2. Lythrum salicaria L. (purple loosestrife, spiked loosestrife)

Pl. 449 a–c; Map 2037

Plants moderately to densely pubescent with short, stiff, relatively broad-based, tapered, unbranched hairs, sometimes roughened to the touch, the lower portions of the stem sometimes becoming nearly glabrous with age, usually producing offsets. Stems 40–250(–300) cm long, erect with ascending branches, wandlike, usually more or less ridged, but not winged, lacking auricles at the nodes. Leaves all opposite, sessile or very short-petiolate. Leaf blades 1–10 cm long, narrowly to broadly lanceolate or oblong-lanceolate or triangular-lanceolate, the upper leaves sometimes nearly linear, truncate to shallowly cordate or rarely broadly rounded at the base, mostly sharply pointed at the tip. Inflorescences appearing as elongate, bracteose spikes or panicles with spicate branches, the leaves reduced fairly abruptly into the leaflike bracts, with a pair of small clusters of usually 3 flowers each at the nodes, the individual flowers sessile or with a minute stalk to 1 mm, usually subtended by a pair of inconspicuous, linear bractlets, these shed early. Petals 7–12 mm long, reddish pink to pinkish purple or reddish purple. Stamens usually 12, the anthers yellow. Pistils lacking a nectary disc. Seeds 0.6–1.2 mm long. 2n=60 (2n=30, 50, 58 elsewhere). June–September.

Introduced, scattered widely in the state, most abundant in counties bordering the Missouri River (native of Europe, Asia; introduced nearly throughout the U.S.; also Canada, Africa, Australia). Banks of streams and rivers, margins of ponds and lakes, oxbows, sloughs, marshes, fens, and bottomland forests; also ditches, roadsides, and wet disturbed areas.

Purple loosestrife remains a controversial plant in Missouri. On the one hand, it has long been cultivated as an ornamental in gardens, prized for its showy inflorescences, long blooming period, and easy care. It also is a favorite plant among bee keepers, as the abundant pollen produces a superior honey. On the other hand, in 1989 the Missouri legislature officially declared purple loosestrife a noxious weed, and by law it is illegal (class A misdemeanor) to import, buy, sell, or plant seeds or plants of Lythrum salicaria or any of its hybrids and cultivars. In spite of this, it is still relatively easy to locate purple loosestrife for sale in plant nurseries in Missouri, in part because of a lack of effective means to enforce state noxious weed laws, inadequate public education on the environmental harm that the species can cause, and the sentimental attachment of many gardeners to heirloom plants. Attempts to develop sterile cultivars have failed thus far because such plants have proven to be only self-sterile and to produce copious seeds when crossed with other strains that may be growing nearby (Ottenbreit and Staniforth, 1994). Additionally, claims by some nurseries that certain cultivars represent the related, but supposedly less invasive Eurasian species, L. virgatum L., have merely confused the issue. Lythrum virgatum, which is easily distinguished from L. salicaria by its glabrous stems and foliage, as well as its less robust and less showy habit (Haining et al., 2007), apparently is rarely, if ever, sold in North America (Harper, 1986). Lythrum virgatum is represented as an escape only by a few historical collections from eastern Massachusetts.

Stuckey (1980) detailed the introduction and spread of Lythrum salicaria in North America. Originally brought into the United States as a garden ornamental from Europe, the plant was documented as an escape from cultivation in New England by the early 1800s. By the mid-1800s, it was invading wetlands in the northeastern States, and it has continued to spread and gain in population density since that time. As an invader of wetland communities, purple loosestrife crowds out other plant species, sometimes eventually developing into large stands that are nearly monocultures. The damage to these wetlands also includes secondary impacts to native pollinators that require particular native plant species and the reduction in food, nesting habitat, and cover for other animals. The effects of purple loosestrife on overall plant species diversity is still a controversial issue, with some researchers claiming that at least at the early stages of infestation, the presence of purple loosestrife in some wetlands can be correlated with a slight increase in the number of observed native plant species (Hager and Vinebrooke, 2004). Such short-term gains appear to be reversed dramatically as the infestation of Lythrum continues to gain in density and coverage.

Currently more than 30 states have legislated against L. salicaria in recognition of its detrimental effects on the environment. Attempts to control the species have relied most heavily on herbicides, but regrowth from surviving stem and root fragments, as well as from a long-lived seed bank in the soil, ensures that infestations must be controlled repeatedly over a span of many years. Various biological controls also have been developed, involving mainly an Asian root-mining beetle and a series of beetles that feed on leaves and flowers (Malecki et al., 1993), and these have proven effective in at least controlling populations in portions of the Great Lakes region and perhaps elsewhere. Concerns about the release of these biological agents into the environment center mainly on the potential for the insects also to attack native members of the Lythraceae, especially native American species of Lythrum and the genus Decodon.

In Missouri, the species originally was reported as an escape by L. P. Jensen (1932), based on a population that had become established at what is now the Shaw Nature Reserve (Franklin County). Palmer and Steyermark (1935) still recorded it only from that report. By the 1960s, Steyermark (1963) knew the species only from one additional population in Newton County. The extensive flooding that occurred in the floodplains of the big rivers in 1993 had a hand in increasing the number of populations, with the result that L. salicaria undoubtedly occurs in every county bordering the Missouri and Mississippi Rivers in Missouri. By 2002, the Missouri Department of Conservation’s eradication program for this species was battling this species at more than 130 sites in 38 counties (personal communication from Preston Stogsdill, then coordinator of MDC’s purple loosestrife control program).

At its original Missouri occurrence in Franklin County, the species first became established in 1930 from seeds contaminating a shipment of peat imported from New York (L. P. Jensen, 1932). Plants were first noted in 1930 beside a greenhouse where the peat had been dumped for storage. Following seasonal rains, seeds produced by these plants were washed down a small gully, resulting in its establishment along the shores of the Pinetum Lake. This population soon grew to form a continuous band along the lake margin, beautiful to behold when in flower but at the expense of most of the other emergent aquatics that had previously grown there. Efforts to rehabilitate the lake margin began in earnest in the mid-1980s with a combination of manual removal of Lythrum plants and annual herbicide applications. After more than three decades of effort, although the species has been controlled effectively, during most years a small number of individuals continue to appear that could result in a reinfestation if not dealt with.

From the preceding, it should be apparent that purple loosestrife represents a tangible and worsening environmental problem in the state. Gardeners and landowners are strongly urged to take steps to attempt to eradicate this species, both where it is cultivated and where it has escaped into nature.