JOHN NELSON’S MYSTERY PLANT: Mystery thistle is an invasive plant

  • Posted: Sunday, May 19, 2013 12:02 a.m.
Photo by John B. Nelson
Photo by John B. Nelson

O thou weed, who art so lovely fair and smell’st so sweet

That the sense aches at thee, would thou hadst ne’er been born!

Shakespeare, Othello, IV, 2

Here we have another Jekyll and Hyde plant...one that is indeed “lovely fair,” but one which, at the same time, is a most loathsome invader. We received this specimen here in the herbarium, mailed to us from a person who found it growing in upstate South Carolina and sent to us for identification.

It’s a thistle and like all thistles, is a member of the daisy family, the Asteraceae. Recall that in this family, the flowers are really tiny and congested into heads. Each head (also called a capitulum) is variously protected, in bud, by a series of bracts on the outside, called “phyllaries.”

When the head is open and the flowers are available to the world, so to speak, these phyllaries are frequently down at the bottom of the head. Here, the head itself is on a long stalk, and sometimes bends over a bit, nodding.

The heads of our mystery thistle can each bear nearly a thousand separate flowers. The individual flowers in the head are narrowly tubular, and purple. If the look of the head reminds you a little of an artichoke, there is a reason for that: they are related species.

At the base of each flower will be a tiny ovary, destined to form a hard, one-seeded fruit, much like a sunflower seed, only pale brown when ripe. And, this being a thistle, there will be a prominent tuft of soft, snow-white down on the top of the achene. That’s what lets these seeds float around in the breeze so easily.

Thistles are also known for spininess, and believe me, this species is true on that mark as well. Sharp, needle-like spines adorn the leaf lobes, margins and bases, and the leaf-like wings go up and down the stem.

Toward the base of the plant these spiny wings are quite broad, and grabbing onto just about any part of a big plant, without gloves on, will be a painful experience. The flower heads are spiny, too – each one of those bracts terminating in a stiff point. Ouch.

All of this spininess might not be so bad if the plants occurred separately, one at a time. But oh, no! This species can grow to be 6 feet tall or so, forming truly dense patches, if it gets the chance, and easily crowding out all the other vegetation. So many flowering heads producing so many ripe achenes...it’s a setting of botanical horror.

So, I think that you will agree with me that this is a charming plant in an aesthetic sense: the blooming heads are very attractive, and they do attract butterflies and goldfinches, which is nice. But the down side of this species can’t be understated. It is a total nuisance, an alien imported into North America from Europe in the 19th century, and by now spread into nearly every state of the union (not reported from Florida yet). Getting rid of it is expensive, and is no easy matter.

John Nelson is the curator of the A.C. Moore Herbarium at the University of South Carolina, in the Department of Biological Sciences in Columbia.

[Answer: “Nodding thistle,” Carduus nutans]

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