Portraits of Wildflowers

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Archive for November 29th, 2011

Frostweed explains its name

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Frostweed ice; click for greater detail.

In the last two posts I gave plausible reasons for the name frostweed and then I batted each one down. Today’s picture provides an answer to a question that may have occurred to you but that remained unasked: So tell us already, why is Verbesina virginica called frostweed? The name comes from one of the strangest phenomena in botany. By the time the first good frost settles overnight on the lands where this species grows, almost all of these plants have gone to seed. Although each stalk stands there unappealingly as it dries out, that first touch of hard frost can cause it to draw underground water up into its base. Now for the bizarre part: the lower part of the stalk splits as it extrudes freezing water laterally, and that process produces thin sheets of ice that curl and fold around the broken stalk and sometimes even unscroll away from it. That’s what you see in today’s picture, which I took just yesterday morning in Great Hills Park; if you’re willing to take your eyes off the pretty ice formations, you can make out a dark section of stalk in the upper left. I’ve read accounts that say some frostweed plants go through a second round of this icy phenomenon when there’s another freeze, but I’ve never tried to verify that. [Update: I’ve verified it for myself.] What I can say from experience is that the extruded sheets of ice are so light and delicate and prone to break when handled that they remind me of phyllo pastry.

And now let me answer another question that may have entered your mind. The last two posts dangled the question of frostweed’s name, but today’s picture, which explains the common name, was taken only yesterday morning. How did I know, when I began this series two days ago, that the temperature would drop enough to trigger frostweed’s ice trick and allow me to take pictures? I didn’t. I was planning to wait until the first or second week of December, which is when Austin usually gets its first hard freeze, and then take a frostweed ice picture that I’d use in an explanatory post at that time. Or if I didn’t manage to get any photographs of the phenomenon this year, I was going to show a photograph from another year. It was just serendipity that the temperature yesterday morning in Austin had dropped close enough to freezing to make some frostweed plants put on their display.

© 2011 Steven Schwartzman

Written by Steve Schwartzman

November 29, 2011 at 5:12 AM

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