Thom Smith | Nature Watch: Watermilfoil and other invasive aquatics


We hear of ponds and lakes being treated for invasive plants, primarily Eurasian watermilfoil (Myriophyllum spicatum). It has only been in the past 40 or so years that it has become a "species of concern," although it was introduced between the 1880s and the 1940s possibly (I think, probably) through the aquarium hobby.

It is moved from one body of water to another, however remote, by watercraft, and is today considered the worst aquatic weed in the United States, having infested lakes in more than 30 states. Also, more money is spent today, at least in Massachusetts, on attempts to control watermilfoils, than all other aquatic species.

And it looked so attractive in home aquariums, where it should not have been used in the first place. Now, although too late, we have learned or should have, that aquarium and pool plants should never be released in any body of water — not even flushed, but remanded to the compost pile or trash can.

Three ways to attempt eradication of this noxious weed include introducing natural predators (weevils, I believe first discovered at Middlebury College in Vermont) into infested waters. The second method is by large floating mechanical dredges that remove the plants from the water, unfortunately leaving broken-off pieces to regrow. Periodic (yearly) treatment controls its ability to overtake a body of water.

Treating with chemical herbicides is the third method, although controversial. These herbicides are very effective, in destroying the milfoils, but also native plants as well. Some states do not recommend chemical use. I believe natural controls to be the best, but I am not an aquatic biologist.

Another, and perhaps the most difficult aquatic weed to eradicate is fanwort, also known in the aquarium trade as Cabomba, a native to our southern states. Again, it may have been introduced into our northern waters by people emptying their fish tanks — fish, snails and all — into a nearby lake. It is aggressive, as well as hardy enough to be resistant to most herbicides, and ranks second to the milfoils for money spent on weed control.

Others include curly-leaved pondweed, native to Europe and arriving on our shores well over a century ago. It seems to thrive in polluted or very alkaline waters.

Elodea or waterweed, is more problematical in the south and we seem to be at its northern range, but with warming waters may extend its range northward. This is another "aquarium plant," that gets tossed into lakes and ponds, again sometimes with fish and invertebrates, all non-native.

The water chestnut, mentioned in an earlier column, is another problem invasive that was introduced intentionally and will be discussed at greater length in a future Naturewatch.

And as the summer progresses, kayaking and canoeing in shallow waters will become all the more difficult as these aliens rise to clog our favorite waterways.

Thom Smith welcomes your questions and comments. Email him at or write him care of The Berkshire Eagle, 75 S. Church St., Pittsfield, MA 01201.


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