many seeds per capsule and each spike can produce up to 120,000 seeds.

Purple loosestrife can be distinguished from the native winged loosestrife (Lythrum alatum, reported from Carbon County in eastern Montana), by the size of the plant, leaves, and flowers. Winged loosestrife plants grow from 16 to 40 inches (0.4 to 1.0 m) tall whereas purple loosestrife plants generally are taller. Leaves of winged loosestrife are 0.79 to 2 inches (2 to 5 cm) long and not cordate-based, and the petals are about 0.2 inches (5 mm) long compared to the 0.3 inch (7-10 mm) purple loosestrife petals. Winged lythrum has only one flower in the axils of opposite leaves, whereas purple loosestrife has pairs or clusters of flowers in axils of opposite leaves.

Purple loosestrife hybridizes with European wand lythrum (Lythrum virgatum), another ornamental that can be invasive. European wand lythrum is smaller, has smaller flowers and narrower leaves than purple loosestrife and the leaves have acute rather than cordate bases. Purple loosestrife is sometimes confused with fireweed (Epilobium angustifolium). Epilobium leaves are alternate, not cordate-based, the flowers lack a hypanthium (or it is not exerted beyond the ovary), and the flowers have four petals and eight stamens.

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Last Modified: 08/12/2008