Resources

SUCCESS STORIES: Water Chestnut in Maryland Maryland

Department of Natural Resources
Contact: Mike Naylor (410-260-8630)

Introduction:
Water chestnut was first observed in Maryland in the Bird River in Baltimore County and the Sassafras River in the 1950s and 1960s. After extensive mechanical and chemical control efforts, the plant did not reappear in either tributary until 1997. Since then, the Maryland Department of Natural Resources has used mechanical control to effectively limit the population size and spread of this non-native plant species. Current populations are less than 1% of their initial biomass.

Problem and Impact:
Water chestnut (Trapa natans) is an aquatic plant native to Asia. In the United States it is has grown and spread very aggressively in many rivers, ponds and lakes. One acre of water chestnut can produce enough seeds to cover 100 acres the following year. The seeds of water chestnut have four hard, half inch spines that are sharp enough to cause injury. In many areas, water chestnuts grow as dense monocultures that outcompete native submerged aquatic vegetation and prevent nearly all water uses. Dense beds can also create breeding grounds for mosquitoes and provide only marginal habitat for native fish and birds.

Project Description:
Because of the many negative attributes of water chestnut, the Maryland Department of Natural Resources (MD DNR) decided to attempt to eradicate the plant from each tributary. The Bird River water chestnut population had spread from approximately 50 plants in 1997 to over three acres in1998, and approximately 30 acres in 1999. The Sassafras River population was slightly larger. DNR coordinated a massive mechanical and volunteer harvesting effort on both rivers in 1999 that resulted in the removal of approximately 400,000 pounds of plants. Even though all visible plants were removed, follow up efforts were necessary because seeds of the plant can remain viable in sediments for up to 12 years.

In 2000, less than 1,000 pounds of plants were discovered and removed from both rivers. As expected, harvest efforts in 2000 substantially reduced the total number of plants. In 2001 and 2002, mechanical and manual harvesting continued to occur. Since then control efforts have been performed on both rivers by small teams of MDNR staff using personal watercraft and small boats. Pairs of DNR personnel survey all shoreline areas in the vicinity of the original populations and for several miles along the shoreline both upriver and downriver. In each year a few hundred plants were found in areas populated in previous years. In 2004, an additional population of plants was discovered in a small pond off the eastern shore of Woodland Creek. Approximately 600 pounds of the plant was harvested from the pond. In 2005, another pond across the same creek also contained several hundred pounds of water chestnut. In 2006 a small population was reported from Still Pond Creek, the next river system south of the Sassafras.

The combination of mechanical and hand removal of plants has been so successful that there has been no need to use herbicides. It is likely, however, that monitoring and control efforts in these two tributaries will have to occur indefinitely to keep the population in check and prevent spread into additional tributaries.

The initial costs for mechanical removal were approximately $20,000 per year. Annual control now takes approximately 100 total man/hours. Typically, this is done by teams of 2-4 people spending two days per river. It requires two Personal Watercraft and one small and light john boat with outboard. Costs for any area will be dependant upon personnel costs, the travel distances involved, and vessel wear-and-tear.

Lessons Learned:
It is absolutely critical that newly discovered populations of water chestnut be dealt with immediately. A delay of one or two months can result in costs that are exponentially higher or a situation where anything resembling eradication is impossible (e.g. Hudson River). As such, resource managers must have the equipment and the personnel available and enough flexibility in their time to do the work. Control must be initiated in the narrow window between when rosettes are on the water surface and when seeds are mature, about 6 weeks in the Chesapeake Bay. Control must also extend well beyond the plants found initially. Sometimes there may be an upstream source, a pond, tidal gut, or shallow bay where a small population thrives and releases seeds to the surrounding area. It will be necessary to visit every single foot of shoreline for at least a mile above and below the plants discovered, as well as any ponds nearby. Also, managers should plan on having to revisit control areas for a minimum of 3-5 years, possibly longer. DNR is still finding individual plants in areas that have been controlled annually for 7 years.