Mystery Plant! #734

John Nelson

Posted 8/2/23

By John Nelson

(Photo by Linda Lee.)

I am thinking that some of you might have been inclined in the past, and even now, to spend time around a pond or lake, perhaps a large impoundment such as our own Lake Murray here in central South Carolina. Perhaps you’ve been in a kayak, paddling around, or a pontoon boat, enjoying a late-afternoon cold refreshment. Or maybe you were fishing somewhere along one of those lakes, or maybe even swimming at a boat landing, enjoying what cool there was. I am further thinking that if you have spent much time around the edges of such waterways you may have seen this plant.

This plant was (and still is, of course), native to central South America. The first botanist ever to pay any attention to it, that is, by discussing its name, was a German gentleman named Carl Friedrich Philipp von Martius. (Can you imagine what it was like in the first grade having to write such a name at the top of your arithmetic quiz? Botanists, when referring to him in a publication these days, just write “Martius”.) Well, it turns out that the King of Bavaria gave him a pretty interesting job, which was to go to Brazil in order to discover and describe new plants. Martius was quite up to this task, and came up with a lot of names for plants new to science. Our Mystery Plant is one of them: it was described in 1825. Martius must have first seen this plant in a quiet backwater or bayou somewhere down there near the Amazon.

Of course, now it is all over the South…also known from California, more recently. It is truly a pernicious wetland species where it occurs now in North America, one of the very worst wetland weeds we have. In fact, it is listed by the Federal government as illegal to import, or transport for that matter. It apparently started showing up along the Gulf Coast in the late 1800’s, probably stowing away in ship’s ballast from South America.

(Photo by Linda Lee.)

The stems are smooth and hollow, with two leaves at each node. The stems can be erect, or just as often, sprawling, and capable of forming dense mats spreading out over the surface of quiet water. Those hollow stems make it easy to float. When blooming, a cluster of small flowers will form a head at the end of a slender stalk, from the axil of one of the upper leaves. Each flower, if you look closely enough, will have 5 papery white petal-like structures, and these heads of flowers, when produced in enough abundance, will be conspicuous from a distance.

I regret to say that there is not much reason to be happy with this plant. It forms dense patches which clog waterways, and it can impede boat traffic, and mess up your fishing line, and is just gross if you happen to swim into it. If broken up, the tiniest portion of a stem will readily make roots and start a new plant, which is the way it gets around. Oh well, the flowers are sort of pretty.

[Answer: “Alligator weed”, Alternanthera philoxeroides]

John Nelson is the retired curator of the Herbarium at the University of South Carolina, in the Department of Biological Sciences. As a public service, the Herbarium offers free plant identifications. For more information, visit or email

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