Water Chestnuts Everywhere (But Not the Kind You Eat) - Sailors for the Sea

Water Chestnuts Everywhere (But Not the Kind You Eat)

By: Chris Mancini, Executive Director of Groundwork Somerville | August 1, 2012

Groundwork Somerville is a community non-profit based in Somerville, MA that works for the regeneration of the urban environment through programs in Healthy Education, Green Jobs and Community Engagement. This article concerns the Green Team’s efforts to remove the Eurasian Water Chestnut from the Mystic River.

On Saturday, August 4, 2012, the Mystic River Watershed Association (MyWRA) and Groundwork Somerville made one last summer push in a multi-year effort to eradicate the invasive Water Chestnut from the Mystic River in Massachusetts, and removed a record-breaking 806 20-lb. baskets of Water Chestnuts in one morning! With the help of hundreds of volunteers every year since 2010, we have managed to remove over 250,000 pounds of this detrimental invasive plant, or the equivalent of about 20 acres. By the end of 2012 we expect to have removed another 310,000 pounds, but even so, are barely able to keep up with the frantic spread of this invasive plant.

For a primer on invasives in general, read, “Invasive Species“, a previous Ocean Watch Essay. Though we often think of fauna (animals) first, it is the non-native flora (plants) that can be the silent killers. They can be beautiful, decorative, and seem harmless until unleashed into the wild.

The Water Chestnut (Trapa natans), pictured left, is a poster plant for invasive species. (Photo by Steve Hurst. Provided by ARS Systematic Botany and Mycology Laboratory.)

  • Native to Asia, Europe and Africa, it was introduced to the Northeast United States in 1897 by a gardener as an ornamental plant in Fresh Pond in Cambridge. (Many invasive species, like the starling, were imported for their appearance).
  • It spreads remarkably quickly. A single plant can produce enough seeds to for 200-300 new plants the following year. Parts of the Mystic River are barely passable due to water chestnuts that are working hard to extend their habitat from shore to shore. These floating-leaved plants form dense, continuous mats over the water surface of lakes, ponds and slow-moving waters.
  • The mats impede boating, fishing, and swimming, and crowd out native plants.
  • Perhaps worst of all, this is not the same water chestnut that makes a delicious accent to stir-fry. Trapa natans provides little to no nutrition for people or animals, and therefore discourages native wildlife from remaining in their natural habitats along the Mystic and other rivers, lakes and ponds in the Eastern U.S.
  • Finally, decomposition of large quantities (and there are very large quantities) of water chestnuts may result in lower dissolved oxygen levels, which can lead to fish kills.

For all these reasons, particularly their ability to spread so rapidly, a comprehensive eradication effort requires diligent, ongoing, and uninterrupted work over many years to have a significant impact. In the Mystic, these efforts began in 2010, and continue through this year. Even a one-year break in these efforts, however, can nullify all the accomplishments thus far, which is why it’s important to continue this work annually to make a major difference on this invasive population over the long term.

The water chestnut grows from a spiny seedpod, sometimes called a Devil’s Head that roots into the muddy bottom of rivers, lakes and ponds. The stems grow up through the water column where a rosette, or multi-leaved pad floats at the surface. The new seeds develop underwater at the rosette, and then are shed and take root in a later year, growing new plants.

The removal is a two-pronged attack on the Water Chestnut, and timing can be the difference between success and failure. The key is to wait long enough for the rosettes to grow, but not so long that they are able to shed their seeds (effectively negating any removal). Early in the summer, around mid to late June, mechanical harvesting takes place upstream to remove massive quantities of early growing chestnuts (pictured right). A mechanical harvester is able to eliminate large swathes of chestnuts in a short period of time. However, the cleared water surface allows light to reach the bottom where waiting seeds may begin to sprout. Within a month there will appear as many water chestnuts as have just been removed. Mechanical harvesting may be repeated in order to offset this regrowth.

Concurrently with mechanical harvesting, manual hand pulling will also occur. While mechanical harvesting is more efficient, it is also far more expensive. Volunteer, community hand pulling events serve two purposes. The first is to attack smaller patches, or new growth that mechanical harvesting misses, and the second is to bring public attention to the water chestnut problem. Groundwork and MyRWA will usually lead about 3-4 community hand pull events a year, with over 50 people coming out each time. We hope that as more people learn about the challenges faced by the Mystic River, the more resources will be directed to addressing the problem.

The Eurasian Water Chestnut is now found all around Eastern North America, from Virginia through Northern Quebec, but it is just one of many invasive plants that have taken root in our waterways. And while an eradication effort can be an overwhelming undertaking, with a few simple cautionary practices, people, and especially boaters, can do much to prevent it from spreading further.


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