A Staff Report from the Straight Dope Science Advisory Board

What's the deal with tumbleweeds?

August 25, 1999

Dear Straight Dope:

Where do tumbleweeds come from? Do they tumble in their "natural" state. or only when they die? Where are their roots? What do they look like when they are growing?

Like so many things in Our Great Land, this is another case where there's more than one thing that has been given the same name. "Tumbleweed" is not one specific plant, though there is technically only one plant that bears that name, a type of Amaranth (a name you may recognize now that some folks are using certain types of amaranth seeds to make flour). The species Amaranthus albus is the tumbleweed.  It's a little shrubby weed common to a fair portion of North America, especially drier areas. Nothing special when alive, it has tiny, greenish flowers and small leaves.

This plant, like the other "tumbleweeds," will blow and roll around after the plant dies and dries up since they commonly have the stem base snap. This appears to be something that has evolved due to the benefits of having the seeds dispersed along with the dead plant. As a tumbleweed rolls, it drops seeds as it goes, so it spreads to new territory easily. There's a tendency for seeds in the middle of the ball to get trapped there, but a good rainfall will cause the dried branches to unfold a bit, and often a large number of seeds will get dumped at once--and watered at the same time. Nice trick, showing that plants effectively invented the wheel long before we did.

Cecil Adams comments:

Good answer, young Doug, but let's not give plants too much credit.  The ingenious machine you allude to is the wheel and axle, which is rare (although not entirely unknown) in nature.  Rolling per se is no great trick. See my column at http://www.straightdope.com/colum ns/980515.html.

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