Mystery Plant! #738

John Nelson

Posted 8/30/23

By John Nelson

(Photo by Linda Lee.)

You might remember a couple of times in this column when the Mystery Plant was all about one of the various “Day-flowers” in a family named Commelinaceae. There are a couple of genera that you may be familiar with, including Commelina, which includes the “regular” dayflowers, as well as Tradescantia, which is a genus of a number of ornamental species, including those yard plants with the purple or blue flowers which we Southerners like to call “spiderworts”. Plants in the dayflower family are characterized, generally, by soft, succulent stems, which often sprawl. These various species have leaves which are alternate, just like you might expect from a grass or a sedge, in being “one at a time” along the stem. As always, the leaf is attached to the stem at a place called a node. The base of the leaf is a tubular affair, forming a sheath around the stem, and then the leaf blade is the free part that sticks out, or up, away from the stem.

The flowers of plants in the dayflower family are rather simple, as far as flowers go. The species we have in the Southeast will have perfect flowers, that is, with both pollen- and ovule- parts (“male” and “female”). There will be 3 sepals and 3 petals, and the petals are usually bright and showy. The reason this family is called the “DAYflower family” is that a single flower lasts only a day: opening in the morning and by the evening shriveling into a sodden little mass. Whatever pollination is going to take place must occur in that intervening time, during the “day”…and there is no nectar being produced for the little insect visitors. Each ovary will produce a small, dry capsule with a few tiny seeds inside.

(Photo by Linda Lee.)

So there you have the good news: the dayflower family, a good one to know about. Now for our Mystery Plant, which is nothing but bad news, believe me.

It is a native species in tropical eastern Asia, where it occurs in wet, marshy areas and forests. Back at the beginning of the 20th Century, botanists started noticing it in the southern states, scattered around in appropriate places, such as old rice fields and other wet places. Of course, the rice industry in South Carolina had basically collapsed well before then, but rice was still being produced in Louisiana and Texas. For various reasons, this little weed was finding itself at home in our area, probably due to its seeds contaminating agricultural shipments. It is an annual species, now a regular component of many natural wetlands, where it forms masses of vegetation with thousands of them. The stems are starting to bloom now, in early autumn, and although the flowers are an attractive shade of pink, that’s no reason to vote for this plant. What’s more is that this species is sometimes sold as an aquarium plant; so if you are an aquarium fancier, please avoid this plant, and for heaven’s sake, never discard aquarium plants into a local pond or stream!.

[Answer: “Marsh dayflower,” Murdannia keisak]  

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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