Two-Horned Trapa: An Invasive Water Chestnut in Maryland and Northern Virginia

Two-horned trapa (Trapa bispinosa) is a highly invasive water chestnut found in waters of Maryland and Northern Virginia. An annual aquatic plant, it was first reported in 2014 in Pohick Bay, on the freshwater tidal Potomac River near Lorton, Virginia.

Why it's a problem

• It smothers waterways. Trapa can form a monoculture that limits recreation, shades out native underwater plants, and can reduce how much oxygen passes from the air into the water.
• Its seed pods hitchhike. The sharp, barbed "horns" on each pod catch on waterfowl, nets, wooden boats and construction equipment, carrying the plant to new waters.
• It's a hazard on land, too. The pods can puncture bicycle tires and shoes.

As of 2024 there were 130 observations of the plant on iNaturalist in the United States, all in Maryland and Northern Virginia. It has been found in Fairfax, Fauquier, Prince William and Loudoun Counties in Virginia and Prince George's County in Maryland — a local problem for everyone in the area.

How to identify it

Two-horned trapa grows in fresh water, usually still or slow-moving.

• Leaves appear on the surface in late May: triangular, toothed, and arranged in a rosette — green on top and reddish underneath.
• Roots anchor in mud up to 15 feet down. The stem and part of the leaf stalks are filled with air, keeping the plant afloat.
• Flowers are light pink and appear in the center of the rosette in June.
• Seed pods — about two per flower — each have two horns.
• The plant dies back after a few frosts, usually in November, but its seeds stay viable for at least two years.

Its look-alike

Two-horned trapa resembles another invasive water chestnut, the Eurasian water chestnut (Trapa natans) — so closely that it was probably mistaken for it before 2014. Both grow in the same habitats and look alike from above. Check the underside of the leaves, the flowers and the seeds:

How you can help

Tracking this plant depends partly on citizen scientists. By downloading the iNaturalist app and joining the project, you can help NatCap PRISM fight trapa and other species targeted for early detection and rapid response.

To report two-horned trapa on iNaturalist:

1. Photograph a floating rosette — both sides of a leaf if you can — plus the flower and seeds if present.
2. Add a photo that shows the location and any landmarks, such as the edge of a path or a large tree.
3. Use the notes to describe the size or location of the patch.
4. You can edit your observation at any time after submitting it.

Sources

• National Park Service: "Two-Horned Trapa (Trapa bispinosa)," citing Chorak et al. (2019), Rybicki, Swearingen and Odenkirk (2019), Swearingen and Fulton (2022) and Fairfax County. The source's photographs are credited to named photographers or licensed CC BY-NC through iNaturalist, so they are not reproduced.