Purple loosestrife (Lythrum salicaria) the introduced, invasive pest plant of wetlands and other habitats, and known for its showy rose-purple flowers, is having a very good year in the wetlands and near-shore areas of Lake Wingra.
Purple loosestrife is a perennial wetland species, first introduced into the United States in the 1800’s as a medicinal and garden plant. Purple loosestrife readily invades new areas and spreads quickly once there. When established, purple loosestrife can dominate a wetland, often excluding some native wetland plants. This pest plant poses such a threat to wetlands that Wisconsin (and several other states) has made it illegal to sell, distribute, plant, of cultivate purple loosestrife or any of its many cultivars.
Recent History of Purple Loosestrife in the Wingra Watershed
A well-established population of purple loosestrife was discovered in the Edgewood Marsh (the 1 ha wetland below Edgewood College) in 1996, having apparently originated from seed transported on storm water from a neighboring garden, possibly a decade earlier. At the time, the distribution pattern was dense, with about 70% of the population found in a relatively small area (about 200 meters square ) near the north end of the marsh. There was a reduced density of purple loosestrife toward the fringes of the marsh. The population was spreading toward the shoreline, and there was imminent potential of spread throughout wetlands surrounding the lake. Isolated individual loosestrife plants of uncertain origin were also discovered in wetlands elsewhere on Edgewood property and in the UW-Madison Arboretum.
Early control efforts from 1996 through 1998 used cutting and application of a 50% solution of roundup to the cut stems. This effort resulted in temporarily reduced loosestrife densities. However, consistent resources were not available to adequately control the spreading population. After 1999 purple loosestrife beetles that ate parts of the plant, were released into the marsh with cooperation of the DNR purple loosestrife bio-control program. Beetle populations were established and their normal feeding on the plant resulted in damage to loosestrife plants in densest growth. However, given the large reserves of dormant purple loosestrife seeds in the soil and without a supplemental and systematic mechanical control program, the loosestrife population eventually rebounded and began to spread.
Near Term Management Prospects
Research has shown that generally within 4 to 5 years after first release, purple loosestrife beetles bring a dense loosestrife population down to the point where seed production is so low that it no longer poses a threat to nearby un-infested areas (Woods, personal communication, 2002.)
However, on the margins of infested areas, where the loosestrife population is too low and scattered to support a predatory beetle population, mechanical and cultural control methods are required. This situation is found on the lake and shore sides of the Edgewood Marsh. These scattered, and hard-to-find plants pose the greatest threat of escape and infestation of nearby areas. In this area systematic, routine, and careful scouting and monitoring by trained volunteers and land care managers is required. Mechanical, cultural and chemical controls will be dictated by the extent of the population and the time, energy and resources available to the land care managers.
Long Term Management Recommendations
So, faced with the management realities, restoration ecologists, concerned citizens, the Wisconsin Department of Natural Resources, and the Friends of Lake Wingra, decided in 2003 to supplement on-the-ground management work with a long-term management plan for purple loosestrife and other invasive species in the Lake Wingra Watershed (see the plan here: FOLW invasives plan.)
The Friends of Lake Wingra pest species management plan recommended values and management principles for consideration by watershed residents, businesses and municipalities. The plan outlined a comprehensive, strategic approach to invasive species management in Lake Wingra and its surrounding watershed. The plan recommended a range of management actions and provided land managers, neighborhood groups and citizens with the tools for developing appropriate tactics to encourage native species and discourage pest species. The plan recommended short-term, mid-term and long-term management actions.
Purple Loosestrife Management Recommendations
The 2003 FoLW Invasive Species Management Plan had this to say about dealing with purple loosestrife in the Wingra Watershed:
“It is recommended that Edgewood, in cooperation with the DNR purple loosestrife bio control program develop target purple loosestrife population levels, and a range of acceptable variation around these targets, for the marsh. Edgewood managers should continue to use predatory beetles in the main body of the densest loosestrife population.”
“Within this containment zone, managers may choose to remove the flowers but leave the plants to support the beetle populations. A rigorous monitoring program should be established to check the success of the bio control program and to inform management decisions. A systematic, rigorous surveillance program, perhaps using watershed volunteers should be enough to keep tabs on the fringe areas. Here again, a set of target population levels should be established; fluctuations around this target level will trigger management action. The exact management action will depend upon the time, energy and resources available to managers and to the degree of threat posed by the population.”
“Precautions should be taken to make sure that as little harm as possible is done to the marsh during the efforts to control purple loosestrife. Steps should be taken to minimize walking in the marsh, soil disturbance, dispersal of seeds and trampling of native vegetation during monitoring and control. Managers might consider such things as temporary, “floating” walkways or small, portable lookout platforms as ways to avoid or lessen impact on the marsh.”
Perhaps it is time to revisit the plan, and to check in with managers to see what steps they are taking to put the plan into action.