Nobody knew how a dandelion seed flew until 2018

Everyone has held one. You find the white globe on its stalk, you blow, and the seeds go out across the grass. It is probably the most-handled wild plant structure in human history. Children in every country where the plant grows have done it, for as long as there have been children and dandelions.

Nobody could explain how it worked until October 2018.

That is not a figure of speech. Naomi Nakayama, a biophysicist at the University of Edinburgh, put it plainly to a reporter that autumn: "It's a weird structure. Nobody really knew how it could fly."

Here is why it is weird. Each seed carries a crown of around a hundred hair-like filaments, all attached at a single point and spreading out into a shallow disk. Look at it against the light and the disk is mostly gap. Almost all of it is empty space.

That should not work. A parachute works by refusing to let air through. Air pushes on the fabric, the fabric pushes back, the thing falls slowly. Punch a hundred holes in a parachute and it stops being a parachute. The dandelion is essentially all holes, and it stays up so well that a seed can cross a field on a light breeze.

What they found in the air

The Edinburgh team found a way to make the air itself visible around a falling seed, and then they looked at what the air was doing.

It was doing something nobody had recorded before. Above the seed, in the low-pressure pocket just downstream of the bristles, a ring of air was spinning and recirculating and staying put. Not touching the filaments. Hovering above them, detached, held in place by the very air rushing up through the gaps.

Their paper describes it as "an extraordinary type of vortex", "a ring of recirculating fluid, which is detached owing to the flow passing through the pappus."

Cathal Cummins, the postdoctoral researcher who led the work, described the reaction of colleagues to Science News: "When you show it to a fluid dynamicist, it blows their mind."

The last line of the paper's abstract is the part worth sitting with. The discovery, the authors write, "provides evidence of the existence of a new class of fluid behaviour around fluid-immersed bodies."

Not a new species. Not a new behaviour in an animal. A new class of thing that fluid can do, found in the weed growing out of a crack in the pavement.

And the emptiness turns out to be the whole mechanism rather than a compromise. The bristles are not a cheap approximation of a solid canopy. By the researchers' own account, the filament design generates something like four times the drag a conventional parachute design would manage with the same amount of material. The gaps are not what the dandelion settles for. The gaps are what it flies on.

The part you can check yourself, in about ten seconds

Four years later, most of the same team published a second finding, and this one you can go and verify in your own yard.

The dandelion clock closes when it is damp.

Not metaphorically. In humid air the filaments fold inward toward each other, and the wetter it gets the further they fold. At 72 percent relative humidity the team measured the bristles closing by about ten degrees; at 87 percent, about twenty-four. Mist a seedhead for an hour and the pappus shuts by a hundred degrees or more.

The aerodynamic consequence is immediate. A closed pappus falls two to three times faster than an open one. The vortex ring above it shrinks and slides down toward the bristles. The seed stops being a flier and becomes, more or less, a small heavy thing.

And it holds on. In the wind tunnel, wetted seeds were significantly harder to pull off the parent plant across every wind speed tested. At the strongest wind they used, just under ten metres per second, a stiff gusty breeze, half the wet seeds were still attached, against about a quarter of the dry ones.

Read that in the other direction and the plant looks less like a passive object. It is not simply releasing seeds and hoping. It shuts in the damp and grips the stalk, and it opens and lets go on a dry day with wind behind it, which is the weather that carries a seed furthest. The paper's word for this is informed dispersal. The plant has no nervous system and no eyes and is nonetheless holding out for better conditions.

So: find a dandelion clock early on a dewy morning and look at how tight it is. Go back to the same one in the afternoon, once the sun has been on it for a few hours. It is a different shape. It has been making a decision all day and nobody told you.

You have time to do this. Common dandelion flowers from March to late fall across most of the United States, which means there are heads going to seed somewhere near you for most of the year. And they are not going far in the tidy sense, one survey in an Iowa tallgrass prairie found dandelion seeds several hundred metres from the nearest plant they could have come from. The Edinburgh team's own account of their work puts it at a kilometre or more on the right day.

The actual remarkable thing

The physics is good. But the physics is not the part that stays with me.

The part that stays with me is that this object has been in the hand of every curious person who ever walked through a meadow, and the answer to the most obvious question about it, how does that stay up?, arrived in 2018. Not because the seed is rare or remote or hard to reach. Because nobody had yet found the right way to look at the air.

There is a version of going outside that is about covering ground. And there is a version that is about stopping in front of one ordinary thing for long enough that it stops being ordinary. The dandelion has been running an undescribed class of fluid behaviour on every lawn in the northern hemisphere the entire time.

Go find one. Blow it, obviously. But before you do, hold it up to the light and look at how much of it is nothing at all.

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Sources

• Cummins C, Seale M, Macente A, Certini D, Mastropaolo E, Viola IM, Nakayama N. "A separated vortex ring underlies the flight of the dandelion." Nature 562(7727):414-418, October 2018. doi:10.1038/s41586-018-0604-2
• Seale M, Zhdanov O, Soons MB, Cummins C, Kroll E, Blatt MR, Zare-Behtash H, Busse A, Mastropaolo E, Bullock JM, Viola IM, Nakayama N. "Environmental morphing enables informed dispersal of the dandelion diaspore." eLife 2022;11:e81962. doi:10.7554/eLife.81962
• University of Edinburgh press release, "Dandelion seeds reveal newly discovered form of natural flight," 17 October 2018 (via phys.org), source of the four-times-efficiency framing and the Cummins quote on material and energy cost.
• Science News, "Dandelion seeds create a bizarre whirlpool in the air to fly," 17 October 2018, source of the Nakayama and Cummins quotes.
• USDA Forest Service, Fire Effects Information System, species review: Taraxacum officinale, flowering season and the Iowa tallgrass prairie dispersal observation.