Barack Obama’s 2009 inauguration brought 1.8 million people to the mall. U.S. Air Force/Public domain

In 2009, after the first inauguration of President Barack Obama, which was attended by a record 1.8 million people, the ground of the National Mall was wrecked. As those hordes had moved over the lawn, they ground the crown of the grass plants, the part where new growth originates, into the frozen soil. After the inauguration, the Mall was more hard-packed drag-race strip of dirt than park.

That won’t happen—can’t happen—this inauguration. The National Park Service has just invested $40 million in refurbishing the long expanse of the Mall with carefully designed turf. The most recently reconstructed section has been closed for the past few months, as the grass settles in and puts down deeper roots. January 20 will be the first time the public is allowed in, and even the smaller crowd of this inauguration, expected to be around 800,000 people, is a threat.

The Park Service now has a challenge—to keep the Mall from becoming a dust bowl or, since a warm spell is thawing the ground, a mud bowl. Its staff is mobilizing to protect its new turf.

The lawn is newly refurbished. National Park Service/Michael Stachowicz

In Pierre L’Enfant’s 1791 plan for Washington, the Mall was designated a grand avenue with a canal running down one side. In the early decades of the city’s expansion, that vision never quite became real, and during the 1800s, the Mall had developed into a different sort of public space, dotted with gardens, greenhouses, a market, and a train station.

In 1901, a congressional commission, determined to redesign the park, swept all that away. The McMillan Commission conceived of a new vision for the Mall, one they said reflect L’Enfant’s original plans. They wanted a giant lawn.

Despite their ubiquity today, lawns were once the height of luxury. The most desirable lawn aesthetic, an expanse of neat, uniform green blades, requires that the lawn be mown. In the era before lawn mowers, that had to be done either by grazing animals (declassé) or by hand-cutting (very chic). Keeping a large lawn, like the Tapis Vert at Versailles, was extremely expensive; aristocrats all over Europe made the investment.

The McMillan plan reimagined the Mall. Public Domain

When the McMillan Commission made its recommendation, its members had in mind those palatial lawns of European royalty. By the early 1900s, though, grass was a more democratic luxury. Frederick Law Olmsted had designed one of the first suburban developments in the U.S. with a tiny lawn for each house, and the lawn mower, first invented in the 1830s, was becoming a consumer product, complete with its own engine.

When the Mall’s green expanse was first planted more than a century ago, though, the space was little used, compared to today. Grass isn’t normally a natural resource that people think about protecting, but most lawns aren’t subject to this level of wear.

“Grass is something that’s taken for granted,” says Michael Stachowicz, the Mall’s turf specialist. “People can grow it in their yard and it’s fine. But I don’t have 30,000 people a day going over my lawn.”

It takes thousands of panels to cover the Mall. National Park Service/Michael Stachowicz

Since the Mall’s last renovation, around 1976, there’s been a major revolution in grass cultivation and technology, spurred by the late 20th-century boom in golf course construction. We know much more about growing grass now—the best mix of plants, the ideal soil conditions, the correct irrigation. Those best practices have been applied to the Mall, where the grass now grows in an engineered, very sandy, and aerated soil. “We’re building 18 acres’ worth of turf like we used to built 5,000 square foot golf greens,” says Stachowicz.

The technology to protect turf is a relatively recent invention, too. For the inauguration, contractors are bringing in special panels, 16 square feet each, to cover up the grass. To the human eye, they look white, but they’re actually translucent, which allows light to reach the grass and keep it healthy. The bottoms of the panels are honeycombed with small, square cells that protect the grass crowns from being crushed and act like mini greenhouses.

Right now, 800,000 square feet of Mall is being covered with approximately 50,000 of these panels, each 16 feet square—so many that they had to be borrowed from sports stadiums across the country. (“Basically we’re looking at nine baseball stadiums worth of flooring,” says Stachowicz.) When they’re lifted back up, the Mall might actually be greener, after the sad winter grass spends a few days insulated from the elements.

Those panels are actually translucent. National Park Service/Michael Stachowicz

The grass isn’t the only element of the Mall that needs protecting from the crowds at inauguration. Trees are boxed off, so that people don’t trample the soil around them or damage the trees themselves. Places like the steps of the Lincoln Memorial are also covered with special flooring to protect the pavers.

Even after the crowds have left, the Park Service staff needs to keep vigilant as the infrastructure of inaugurations is removed. “Leaving is a lot more chaotic than set-up,” says Stachowicz, and they have to make sure that trucks don’t run over lampposts or post-and-chain fences—“That happens!”

The Park Service has spent basically the entire Obama administration making the Mall look nice again. The job now is to make sure it doesn’t get ruined on Trump’s first day.