The National Mall is in the final stages of a $40 million reconstruction of the iconic central grassy spine of the Mall in Washington, D.C. Consisting of eight grass panels totaling 18 acres, the first three, on the Capitol side, were completed in 2012. The fences are coming off the other five in the coming weeks, although barriers will continue to keep people away from the grass to allow its root system to grow over the next few months.
The grass panels — each is more than two acres in area — will be opened (with protection) for the presidential inauguration next year but won’t be accessible until spring, said Michael Stachowicz, the National Park Service’s turf guru. Stachowicz was hired in 2012 to oversee the nation’s most conspicuous front yard, which includes the area between the Lincoln Memorial and the Capitol. He had previously worked on golf courses.
Ironically, as the science and practice of turf care advanced over the past three decades, the condition of the Mall deteriorated to a point where standard lawn renovation techniques - aeration, dethatching and reseeding - couldn’t possibly reverse the decay.
“Everyone has a vision of the National Mall they see from a helicopter, and everything looks wonderful,” said Teresa Durkin, senior project director of the Trust for the National Mall. “It’s not until you get down and walk around and see the impact of the sheer use - 3,000 permitted events, sometimes up to 33 million visitors per year.”
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The resulting bare patches had the density of cinder block, and even the weeds had difficulty surviving.
“I couldn’t believe the weeds were even growing there,” said Peter Landschoot, a professor of turf-grass science at Penn State and an early consultant on the project, which began in 2007. “It was in pretty bad shape.”
The central grass panels are flanked by other lawns shaded by American elm trees. Together they form the grand avenue that connects the monumental core of the nation’s capital, laid out in the 1902 blueprint known as the McMillan plan. But the Mall also functions as America’s common, a place of public gathering, for entertainment, for demonstration, for great civic events. That has always been the underlying tension and one, post-restoration, the Park Service is trying to balance more in the lawn’s favor. A new plan for the Mall, along with a Park Service operating manual, is designed to minimize the damage caused by human activity.
The key to the new turf’s vitality is the dominance of coarse sand in the mix, which will promote a vigorous root system. The sand resists physical compaction but also contains much more oxygen than a regular loam soil, which roots need to grow, said Norman Hummel, a consulting soil engineer who worked on the project.
“You get a better, aerated soil,” he said. Could we try this at home? Not easily, and not on this scale. Sandy soil dries out quickly and doesn’t hold nutrients in the same way as clay or loam soils. To build a Mall-like system at home would require deep pockets. “In some situation where somebody with a lot of money wants an event lawn on their property, they’ll go to this type of system, but for the average homeowner, it’s kind of tough,” Hummel said.
One of the most remarkable aspects of the turf renovation is that so much of the work is not obvious or visible. Peeling back the layers offers lessons in landscape architecture, soil science and turf-grass breeding.
Protecting the Mall from abuse will be key to its longevity. So the grass panels are framed for the first time in a light granite curb, and the paths that crisscross the spine of the Mall have been broadened to keep feet, equipment and event structures off the grass as much as possible. The old paths were approximately 45 feet wide, and the new design increases them to about 60 feet. Above the 12th Street tunnel, the paved area has been broadened to 105 feet.
Soil: The old soil - lifeless dirt, really - was scraped off and trucked away. After the subterranean infrastructure was built, the panels received a four-inch-deep sublayer of pea gravel, on top of which was laid a soil mix that consisted of 80 percent sand blended with 10 percent topsoil and 10 percent peat. Each panel received about 3,000 tons of new soil mix.
Grass: Direct seeding is the preferred method of establishing a new lawn, but the Park Service team laid sod so that the work could progress at times of the year when seeding isn’t optimal, and to be ready for its first big event: January’s inauguration.
The grass mix was customized for the Mall and grown from seed at a turf farm in southern New Jersey. The farm was selected for its native sandy soils, to match the Mall’s engineered soil. The grass consists of 90 percent by volume turf-type tall fescue varieties and 10 percent Kentucky bluegrass. However, because bluegrass seed is much smaller, the seed mix is roughly equal parts bluegrass and fescue.
The designers picked grass varieties bred and tested for superior wear and drought resistance in the mid-Atlantic region. Bluegrass, though harder to maintain, is valued for its ability to fill in bare spots in a way the clumping fescues cannot.
Drainage: Good drainage is essential for healthy turf and soil vitality, and the new system allows the grass to receive as much as four inches of rain without waterlogging. When the water load in the soil reaches a certain level, the suspended water drops into the gravel layer below. “It can actually be drier a day after a heavy rain that causes a flush than a lighter rain that doesn’t,” Stachowicz said.
Each panel is edged with French drains, as well as a grid of drainage lines laid 15 feet apart and four feet deep, out of the range of tent stakes. Subterranean columns of gravel connect the turf to the buried pipes. Rain and irrigation water is collected and stored in four new 250,000-gallon underground cisterns. This conserves water but also minimizes stormwater runoff. The water is filtered and disinfected with UV treatment before reuse in the irrigation system.
Irrigation: The grass panels have three rows of powerful sprinkler heads that can throw water as far as 90 feet each. The system is linked to an automated weather station that monitors such data as temperature, humidity, wind and soil moisture.
The new grass panels will receive an aggressive level of maintenance to counter the effects of usage and the natural stresses facing cool-season grasses in hot, humid Washington. The maintenance regime, now employed on the three panels of Phase 1, will be applied as well to the remaining five panels.
Mowing: The new Mall turf will be mowed at least twice a week during the growing season. Frequent mowing (at the right height) encourages the grass plants to put their energy more into root development than top growth. .
Watering: By watering well and then allowing the soil to dry, you encourage the grass to develop deep roots.
Overseeding and aeration: Overseeding is the term for spreading fresh seed on established lawn. One thinks of overseeding as an annual or semiannual step to lawn care, but the new Mall grass will get a weekly overseeding from mid-May to the end of October. Each panel will receive some 400 pounds of tall fescue seed per application. Aeration, in which plugs of soil are removed to ease soil compaction, will occur about twice a month.
Weed and feed: In March, April, September and October, crews will spread a chicken manure compost. Every two weeks during the growing season, they will spray a weak solution of urea as a foliar feed. Stachowicz uses a low-impact herbicide - applied at five ounces per acre - that controls crabgrass and other weeds without interfering with grass seeding in the way that pre-emergent herbicides would.
The third element in the turf renovation is a strategy to manage more tightly the events on and around the Mall: The footprint of the Smithsonian Folklife Festival, for example, has been reduced in size and relocated eastward. Event-related closures of Madison and Jefferson drives also allow a move to paved surfaces. Other events have been moved off the Mall. For those that remain, the thinking has shifted from one of fixing damage after the fact to preventing it beforehand, said Stachowicz, whose official title is turf management specialist. This includes keeping vehicles off the grass and requiring pedestrian decking that is translucent, allowing light to reach the grass. “We don’t allow plywood anymore,” he said.
Areas will be closed afterward to allow the turf to rebound from use. When all eight panels are opened next spring, Stachowicz will have more flexibility in juggling areas to open and those to close, he said.
“It’s there to get used,” he said. “But that means we have to give it every advantage we possibly can.”