Newly laid turf along Melisande’s Allee offers just a hint of a major waterline infrastructure renewal at Dumbarton Oaks in Georgetown. (Adrian Higgins/The Washington Post)

You may view the garden at Dumbarton Oaks as a fine historic landscape. I think of it more as an oasis for the mind.

Its quiet, shady spaces and highly crafted neoclassical character offer an environment where calmness prevails and the clamor and noise of the world melt away. It’s a good place for soul searching.

Those of us who are drawn to landscape architect Beatrix Farrand’s design masterpiece have been cooling our heels since last summer, when the garden closed for major infrastructure repair. The good news is that it has reopened.

To understand why the garden had to close, you have to expand your vocabulary. I didn’t know what “tuberculated” meant either (and yes, it’s related to the description of TB), but it was what was occurring in the network of underground water-supply pipes that are the garden’s lifeblood. The insides of the 90-year-old cast-iron pipes — three and six inches in diameter — were becoming encrusted and failing.

Yanking the old pipes out and replacing them with modern PVC four-inch pipes may seem fairly simple, except the old network had become covered with the roots of the now-mature trees that bestow Dumbarton Oaks with its sense of age and history.

As a result, new routes had to be worked out for much of the new system. In sensitive areas, workers used pneumatic spades, which employ high-pressure bursts to dislodge soil without harming roots. In a couple of areas, they used directional diggers that tunnel like moles beneath subterranean obstacles.

“We are telling people we’ve had laparoscopic surgery and the scars are visible, but it’s healing,” said Jonathan Kavalier, director of gardens and grounds.

As part of the project, which was planned for years, key water features were repaired and in some cases re-plumbed so that their water recirculates. In addition, extensive storm-water drainage was installed on the north side of the 16-acre estate to help with problems of erosion and siltation that have afflicted adjoining Dumbarton Oaks Park for decades.

Visitors will find areas where the work is still obvious, but much of the project is already hidden. Corridors of newly laid sod signal the direction of new waterlines, but even there you have to be alert to the darker shade of the grass.

“This was a war zone,” said Kavalier, as he led me into the Ellipse, famous for its aerial hedge of clipped hornbeam and central fountain. “And now you wouldn’t know it.” You can just about make out the seams in the new turf of the lawn. The fountain’s outer structure was dismantled. Its central column, with its spouting faces, was re-plumbed, and the stonework repaired and cleaned.

The two water basins in the Fountain Terrace were converted to a recirculating system, as was the enchanting Lovers’ Lane Pool nestled in the corner on the eastern edge of the garden. Previously, the pool’s water was stagnant except during storms, when the overflow drained away. The pool is framed by an amphitheater whose brick paths converge to form a walk through Melisande’s Allee, a tree-lined hillside meadow of grass and spring bulbs.

The brick path was lifted to make way for a waterline. The antique bricks were reset in fresh mortar after cleaning. The narrow, sinuous path is bordered by virgin turf that in time will become integrated into the surrounding meadow.

The new supply line restores water across the garden and avoids the problems gardeners increasingly faced with lumbering hoses over large areas. In addition, the unreliability of the old system had made the desire to extend lines into new areas inherently unfeasible.

Functioning parts of the old system were left because they were too inaccessible or in areas too sensitive to dig up. The garden’s horticulturists, led by Kavalier’s predecessor, Gail Griffin, helped shape the final layout of the system to minimize harm to plants and landscape. The project manager was Alan Dirican, director of facilities, and the general contractor was Whiting-Turner. Workers installed more than a mile of replacement pipe but also added new connection boxes for the gardeners to hook up sprinklers and hoses. More than 50 are now positioned around the garden.

The pneumatic spade allowed technicians to create trenches that ran below exposed surface roots of old trees and shrubs. It’s slow and dirty work — soil particles fly about — but trauma to the roots is minimized.

The most obvious sign of the work can be found on the garden’s northern fringe, where gutters and drains direct storm water to buried catch basins. Excavated areas have been draped in biodegradable fabric to hold the soil while vegetation grows back. This area is home to a stone pavilion called the Catalogue House. Kavalier and his colleagues are in the process of installing a small exhibition in the structure explaining the nature and scope of the work.

The post-project cleared swaths are most pronounced through the nearby hillside garden known as Forsythia Dell. It is dominated by forsythia, an old-fashioned shrub that brings a garish splash of yellow in early spring. It grows happily in poor soil and holds slopes together.

In the wake of the project, you can see forsythia cuttings rising like little antennae from the landscape fabric, and within a year or two they will cover the ground. Forsythia is so hard to kill that twigs of it, stuck into the ground by the side of the path to act as a barrier, have simply burst into growth.

They are a metaphor for a garden bouncing back from major surgery, and for a spring in Washington that has been painfully slow to get going. But the growing season is back, and so is this beautiful garden in Georgetown. “It was hard for everyone to go through here, and kind of scary for the gardeners,” Kavalier said. “But it was successful and will help things in the long run.”

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