Purple Loosestrife: An Insidious Invader in the United States Ecology and Management of Invasive Plants NRES 641 By Brian R. Wright

In Blossey (1995), purple Loosestrife (Lythrum salicaria) is a: – Invasive emergent wetland perennial plant that has been introduced worldwide. – Plant that is native to Europe and Asia – Plant also has been introduced into Australia, New Zealand, Peru, Chile, and Canada – Plant that has been introduced into the lower 48 states, except Florida, in the United States – Plant that has been designated as a noxious weed in at least 19 states

In Anderson (1995), purple loosestrife is a plant that has infested approximately 300,000 acres of wetlands, and approximately 100,000 acres of riparian pasture causing an estimated $45 million dollars in damage in the United States each year The success of this plant species in the United States is due mainly by the lack of North American insect predators Further, continued expansion of the range of this species coincides with increased development and use of road systems, commercial distribution for horticultural purposes, and regional propagation of seeds for bee forage Purple loosestrife was introduced in the United States in the early 1800’s

Purple loosestrife was a plant that was brought to the United States by settlers for their flower gardens, and seeds were present in the ballast holds of European ships that used soil to weigh down ships for stability on the ocean (University of Minnesota 2006). Purple loosestrife was also used as a treatment for diarrhea, dysentery, bleeding, wounds, ulcers, and sores (Blossey et. al 2001). This species is an aggressive weed that rapidly displaces wildlife-supporting indigenous plant species in wetland communities while offering no value to many animal species

Purple loosestrife is in the family Lythraceae This species is rhizomatous and can invade wetland habitats by roots, root fragments, and stems as well as seeds (Lym 2004). Leaves of purple loosestrife are classifed as simple and are small linear to lancelate. Further, the leaves are from 0.75 to 4 inches long, 0.2 to 0.5 inches wide and can be opposite or whorled (Lym 2004). Leaves attach directly to the stems at their bases and are oblong with heart-shaped bases (University of Maine Cooperative Extension 2008). Leaves are usually arranged opposite each other in pairs which alternate down the stalk at 90 degree angles (University of Minnesota 2006).

According to the University of Maine Cooperative Extension (2008), the most identifiable characteristic of purple loosestrife is the rose to purple to white colored flowers. Flowers have 5 to 7 pink purple petals surrounding small, yellow centers and are clustered on spike-like stems. The size of the spikes can range from a few inches to 3 feet (Lym 2004). The stems of this species are square and can be smooth to somewhat fuzzy. Flowers open in July and continue to bloom through September or October. These plants can reach a height of 2 meters, and one mature plant can produce more than 2.5 million seeds annually (Blossey 2002; Wisconsin Department of Natural Resources 2004).

Seeds of purple loosestrife can remain viable for more than 20 years and seed survival can range from 60 to 70% resulting in an extensive seed bank (Wisconsin Department of Natural Resources 2004). Under natural conditions, seedling density can approach 10,000 to 20,000 plants/meter squared. The fruit of this plant is composed of a dry capsule generally containing 100 or more tiny, dark-colored seeds (University of Maine Cooperative Extension 2008). These seeds are one millimeter or less in size (Lym 2004). This plant species has a characteristically large taproot

Purple loosestrife seeds are dispersed by water and in mud adhered to aquatic wildlife, livestock, and people (Blossey 2002). This species can be very opportunistic in areas where recent soil disturbances have occurred. This plant species are commonly are found growing in man-made storm-water retention ponds, ditches, next to parking lots, and roads (University of Maine Cooperative Extension 2008). Further, this invasive wetland plant’s seeds fall near the parent plant and are transported long distances by water, animals, boats, and humans (Wisconsin Department of natural Resources 2004).

Additionally, this species follows a pattern of establishing populations on disturbed sites, maintaining low population numbers, and then dramatically increasing populations when conditions are optimal (University of Maine Cooperative Extension 2008). In Malecki 1993, monotypic purple loosestrife stands reduce biodiversity of wetland systems by replacing native plant species, thereby eliminating the natural foods and cover that affect many wetland wildlife species. Purple loosestrife plants need soil temperatures above 20 degrees centigrade for successful germination (Blossey 2002).

Control measures are broken down into four categories, including prevention, mechanical, chemical, and biological. These measures are used to combat purple loosestrife infestations (Blossey 2002; Lym 2004; University of Minnesota 2006). Prevention of infestations of purple loosestrife is by far the best and most effective control measure available to land managers, government agencies, state agencies, and the general public (Wisconsin Department of Natural Resources 2004). Mechanical control measures include: cutting, pulling, digging, mowing, and burning (Wisconsin Department of Natural Resources 2004).

The best time to cut purple loosestrife down is before the plant flowers. Cutting too early encourages more flower stems to grow, and cutting too late may mean that seeds may have already fallen (Wisconsin Department of Natural Resources 2004). Pulling and digging can be effective in controlling purple loosestrife infestations, but this measure may create disturbed bare spots that may allow for re-growth, or infestations by a different species of invasive plant. Pulling and digging are recommended for small populations (3 acres or less), low density situations, and for isolated stems.

Pulling and digging is recommended for young plants (up to 2 years old) or in loose soils. Mowing has not been shown to be effective in controlling purple loosestrife infestations due to the fact that this species can reproduce vegetatively. Further, mowing needs to be combined with flooding in order to be effective against purple loosestrife. Mowing and flooding are not recommended because this method could contribute to dispersal of the species by disseminating seeds and stems (Wisconsin Department of Natural Resources 2004). Burning is also ineffective in controlling purple loosestrife infestations.

Chemical control methods can be effective and are perhaps the best way to eliminate purple loosestrife infestations; however, chemical control methods have limited utility when large areas are to be treated due to: the high cost of herbicides, the potential for environmental contamination, and the potential impact on aquatic organisms. Currently, glyphosate herbicides (Round-up, Rodeo, and Glyfos) are the most commonly used chemicals to kill purple loosestrife. Glyphosate must be applied in late July or August prior to flowering and seed drop in order to be most effective. Triclopyr (Garlon 3A) is another herbicide used to control purple loosestrife, is formulated to be used around water, and does not kill grasses and sedges. The herbicide 2,4-D has also been shown to be effective against infestations of purple loosestrife.

Multi-year chemical treatments are required for residual control of seedlings and vegetative re-growth. Biological control methods are another effective treatment to combat purple loosestrife infestations. Three biocontrol species have been released in 25 states and 7 Canadian provinces. These species include a root-boring weevil (Hylobius transversovittatus) and two leaf-feeding beetles (Galerucella pusilla) and Galerucella calmariensis) (Blossey 1995; Lym 2004). Adult and larvae of the Galerucella insect species feed on leaves and flowers (Lym 2004). These insects over-winter as adults and lay eggs in June (Lym 2004).