Saturday, July 16, 2016

Water Chestnut: Invasive Aquatic Plant

Water chestnut plant at surface, rooted to bottom. 
Often, the root supports multiple rosettes, each  
bearing several nuts. The plant is an annual, so it 
does not regrow from the same root the 
next year. Rosettes are 6-12 inches across.
Water chestnut, an invasive water plant, has a nature akin to lily pads on steroids, growing rapidly in nutrient-rich fresh water ponds, lakes and slow-flowing rivers. Unchecked, it will almost completely cover water surfaces, making boating, swimming and fishing impossible. The dense floating mat of overlapping leaves also blocks sunlight penetration, causing oxygen deprivation lethal to fish and other animal life. In addition to this ecological horror story, the large, sharply pointed seeds, which mature in early August, fall to the bottom, and can cause painful wounds if stepped on.

This species, Trapa natans, is not to be confused with the edible water chestnut common to Chinese cuisine. The plant was initially brought to the Harvard University Botanic Garden, possibly from southeastern Europe or western Asia. In the 1870s staff gardener Louis Guerineau took it upon himself to throw seeds into Fresh Pond and other Cambridge waterways. This came to the attention of botanist George E. Davenport, who decided to bring seeds and live plants to his friend Minor Pratt, in Concord. He and Pratt seeded a pond near the Sudbury River, and he suspected Pratt conducted additional distributions. Thus, Cambridge was point zero and Concord the plus one. Current distribution ranges from Canada to Maryland, and westward into New York and Pennsylvania.
Click on any photo to enlarge

As early as 1879 there was a concern voiced by botanist Charles S. Sargent, Director of Boston's Arnold Arboretum, that this non-native species threatened to become a nuisance, based on dense growths reported in Cambridge. Davenport fessed up in the Bulletin of the Torrey Botanical Club, Vol. 6: "I have several times had plants of Trapa natans that were collected in the vicinity of Boston, during the present year, brought to me for identification, and I have entertained no doubt as to the manner of its introduction into waters outside Cambridge Botanic Garden. But that so fine a plant as this, with its handsome leafy rosettes and edible nuts, which would, if common, be as attractive to boys as hickory nuts now are, can ever become a 'nuisance' I can scarcely believe."

This past Saturday a doughty band of twelve volunteers, organized by OARS (Organization for the Assabet Sudbury & Concord Rivers), launched canoes onto the Assabet River in Maynard, upstream from the dam next to Powdermill Road. What this involved was paddling upstream about one-quarter mile. Two occupants per canoe would steer into an area with plants to pull them by hand, each yank resulting in a dripping, muddy mess dropped into laundry baskets in the middle of the canoe. The laden canoes would be paddled back to the launch site, the baskets lugged ashore to a compost pile, the canoes bailed out, the process repeated. Messy, messy, messy! 

Pile of water chestnut plants, to be hauled away to landfill. 
Years of these visits, conducted every July before the nuts mature and fall to the bottom, have done a great job of eradicating the plants from long stretches of the Assabet River and reducing density in the still impacted parts. Surveillance visits are repeated each year, because while most seeds sprout next spring, some are still viable as much as 8-10 years later.     

 To get an idea of how bad it can get, Vermont spends over half a million dollars a year hiring companies with mechanical harvesters to manage the worst parts of Lake Champlain, plus paying dozens of people to do hand-pulling in less-infested waters on the big lake and elsewhere. The 2013 report described 1,200 tons collected by the harvesters and more than 21 tons by hand.

This posting repeats in part a column that was in the Beacon-Villager newspaper (serving Maynard and Stow, Massachusetts) in July 2015.



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