Where do
lawn weeds
come from?

The full narration of the video.

Kuansong de Xianggu asked: how do weeds get into a city lawn? You've seen it: the gardener pulls out every weed. A few days later, a new batch is back. Nobody sowed any seeds, so where do they come from? Try a small experiment.

Dig a shovel of soil from the lawn, with not a single plant in it. Spread it in a tray on a windowsill and water it every day. A week or two later, the tray is covered in tiny sprouts. Researchers at Beijing Forestry University counted this way: they grew soil from seventy-five parks. Each square meter held three hundred fifty to nine hundred seeds.

So under the lawn, a long line is waiting for an open spot. It's like a supermarket checkout: the grass fills every register. With every register taken, nobody in line moves up. So when is it their turn? Three moments.

One: the grass gets trampled. The soil packs hard, roots can't dig in, and a bare patch opens. Two: grass cut too short can't block the sun. Some weeds hug the ground, and the mower passes right over them. Three: weeding itself. The hoe turns up deep soil, and buried seeds suddenly see light.

Many weed seeds wait for exactly that flash of light: one experiment found daytime plowing raised up to three times the weeds of plowing at night. So every time you weed, you're calling out: next, please! And the line keeps growing. One dandelion puff holds over a hundred seeds, each with a tiny parachute. Most land within a few meters; a few ride warm updrafts past a hundred.

Birds eat seeds and drop some in their droppings. Shoes and wheels carry a few along too. The newcomers all join the back of the line. But this line isn't first come, first served: whoever's nearest the surface and sees light first goes first. And they can wait a very long time.

In eighteen seventy-nine, an American botanist buried bottles of weed seeds. In twenty twenty-one, one bottle came up: after a hundred forty-two years, some seeds still sprouted. Want fewer weeds? Don't just pull; keep every register busy. Leave the grass taller. Oregon State University reports: going from about four centimeters to seven or eight cut dandelions by up to forty-five percent.

Patch bare spots early. Thick grass blocks the light, and the seeds below keep waiting. So the weeds that pop back up didn't fall from the sky; they're just next in line. They were under your feet all along. Weeds in sidewalk cracks and on top of walls are the same: wherever a sunny gap opens, the next in line steps up.

Kuansong de Xianggu, where's the most unlikely spot you've ever seen a weed growing?