Public Works and Environmental Services

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Joni Calmbacher
Director, Stormwater Planning

Trapa bispinosa Early Detection and Eradication

Since 2000, the aquatic invasive species known as Trapa bispinosa (two-horned water chestnut or two horned trapa) has spread to more than 60 known locations in Fairfax County. The county is raising awareness and asking for help detecting, reporting, and eradicating the invasive plant.

The plant is found in rivers, tidal bays, and ponds throughout Fairfax County and is distinguishable by its triangular-toothed leaves. Typically, infested waterways suffer from degraded water quality and detrimental impacts to surrounding habitats. Spring is the peak season in which seedlings sprout and it rapidly spreads within a water body. The plant typically emerges from seeds between April and May and once summer comes, infestations spread across the water body rapidly and become more difficult to eradicate. It will flower, fruit and the leaves become dense and visible on the water surface from June to September. All the leaves die back with a hard frost in December, leaving just the viable seed in the bottom sediment. The previous year’s black, two-spined seed hulls (1 inch wide) can be found adrift along the shoreline any time of year.

Trapa Identification Image

Coordination among interested parties requires communication and we hope this site provides a platform where you can report a possible sighting (USGS - Sighting Reporting Form), ask questions and seek advice (Northern Virginia Soil and Water Conservation District), request control help from local environmental groups, and share experiences among social media groups. If you think you have trapa in your pond, we strongly urge you to follow these steps:

Detect the issue early. This means becoming familiar with the plants. Fortunately, these species lend themselves quite readily to identification with just the leaves, which typically emerge in April or May. A couple helpful identifiers are location in freshwater ponds or large waterways and having floating rosettes of roughly triangular-shaped leaves with a saw-tooth edge. Additionally, T. bispinosa typically exhibits a pink hue to the bottom of its leaves.

Click photo to enlarge

Photo - Trapa - ID1


Photo: trapa - ID2 - small


Report any potential populations as soon as possible. This is a regional issue, and we are working to track locations of trapa observations to better allocate resources toward a coordinated response. To report sightings of trapa, please click on this link USGS-NAS-Reporting or navigate to the USGS Non-indigenous Aquatic Species Reporting address referenced below to access the online reporting form.

Confirm any potential identifications of trapa populations because we want to immediately avoid any collateral damage to non-target native species. If you submit a report to the USGS-NAS they will be able to confirm the identification. Early detection and rapid response are making a significant positive impact in the management, removal, and possible eradication of this non-native, nuisance plant.

Assess the population size, distribution, and local environmental conditions to determine the best practice for getting trapa out of your pond. There are several options for treatment that depend on this context. Please feel free to check with technical experts at the NVSWCD with general questions regarding assessment of your trapa population or if you would like assistance with treatment options.

Photo: Trapa - spreadAllowing T. bispinosa to proliferate in your pond will create an abundant seed bank, which can in turn lead to this plant spreading to other water bodies. These seeds may transport by streams, creeks and flooding events that connect water bodies; but the most common vector is seed transport by waterfowl. The ‘horns’ of the seed pod become lodged for travel on a bird, leading to an unpredictable spread of this plant. If you have T. bispinosa in your pond, the problem may not be as isolated as you’d reasonably expect. If you have waterfowl visit your water body, you may one day have T. bispinosa.

Photo - You can make a difference

Treat trapa with a plan for eradication. Given the annual life cycle and growth habit, small populations of trapa may be easily treated at low cost by hand pulling or other means of mechanical removal. Removing very small populations safely may only require one or two people, a few hours, and possibly some hand tools and a small boat. The key to this approach is timing. Remove it before it can produce seeds that feed the seedbank and propagate subsequent generations. If there are no other competing difficulties, ideally removal should be timed around flower set, as the plant is pouring its energy upward. Too early in the season, and you could miss some plants that have yet to emerge or any remaining cut plant fragments could have enough growing season to produce another cohort. Too late in the season, and you run the risk of seed production. Pulling is best, but if can treat it during this goldilocks window in the season, you probably only need to target the upper floating leaf/flower parts. For best results, harvested plants should be carefully removed from the water body to limit loose plant propagules and keep oxygen levels at a more natural or normalized level.

For larger populations, removal can be especially difficult with aquatic plants. Northern Virginia Soil and Water Conservation District (NVSWCD) has offered to provide advice for management that will be specific to your situation. You may want to consider enlisting your community environmental organizations or a volunteer group to donate their services. Please see the links below for NVSWCD and volunteer organizations that may be interested in helping with large removal efforts. As this program grows, we hope others will be interested in helping. Otherwise, you may want to contact a professional for consultation and assistance with alternative treatment options.

For all these options, monitoring the water for recurrence or changes in quality is paramount to long-term success. Keep an eye on your pond and enjoy the fruits of your labor.

This is not a requirement, but an opportunity for environmental stewardship and connection with your community, helping to keep your neighborhood pond and common water resources healthy.

Fairfax Virtual Assistant