Purple loosestrife; Spiked loosestrife; Salicaire; Bouquet violet; Rainbow weed

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental

Purple loosestrife adapts readily to natural and disturbed wetlands. As it establishes and expands, it outcompetes and replaces native grasses, sedges, and other flowering plants that provide a higher quality source of cover, food, or nesting sites for native wetland animals (U.S.EPA 2008). The highly invasive nature of purple loosestrife allows it to form dense, homogeneous stands that restrict native wetland plant species, including some federally endangered orchids, and reduce habitat for waterfowl.

Specialized marsh birds avoid nesting and foraging in purple loosestrife (Blossey et al. 2001). The federally endangered bog turtle (Clemmys muhlenbergi Schoepff) loses basking and breeding sites to encroachment of purple loosestrife (Malecki et al. 1993). While some avian fauna, such as the swamp sparrow (Melospiza georgiana), have successfully utilized purple-loosestrife dominated habitat around Lake Huron, overall avian diversity in these sites is much lower compared to other wetland habitats (Whitt et al. 1999). Additionally, species richness in wetland moth populations has been shown to be negatively correlated with purple loosestrife cover (Schooler et al. 2009).

Numerous studies demonstrate the aggressive and competitive nature of purple loosestrife. Fernald (1940) reported a loss of native plant diversity in the St. Lawrence River floodplain following the invasion of purple loosestrife and the exotic flowering rush, Butomus umbellatus. Purple loosestrife was determined to be the most common exotic species of the St. Lawrence River wetlands, appearing at more survey stations than any nonindigenous plant (although not at the highest densities) (Lui et al. 2005). However, L. salicaria appeared to have a lesser effect on plant diversity at colonized sites relative to grass exotics, reed canary grass (Phalaris arundinacea) and common read (Phragmites australis) (Lui et al. 2005).  Surveys of coastal wetlands on the Great Lakes found L. salicaria to be the one of the most common emergent exotic plants across the Lakes and indicated that L. salicaria presence was associated with a significant reduction in species richness (Trebitz and Taylor 2007). Gaudet and Keddy (1988) report declining biomass for 44 native wetland species in a laboratory setting with the establishment of L. salicaria.  Among twenty tested wetland plants, (Keddy et al. 1994) found purple loosestrife to be among the most competitive, causing an average yield reduction of 60% in its neighbors across different habitats. Keddy et al. (1998) also found that competition with L. salicaria could reduced the above-ground biomass of adjacent target species by over 80%, more than 6 other tested competitors, including T. augustifolia. However, despite growth reduction, target species survival was also highest in L. salicaria pots (Keddy et al. 1998). In the Hamilton Marshes adjacent to the Delaware River, annual above-ground production of L. salicaria far exceeded all other plant species’ production combined.

Purple loosestrife causes annual wetland losses of about 190,000 hectares in the United States (Thompson et al. 1987; Mal et al. 1997). At the Effigy Mounds National Monument (EFMO), combined populations of purple loosestrife cover an area of 5 to 10 hectares growing in regularly disturbed sites.  This species has a major visual impact on the vegetation of EFMO, and it has the potential to invade and replace native communities endangering the areas' primary resources (Butterfield et al. 1996).   

The invasion of L. salicaria alters biogeochemical and hydrological processes in wetlands. Areas dominated by purple loosestrife (Fig. 2) show significantly lower porewater pools of phosphate in the summer compared to areas dominated by Typha latifolia L. (Templer et al. 1998).

Purple loosestrife leaves decompose quickly in the fall resulting in a nutrient flush, whereas leaves of native species decompose in the spring (Barlocher and Biddiscombe 1996; Emery and Perry 1996; Grout et al. 1997). This change in timing of nutrient release at a time of little primary production results in significant alterations of wetland function and could jeopardize detritivore consumer communities adapted to decomposition of plant tissues in spring (Grout et al. 1997).

Current research on the socio-economic impact of Lythrum salicaria in the Great Lakes is inadequate to support proper assessment.
Realized:

Since the 1980s, purple loosestrife has received an increasing amount of attention from the media nationally, almost always in a negative light (Lavoie, 2010). However, no quantitative studies are known to have measured the societal perception of purple loosestrife. Because of its ability form dense homogeneous stands and reduce waterfowl habitat, it is perceived as a species which is dominating and inhibitive to duck hunting.

Current research on the benefits of Lythrum salicaria in the Great Lakes is inadequate to support proper assessment.
Realized:

Although sale, purchase, and distribution of purple loosestrife is illegal in all Great Lakes states, a positive perception of purple loosestrife as a gardening/ornamental species butterflies is still represented in some media sources due to its beauty and ability to attract bees and other pollinators (Lavoie 2010).

Now regulated in all Great Lakes states. Lythrum salicaria is listed as an exotic weed in Illinois (525 ILCS 10/3, 10/4) making it illegal to buy, sell or distribute plants, its seeds, or any part without a permit.  Planting, sale, or other distribution without a permit is also prohibited in Indiana (312 IAC 14-24-12).  Purple loosestrife – including all cultivars – is a prohibited invasive species in Minnesota (MN Administrative Rules, 6216.0250 Prohibited). The species is restricted in Michigan, with an exemption for sterile cultivars (MI NREPA 451, Section 324.41301). Planting or sale of the species without a permit is prohibited in Ohio (O.R.C. ' 927.682), though the director may exempt varieties ‘demonstrated not to be a threat to the environment’.  Pennsylvania has designated all nonnative Lythrum species and their cultivars as noxious weeds (7 PA Code 110.1). Purple loosestrife is designated both as a restricted species (NR40.05: Restricted) and as an invasive aquatic plant (NR 109.07 (2)) in Wisconsin. 

Control

Biological
Biological control agents do not eliminate the target weed, but when successful, can sup- press weed populations to a nonsignificant level (Rees et al. 1996). Five species of beetles have been approved for the biocontrol of Lythrum salicaria (Blossey et al 1994ab). 
Galerucella calmariensis and G. pusilla are both leaf-feeding chrysomelids. These beetles defoliate and attack the terminal bud area, drastically reducing seed production. The mortality rate to purple loosestrife seedlings is high. Evidence of Galerucella ssp. damage are round holes in the leaves. Four to six eggs are laid on the stems, axils or leaf underside. The larvae feed constantly on the leaf underside, leaving only the thin cuticle layer on the top of the leaf.  Initial introductions in eastern North America occurred in Virginia, Maryland, Pennsylvania, New York, Minnesota, and southern Ontario in August, 1992 (Hight et al., 1995).  In 1992 these three beetles were released in Washington.  By 1996 populations of Galerucella ssp. visibly impacted purple loosestrife stands (Washington.gov 2012).  In the Great Lakes region, Sea Grant conducted an extensive, multi-state program involving youth in raising and releasing Galerucella beetles for control of purple loosestrife (Sea Grant 2001).

Hylobius transversovittatus is a root-mining weevil that also eats leaves. This beetle eats from the leaf margins, working inward. The female crawls to the lower 2-3 inches of the stem then bores a hole to the pithy area of the stem where 1 -3 eggs are laid daily from July to September. Or, the female will dig through the soil to the root, and lay eggs in the soil near the root. The larvae then work their way to the root. H. transversovittatus damage is done when xylem and phloem tissue are severed, and the carbohydrate reserves in the root are depleted. Plant size is greatly reduced because of these depleted energy reserves in the root. The larvae evidence is the zig-zag patterns in the root. 
Nanophyes marmoratus and N. brevis are seed eating beetles. Young adults feed on new leaves on shoot tips, later feeding on the flowers and closed flower buds. Sixty to one hundred eggs are laid in the immature flower bud. Seed production is reduced by 60%. There were two test sites releases in 1996.  Approval to introduce N. marmoratus was granted followed by introductions in New York and Minnesota in 1994. Additional releases occurred in New Jersey in 1996. N. marmoratus has also been released in Ohio (Ohio EPA 2001).  Release of N. brevis planned for 1994 was delayed due to contamination of the original shipment with a parasitic nematode (Piper, 1997).  This infection appeared benign for N. brevis, however, due to the potential for non-target effects of the nematode after introduction into North America, only disease free specimens should be introduced, which, at present, effectively precludes the introduction of N. brevis (Blossey 2002).

Bayeriola salicariae, a gall midge, was studied and screened between 1990 and 1992 (Blossey and Schroeder, 1992).  Based on results indicating a potential wider host range, the gall midge B. salicariae was not proposed for introduction (Blossey and Schroeder, 1995).

Targeted grazing by sheep has also been used as a biocontrol (Kleppel and LaBarge 2011).

Revegetation of disturbed riparian sites can be used to prevent purple loosestrife establishment and to reduce re-establishment after control procedures are applied.  Fowl mannagrass (Glyceria striata), foxtail sedge (Carex alopecoidea), and reed canarygrass (Phalaris arundinacea) have achieved dominance and prevented re-invasion in plots where purple loosestrife was experimentally removed. Smartweed (Polygonum lapathifolium) is reported to out-compete purple loosestrife during its first year of growth.  Seeding Japanese millet (Echinocloa frumentacea, also called billion-dollar grass) at 30 pounds/acre on exposed moist soil after drawdown and before purple loosestrife seedlings began to grow provided control. Japanese millet is considered an exceptional wildlife plant (Jacobs 2008).

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

Physical
Most mechanical and cultural attempts to control purple loosestrife are ineffective. A single known exception is cutting followed by flooding.

For small infestations and isolated plants, hand pulling may be effective.  Pull individual loosestrife plants by hand before seed is set . The entire root system must be removed, but do not dig out roots because soil disturbance may release seeds buried in the soil and break off plant parts, which then reproduce.  Instead, a cultivator may be used to tease roots from the soil.  All plant parts should be bagged to prevent dispersal or resprouting and preferably burned.  Follow-up treatments are recommended for at least 3 years.

Frequent cutting of the stems at ground level is effective but must be continued for several years. (Courtney 1997).  Mowing is generally not effective as it exposes the seed bank. 

Flooding is generally ineffective at controlling purple loosestrife, though some success has been reported for control of seedlings when using flooding regimes in excess 30cm for over 7 weeks (Balogh 1985). 

Prescribed burning is not an effective management tool for purple loosestrife.  The dead upright stems do not carry fire well and the fine fuels are often lacking. The growing points of the root crown are about 2 cm (0.8 inch) below the soil surface, so surface fires are not likely to inflict much damage. Purple loosestrife begins spring growth about a week or 10 days after broadleaved cattails, so a fire of sufficient intensity to damage purple loosestrife could also damage desirable native species (IL DNR 2007).

Chemical
Only herbicides permitted for wetland use may be used to control purple loosestrife.  There are four chemicals that can be used to manage purple loosestrife on sites with standing or moving water typical of where it invades. Triclopyr and glyphosate are used most commonly. However, 2,4-D, and imazapyr are also formulated for aquatic applications.  Widespread elimination of standing biomass may result in the exposure and sprouting of the immense purple loosestrife seed bank. Thus broadleaf-specific herbicides which do not harm monocot species (such as common wetland grasses and sedges) are preferred. The most species specific way to apply herbicide is by cutting and treating the stems. Foliar spray can be used by applying herbicide after the period of peak bloom, in late August. Any control method should be followed up on a yearly basis to catch any missed plants or new sprouts. (Ohio EPA 2001)

See also:

Midwest Invasive Plant Network - Invasive Plant Control Database

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.