Invasive Water Chestnut

 

An invasive species of plant has arrived in our Potomac River Watershed. It was first observed by John Odenkirk a biologist with the Virginia Department of Wildlife Resources who discovered the beginnings of the current outbreak in Pohick Bay in 2014. With the help of Dr. Nancy Rybicki formerly of the U.S. Geological Survey and now teaching at George Mason University, the plant was identified as a native of East Asia . This is not the same type of Water Chestnut (Eurasian) that blanketed much of the Potomac in the 1950s. This is a new species of water chestnut. The new plant has leaves with the same serrated pattern, but the underside of this new species is red. This species has pink flowers and the seed pods which form this time of year have two hooks on them.

Once water chestnut shows up in a water body, it spreads to cover large areas chocking out all other life. If allowed to flower and reproduce, can spread far and wide carried by Canadian geese, other wild life and the flow of the rivers and streams. This plant forms dense floating mats that cover the water surface, blocking sunlight and killing aquatic grasses and vegetation. Dr. Nancy Rybicki as been trying to alert owners of ponds and other water bodies that have been impacted encouraging them to take action before the Water Chestnut blankets the region.

Across Northern Virginia, this invasive water chestnut has spread to 81 known locations. Dr. Rybicki believes it can still be stopped before it takes hold, but we need to act before it spreads to the Potomac River. The Virginia Noxious Weed Advisory Committee nominated the water chestnut to be designated as a Tier 2 weed in 2019 and it remained mired in the bureaucracy during the pandemic. Since that time it spread from a handful to 54 locations in 2020 to 81 this year. The Attorney General’s Office is reviewing the proposal. 

It is spreading in stormwater ponds, farm ponds, golf water hazards, and ponds in parks. Once it flowers with a pink flower it produces its 2 horned fruit with barbs that attach to wildlife, but are sharp enough to puncture a shoe. The water chestnut is an aquatic annual herb. It sprouts from seeds in spring and the plants die off in winter after a hard frost, but the seeds can lie dormant for several years and be washed from a spill way down river.

If this new water chestnut is allowed to establish itself in ponds around Virginia it could spread into the tidal waters, and we will face an epic control challenge that would both undo past decades of successful eradication of invasive species and undermine all our efforts in restoration of the Chesapeake Bay. Once established, a colony can smother an entire pond or lake. The dense mats can block the passage of oxygen in the atmosphere to the water below and create oxygen-starved expanses where aquatic life is wiped out. The plant’s long tendrils also impede boat navigation.

Northern Virginia Soil and Water Conservation District recently obtained about $300,000 from Fairfax County’s Environmental Improvement Program to fund a staff position to oversee eradication in at least 30 ponds countywide. That amount also covers the cost of contractors to do the work to hand remove the Water Chestnut.

If added to the Noxious Weed list, the Virginia Department of Agriculture could use its own resources to suppress populations or reduce its spread. The classification also would prohibit the movement and sale of those plants into or within the state without a permit.