Last Friday, Sept. 5, I spent the morning with Dr. Nancy Rybicki, aquatic plant ecologist with the U.S. Geological Survey, and John Odenkirk, fisheries biologist with the Virginia Department of Game and Inland Fisheries, as part of an emergency harvest crew assembled to manually remove a recently discovered infestation of invasive water chestnut at the Pohick Bay park boat rental area in Gunston Cove just off the Potomac River.
This aggressive plant is a prolific reproducer: One acre of water chestnut can produce enough seeds to cover 100 acres the following year. On Aug. 21, the water chestnut (Trapa spp.) covered 1100 square meters (about 0.3 acres); by Sept. 5, the plant had expanded to almost 1500 square meters.
The last time this highly invasive plant was seen in the Potomac was in the 1920s. The water chestnut grew rapidly from a small patch, and by 1933, thick mats of it covered more than 10,000 acres of the Potomac River from Washington to near Quantico, Va. The U.S. Army Corps of Engineers mechanically harvested the plants through 1945 at a cost of nearly $3 million (in 1992 dollars). They continued hand harvesting until the mid-1960s. We’re hoping to prevent that from happening again.
In addition to its reputation as a prolific spreader, this particular invasive plant can cause several problems for the environment and recreational activities. The seedpods have hard half-inch-long spines, which are sharp enough to penetrate shoe soles and large enough to keep people off beaches. Large mats of the nuisance plant can make boating almost impossible. Additionally, as the water chestnut leaves fan out, they block sunlight from reaching native bay grasses, which provide critical habitat for native fish and bird and help to maintain and improve water quality
Odenkirk and Rybicki assembled a team of 30 volunteers from DGIF and USGS, George Mason University (GMU), Pohick Bay park staff, Virginia Master Naturalists, and local hunters, fishers, and boaters to assist in removing the plants and collecting scientific information about the plant. Participants waded through the water and removed 751 bushels of plants weighing over 3.6 tons. The discarded plants were placed into piles for onsite composting in the park.
Water chestnut seeds can remain viable in sediments for many years, which means this area will need to be followed closely for several years moving forward. Rybicki is collecting information on the ecology of this population of water chestnut to inform park managers, natural resource agencies, and others who need scientific information to address the threat.
I worked with Cindy Smith, Ph.D., a fellow researcher from George Mason University’s Potomac Environmental Research and Education Center, to collect plant samples. We determined that one seed can produce a single multi-branching plant with up to 25 rosettes, over 30 seed pods, and stems that are up to 3 m long. The plant, which is rooted in the sediment, has green, triangular leaves that are shiny and waxy above and coated with fine hairs below. Here is a short video we made:
Although it is unclear how the water chestnut arrived in Gunston Cove, it was likely transferred via boat, particularly since the mat was discovered near the boat launch. A plant fragment or seed may have been inadvertently transported on a boat that had previously come from waters with an existing population. In addition, the seeds adhere readily to many surfaces, including the legs of volunteers during the emergency harvest. Because of this “stickiness”, the seeds could also have been transported on the feathers of waterfowl. Another possibility being investigated is whether this is a new introduction from one of the countries where it is native, rather than the spread of water chestnut from another state.
Because of limited funding for invasive species monitoring and removal, Rybicki and Odenkirk are hoping to get the word out about this potentially large problem, so please share as widely as possible. I’ll provide regular updates here as well.