Control and Science of Invasive Water Chestnut
Using grants from New York State’s Department of Environmental Conservation and the New York Power Authority’s St. Lawrence River Research and Education Fund my team is studying the environmental impacts of the water chestnut invasion in rivers and lakes of the north country. Because the invasion is most intense in the Oswegatchie River at Heuvelton we’re focusing on this location to find out whether chestnut excludes native vegetation and diminishes environmental conditions that support fish populations. We’re also trying to reduce chestnut populations here, which will prevent their spread into other nearby waters, such as Black Lake and the St. Lawrence River. Finally, we are preserving areas of native vegetation that can recolonize shorelines previously disturbed by thick chestnut beds. Read on to find out what we’ve accomplished thus far! Any questions? Please reach me at bbaldwin@stlawu.edu.
What’s the problem with this aquatic plant?
And because water chestnut can quickly reproduce and grow across shallow nearshore areas (especially those <10 feet) it can create large “fields” that choke off access to docks. This is much worse than another invasive plant, Eurasian water milfoil! To move around, property owners need to rent weed harvesters or frequently cut paths of their own, as seen in this June 2024 drone photo of the Oswegatchie River shoreline at Heuvelton. Mowing your yard occasionally is one thing but this is much more challenging (and expensive)!
Too much of a good thing! Plants fuel ecosystems but as a newcomer to our area water chestnut can grow out of control, crowding out the native plants that our fish have adapted to while hindering recreational boating and fishing. The nuts can also wash up along the shoreline and hurt your feet (and your dog’s paws)!
Where is water chestnut in the North Country?
As of early 2025 this invasive plant had mainly colonized (right, yellow symbols) the Oswegatchie River at Heuvelton (red box). But smaller outbreaks had also been reported in Black Lake and in a small bay on the St. Lawrence River near Ogdensburg.
Heuvelton: Since about 2020 our neighbors there have reported colonies of water chestnuts growing in Lisbon Creek, a feeder stream that flows (yellow arrows) into the larger Oswegatchie river, just above the dam in Heuvelton. By summer 2024 this plant, which can grow across the surface of waters, blocked off about 40 acres of formerly navigable waters in this area (see photos below). During 2024 we established study sites in these 2 waterways, some of which were dominated by the growth of water chestnuts (red dots) and others that were (thankfully) still dominated by native aquatic vegetation (green dots). We measured environmental conditions at all sites and harvested water chestnuts from red sites. We also set up conservation zones (green boxes) and hand-pulled small outbreaks of chestnuts from those locations. If all goes well, we hope to save native vegetation at green sites (important nursery areas for fish) and gradually convert red sites back into green sites!
Black Lake: our goals are to save this downstream lake from the same fate as Heuvelton waters! We will continue to remove chestnuts from SW and NE ends of the lake and survey other shorelines for new plants. Want to help defend the lake? See below!
St. Lawrence River: We will continue to remove chestnuts from Tibbitts Creek bay and survey other shorelines for new plants. Want to help defend the river? See below!
Unfortunately…during 2024 surveys of several shoreline bays along the St. Lawrence River, we did find (and remove) water chestnuts from Tibbitts Creek bay, just downstream of Ogdensburg. But most other bays were fine and undisturbed by this invasive plant! During 2025 we will continue to protect these important nursery areas for sportfish. Unless we (and shoreline residents) do so, this population will only grow, reducing fishing value and recreational boating, all while sending more seeds/nuts downstream toward Waddington and Massena. Let us know if you can help remove these plants (see below)! We will continue surveys of other important bays along the St. Lawrence thanks to the guidance of area wildlife experts like Mike Morgan of DEC (pictured here with my summer research students Maggie Layman and Lilia Smith).
Where else might this invasive plant spread?
We are most concerned about downstream Black Lake and other bays along the St. Lawrence River. We also have to keep it out of Lower Lake, upstream near Rensselaer Falls. Thankfully, limited surveys of Lower Lake in 2024 were negative. But if the plant infests either Lower or Black Lake – both of which are shallow, productive ecosystems – they could become potent “chestnut factories” that will constantly seed other downstream waterbodies. Interviews with local citizens suggest the invasion began about 2020 within Lisbon creek (images above), which, flowing into the Oswegatchie River, colonized large areas of the impoundment created by the hydro-electric dam in Heuvelton. From there it is has spread below the dam, down the Oswegatchie and into the St. Lawrence River (although Lake Ontario colonies could also be sources). Most likely, since 2020, chestnuts have been tumbling downstream (or hitching rides) to Ogdensburg, Waddington, and even Massena. And with some record rains and floods during summer 2024 (see map) this dispersal likely increased. But thanks to our major harvesting efforts, begun in 2024, we feel we’ve begun to turn the corner on this invasion (see below).
What are we doing to suppress the current invasion?
Since we had over 40 acres of water chestnut “fields” to remove at Heuvelton in 2024 we paid a local weed harvesting company to cut the top halves of the plants off before they could make new nuts (each plant can make 10-20 nuts in our location). This also reopened vast areas to boaters. Our collaborator, Bill Dashnaw (on behalf of the Town of Oswegatchie), used his own DEC grant to purchase a second harvester (below). Together these machines cleared about 90% of the infested area and prevented the production of about 85,000,000 new nuts! In 2025 we should be able to curtail it further. Boosting our confidence is our discovery that entire plants (leaves, stems, roots and source nut) can be easily pulled out during May, when they are juveniles. Currently, midsummer cutting removes only the top half of the plant leaving the bottom half to send up new leaves (as grass does in your lawn). But the new harvester, which has a pulling apparatus, has the potential to prevent such regrowth, especially in May, and this in turn should keep more waters open to recreational boating and fishing. Done repeatedly over the next few years we should be able to deplete any new juvenile plants that arise from residual nuts produced prior to 2024. This should allow us to finally turn the tables on the chestnut invasion.
Restoring Native Vegetation for Fish! We’re also working just upstream of the worst chestnut outbreaks in the Oswegatchie River, hand-pulling the occasional chestnut plants that have managed to colonize weed beds (shoreline vegetation zones) still dominated by native vegetation. This effort will ensure that upstream areas support fish populations while also seeding native plants back into the cleared out chestnut “fields” just downstream. We hope the local community can join us in this effort, hand-pulling any chestnut plants they encounter in these priority management areas (PMAs). Its easy to do (see below) and lets residents be proactive in protecting this beautiful stretch of the river.
How can you stop an invasion near you?
An easy way to start is to search shorelines where you see lily pads growing (shallow and muddy). Among those floating oval leaves you may see clusters (rosettes) of chestnuts, which also float. But as you can see, they have toothed, triangular leaves that originate from a central stem. Like lilies, that underwater stem is rooted into the sediments. To remove, simply grab these rosettes and pull them up into your boat. Ideally, pull up the stem and the roots (which can be muddy; rinse them off in the water first) but minimally, break the stem and keep the rosette as this is where the flowers and baby nuts form. Then bag the removed plants and dispose in the trash. Soon after, please reach us so we know where you found them (bbaldwin@stlawu.edu).
You are now a hero in your neighborhood because you cleared the water of these plants (easier to boat and fish), gave native plants a competitive edge, and prevented those chestnuts from making 10-20 new nuts, which would otherwise lead to exponential growth. As you can see to the right, these plants form the 4-pointed nuts on their undersides and they will fall off after 2-3 weeks, creating a “seedbank” along the bottom that may germinate next year (or even sometime over the next decade). If we as a group don’t survey for and remove these plants they will happily multiply and (gulp) create more infestations like the one we are trying to control in Heuvelton (above). If you want more information about volunteering to prevent invasions such as this please contact us or visit the SLELO PRISM site, where many successful initiatives are taking place (https://www.sleloinvasives.org/learn/volunteer/)!
Don’t be distracted with this other plant, which may look like chestnuts
Chestnut plants
>>Triangular leaves (1-2″) with toothed edges. Rosettes of interconnected leaves
>>swollen floats on leaf stems
>>may have 4 pointed seeds/nuts below leaves
>>small, white, 4-petaled flowers
Frogbit plants
>>”tiny water lilies” with round/oval. smooth-edged leaves. Rosettes of interconnected leaves
>> spongy leaf undersides that may appear red-purple
>> small, white, 3-petaled flowers
Is water chestnut bad for fish?
The jury is still out. So far we have found that juvenile fish swim inside the chestnut “forest” during the daytime (see https://fb.watch/tMDCSsX8i0/), just as they do in native plant weed beds. We’ve also found a food base for them that is similar to that seen among native plants (see images below). That’s encouraging and suggests that chestnut plants might still support native fish communities, even if its difficult for us to actually FISH in these areas. BUT, what about the “underwater weather” within chestnut beds? Do fish have enough dissolved oxygen (DO) and is the pH of the water neutral or acidic (similar to ADK lakes that were stressed by acid rain)? Turns out, conditions look good during the daytime but stressful at night, at least during hot summer weather (see graphs below). Perhaps juvenile fish use these habitats for food and shelter during the daytime (beneficial!) but leave these forests during hot summer nights. Currently, we’re not sure whether native fish spawn within chestnut areas as they evolved to do in native plant weed beds. But, if their microscopic larvae and slightly older juveniles use chestnut beds that will help native fish communities persist over time. Ultimately, though, our goal is to remove most of the chestnut plants so native plants can reoccupy all the shoreline areas and support normal levels of reproduction.
Dinner is Served! Like a kelp forest along an ocean coastline, river and lake weed beds can host a rich variety of fish and other smaller critters. Here’s a sampling of the nutritious fish chow on offer for young fish. There are (going clockwise) tiny zooplankton for fish larvae to eat (a 2 mm copepod with orange antennae), and larger (10-20mm) amphipods (4 antennae & lots of legs), elongated fly larvae, flexible flatworms, and mites (8 legs) that can swim!
Fluctuating “underwater weather”. We deploy underwater weather stations (hydrolabs) within a variety of weed beds in this area to describe the environmental conditions that impact fish. In July 2024 we recorded this nearly 4 day long variation in oxygen levels within a typical water chestnut “field”. As you can see, oxygen can rise to healthy levels (>50%) during some afternoons as plants crank up photosynthesis. So fish would be fine at these times. But other afternoons are cloudy, limiting the production of oxygen by these plants. And things get mighty stressful most nights when all organisms – even the plants – are using up the daytime oxygen. If oxygen plummets below 20% (pale red box) conditions become hypoxic. Far worse is when it crashes to 0%, which is anoxic! Both are incredibly stressful for fish. Most likely, any fish within a water chestnut bed would sense such low levels and swim out of an underwater forest of these plants. Meanwhile, over in native weed beds, conditions can also fluctuate but thankfully oxygen doesn’t plummet to dangerous levels.
As if oxygen levels aren’t bad enough in chestnut beds during summer, pH levels can also decline to stressful levels. Imagine the synergistic effect of this one-two punch on fish! These two stresses, happening together during warm summer nights, are why we believe fish may leave at those times in a 24 hr day.
What makes this invasive plant so successful?
Coming Soon… hitch-hiking seeds, yearly production of about 10-20 new nuts, each seed able to germinate over the next decade, rapid plant growth, floating leaves that form canopies able to shade out competing plants, etc!
