In time, the aliens would come to understand almost everything about us—our ceaseless yearning, our habit of wandering, how we love the feeling of the sun’s light on our skin. At last, they would have only one question remaining: “We have noted that there is a green god that you keep in front of and behind your houses, and we have seen how you are devoted to the care of this ornamental plant god. You call it Kentucky bluegrass, although it is neither blue nor from Kentucky. Here is what we are wondering: Why do you worship this species? Why do you value it over all the other plants?”

Poa pratensis, as it is known to the scientific community, is ubiquitous the world over. Much of the time when you see a soft, green expanse of lawn, you’re looking at least in part at Kentucky bluegrass. The plant is native to Europe, northern Asia, and parts of North Africa, but according to the Invasive Species Compendium, it is now present on every continent, including Antarctica.

The typical shoot of Poa pratensis has three to four leaves, shaped like little canoes, and if left unmown, it can grow to three feet tall and sprout blue flower heads. But it is rarely left unmown, at least not in my neighborhood, where it is illegal to allow your grass to grow more than six inches long.

If you’ve ever driven through my home state of Indiana, you’ve seen mile after mile of cornfields. Amber waves of grain are enshrined in the song “America the Beautiful.” But more land and more water are devoted to the cultivation of lawn grass in the United States than to corn and wheat combined. There are around 163,000 square kilometers of lawn in the U.S., greater than the size of Ohio, or the entire nation of Italy. Almost one-third of all residential water use in the U.S.—clean, drinkable water—is dedicated to lawns. To thrive, Kentucky bluegrass often requires fertilizer and pesticides and complex irrigation systems, all of which we offer up to the plant in abundance, even though it cannot be eaten by humans or used for anything except walking and playing on. The U.S.’s most abundant and labor-intensive crop is pure, unadulterated ornamentation.*

The word “lawn” didn’t even exist until the 1500s. Back then, “lawns” referred to expanses of grass shared by communities to feed grazing livestock, as opposed to “fields,” which denoted land used to grow plants for human consumption. But by the eighteenth century in England, ornamental lawns similar to the ones we know now had emerged—back then, lawns were maintained by handheld scythes and shears, and so keeping a lawn without the help of grazing animals was a sign you were rich enough to hire lots of gardeners, and also to own land that did nothing but look good.

The ornamental lawn fad spread throughout Europe, and also to the United States, where people enslaved by Thomas Jefferson maintained a closely mown lawn at Jefferson’s estate, Monticello.