The science behind perfect grass on World Cup’s 16 fields

The World Cup’s first whistle will sound June 11 at Estadio Azteca in Mexico City, kicking off a summer-long spectacle that will stretch across three countries and draw a global audience.

For the grass they’ll play on, it all started years earlier in North Carolina. Or Colorado. Or Canada. Or whichever sod farm had been assigned to grow a particular, essential piece of the tournament.

Unique challenges

Across North America, FIFA’s pitch experts have spent years trying to make 16 new fields in three countries feel like one playing surface - indoors and out, in heat and shade, at sea level and altitude, inside stadiums that were not always built with grass in mind.

Along the way, the playing surfaces that will serve as the stage of this summer’s tournament have made an unlikely journey: from research plots to sod farms, from refrigerated trucks to stadium floors, from a living crop to soccer’s biggest show.

The hardest question may not be answerable until it’s too late to do anything about: Can a living field thrive for weeks inside a domed stadium, under artificial light, on a temporary surface - then hold up when the best soccer players on the planet ask every bounce, roll and blade beneath them to behave properly?

For World Cup organizers and their team of grass experts, each venue presented unique challenges.

Several host venues normally use artificial turf, and eight stadiums required a temporary natural-grass field to be installed over, or in place of, an existing artificial surface. Five are domed or covered. Mexico City brings altitude. Vancouver, Seattle and Boston bring cooler, cloudier conditions. Miami and Monterrey bring heat. Houston brings … a rodeo to town, which presents its own surface challenges.

But for players, the ball should roll the same, a plant foot should hold the same and a pass should bounce the same.

The field prep began nearly five years ago when FIFA partnered with turfgrass experts at the University of Tennessee and Michigan State.

At the time, the researchers did not yet know the full map of the tournament or the exact shape of the challenge. As the venues came into focus, so did the scale of the assignment, which FIFA has only described as a multimillion-dollar undertaking.

John "Trey” Rogers, a Michigan State professor and a turfgrass expert, recalled wincing with the announcement of each new venue.

"‘There’s three dome stadiums. No, there are four. No, there’s even more,’” he said.

The question was not simply what grass would look best to millions of viewers across the globe. It was what surface could be grown, transported, installed and maintained across North America - then asked to perform under World Cup pressure.

"It was hard to comprehend how big it was,” said John Sorochan, a turfgrass scientist at Tennessee.

The project reunited two turfgrass scientists whose careers have overlapped for decades.

Rogers helped develop the temporary natural-grass field used inside the Pontiac Silverdome during the 1994 World Cup. Sorochan was his student then.

Until then, a World Cup match hadn’t been contested indoors on natural grass before. "The holy grail has always been: Could you do something inside?”

Rogers said. They pulled it off, helping prove that natural grass could be brought indoors and survive for several World Cup matches.

Understanding root behaviour 

The science has evolved since then. The indoor covered stadiums have grow lights, ventilation systems, carefully designed rootzones.

"There’s a lot more tools in the toolbox,” Sorochan said.

For this World Cup, Sorochan said researchers looked at the stadiums individually, then grouped them by the challenges they presented: domes, outdoor synthetic-turf conversions, existing grass venues and the altitude of Mexico City.

Every additional grass species would make consistency harder. So the World Cup surfaces were narrowed to two main systems: Bermuda grass for warmer climates and a mix of Kentucky bluegrass and perennial ryegrass for cooler ones.

"When you add another grass,” Sorochan said, "that just adds another variable.”

Together, the stadium fields comprise a total 88 square miles of natural sod.

The fields were grown on plastic.

For a conventional field, roots grow down into soil and are cut when the sod is harvested and moved to a stadium. But the variety of existing surfaces in World Cup stadiums demands a different approach.

Growing on plastic changes the root behavior. The roots grow down, hit the plastic barrier and turn sideways, forming a dense mat.

That allows the sod to be harvested with much of its root system intact. Once the sod is moved into the stadium and placed on sand, the roots begin growing downward again.

The World Cup fields started at one of nine sod farms: six in the United States, two in Canada and one in Mexico.

Some grass was seeded about a year before installation. Rogers said a Kentucky bluegrass/perennial ryegrass field grown on plastic can be ready in about nine to 11 months.

"We can now cut it like a pizza and roll it up,” Sorochan said.

A typical piece of sod is about 3½ feet wide and 30 to 45 feet long. The rolls are loaded into refrigerated trucks, much like produce headed to a grocery store.

"It just takes a lot more trucks,” Sorochan said.

The grass then goes on a long journey.

The pitch for the final traveled from Carolina Green Turf Farm in North Carolina to the Meadowlands in New Jersey. Grass from Colorado is now in Atlanta, Dallas and Houston. In Mexico, sod grown north of Monterrey faced a roughly 10-hour ride to Guadalajara.

The timing matters because the cargo is alive. Too much heat, weight or time can stress the sod before it reaches the stadium.

When the sod arrives, crews are not just rolling out a green carpet.

At some venues, artificial turf came out and the field was built over concrete. At others, the existing surface was protected, covered and topped with layers: drainage, geotextile fabric, sand, irrigation and reinforced sod.

The benefit of sod grown on plastic is that it arrives mature and stable, not as a fragile new planting. Rogers said fans might be surprised how quickly a surface can be installed and used.

"You’d be surprised how many games you’ve watched in the National Football League got laid on a Thursday and the game on Saturday or Sunday,” he said.

He likens the installation process to a gourmet meal moving from ingredients to kitchen.

"Eventually, we’ll turn it over to the chefs,” he said, "and away we’ll go.”

To a turfgrass scientist, the magic lies beneath the surface.

Surface and playability 

FIFA’s detailed specifications call for 100 percent natural grass, typically grown on a sand-based rootzone designed for drainage and stability. The grass is reinforced - either through stitched synthetic fibers or a hybrid backing - so it can handle elite-level cutting, sliding and sprinting.

In permanent systems, that can mean 12 inches of sand above a 6-inch gravel drainage layer. In temporary systems, the profile is shallower, often with 6 to 10 inches of sand and a plastic drainage module beneath. Some systems also include vacuum and ventilation components that pull excess moisture out and move air through the profile.

The grass is the visible part. The field is the system underneath.

In domed venues, field experts also have to replace the sun.

That means grow lights - sometimes a lot of them. Some stadiums can light the entire field at once. Others may have to light one section, move the equipment, then light another. In Dallas, a system of lights is suspended from the roof and lowered closer to the field.

The lights are not just to keep the grass green. They help the plant produce energy, recover from wear and hold up through weeks of World Cup traffic.

At Tennessee, Sorochan’s team used a soccer-ball launcher, high-speed cameras and video analysis to study how the ball behaves when it strikes the surface. In one test, he said, researchers fired balls into the grass at 55 kilometers per hour and a 17-degree angle, then measured whether the bounce stayed low enough for a player to control it - ideally, not rising above the knees.

A long pass should hit the surface and rise predictably. It should not jump over a player’s control in one city and skid differently in another.

The goal, Sorochan said, is simple: "We don’t want the surface to compromise the playability of the game.”

When the matches begin, most fans will watch the players.

Rogers will watch the ball roll.

In American football, he said, the ball hitting the ground usually means a play is dead. In soccer, the ground is part of every play. The pitch is in every pass, every touch, every cut, every slide. Across the tournament - on similar fields in 16 dissimilar stadiums - Rogers is hoping his fields perform so well that no one feels the need to mention it.

"Silence is golden for me,” Rogers said.