Bad news, I'm afraid.
They are interesting little things, very hard and very sharp. Shaped sort of like the horns of a bull. But of course, they come from a plant. And if you step on one of these things, it will hurt. Ouch!
These are the oddly-shaped fruits of a curious aquatic plant that is native to large parts of western Asia and the warmer parts of Europe. Because they are aquatic plants, and due to their odd shape, these little fruits have been termed "water chestnuts." And of course, that is a perfectly OK common name...because all common names are "OK", in a sense. There are indeed no rules that govern common names for plants (or animals), and if you are a faithful reader of this column, you may remember the author's near-obsession (completely justified, of course!) with scientific names. Again, there can be only one scientific name, in a proper sense, for any plant. Common names, though, are completely up to the imagination (or lack thereof) of whomever wants to talk about a given plant species in a vernacular, “over the counter” way. And, more than one plant species...as in this case...can have the same common name. Here, the name "water chestnut" is also used for a completely different species (a sedge, actually), and this is the one that you can buy in cans at the supermarket, the sliced stuff that goes so well in Oriental stir-fry dishes. So, I'm sure that you will agree with me that the best way to refer to plants is by their scientific names.
Our Mystery Plant, though, is more closely related to the various members of the evening-primrose family. It is an absolute aquatic species, growing in quiet ponds and backwaters, preferring places with mucky bottoms. The leaves produced below the water are dissected and fern-like. The floating leaves, though, are much different in shape. Their blades are diamond shaped, or triangular, and their leaf stalks tend to be swollen, containing air-filled tissues that help the plant float. The flowers, produced at the top of the stem, are small, each one with four tiny white petals, as well as four little stamens. There is a single ovary, which, of course, will yield the odd-looking fruit. A single, starchy seed develops inside the fruit. (These are edible, after being cooked a bit.)
This species showed up in New England just after the Civil War, no doubt as an aquatic garden (aquarium?) plant. As with many cultivated aquatic species, it found its way --accidentally or not-- into the various waterways of New England, and now, much of southern Ontario. There are reports of it as far south as Virginia. For heaven's sake, we don't need yet another aquatic nuisance, likely to choke our lakes, ponds, and rivers, if it ever gets introduced.
If you are interested in growing aquatic plants as a hobby, please don't grow this one...and make sure that whatever aquarium plants you are growing don’t somehow end up in our waterways. There are plenty of native aquatic species that are attractive, easy to grow, and perhaps most importantly, non-invasive.
[Answer: “Water caltrop," Trapa natans]
John Nelson is the curator of the A. C. Moore Herbarium at the University of South Carolina, in the Department of Biological Sciences, Columbia SC 29208. As a public service, the Herbarium offers free plant identifications. For more information, visit www.herbarium.org or call 803-777-8196, or email email@example.com.