The renovated Friendship Garden reflects advances in perennials since the early 1990s, when the first garden was planted. Giant coneflowers and wild quinine bloom in front of the Garden House shed. (Adrian Higgins/The Washington Post)

Time is the fourth dimension of gardening, and sets landscape design apart from other art forms.

In a new garden, the plants go in young and reach an optimum state of scale and ornament after a few short years. Even with expert care, the original design cannot remain static.

When a garden gets old and neglected, its lines are blurred by its disorder. This entropy takes many forms. Trees cast gathering shade, shrubs elbow out lesser plants, some plants die off, and others invade territory. The garden’s architecture — the hardscape — decays. The unfed soil loses its vitality.

I have observed this decline at many familiar gardens over the years, including a demonstration garden around the National Arboretum’s Arbor House, a ranch-style structure, formerly a house, now used as a gift shop and offices. The original garden, named the Friendship Garden, was planted in 1991. It was funded by the National Garden Clubs, which also paid for an intern to care for it. But after a decade or two, nature took its course.

Several old trees in the one-acre garden came down; some plants petered out while others spread with abandon. The worst was a leafy plant named butterbur, whose botanic name, Petasites japonicus variety giganteus , gives a clue to its ambitions. It loves boggy areas and it spreads by rhizomes. It became, by default, the dominant perennial, particularly after the trees went and the sunlight flooded in.

The renovated Friendship Garden reflects advances in perennials since the early 1990s, when the first garden was planted. This section includes giant coneflower and wild quinine, both in bloom, and drifts of July-flowering black-eyed Susan, variety deamii. (Adrian Higgins/The Washington Post)

The butterbur colony required a backhoe to extract, and its runners and roots, once piled on the back of a truck, were so thick they looked like “great big hoses,” Ellen Spencer said. Spencer is the chairman of a $130,000 project by the National Garden Clubs and its local branch, the National Capital Area Garden Clubs, to restore the garden.

It was replanted (and re-turfed) last year and dedicated in May. Three large beds radiate from the back of the Arbor House and are full of extravagant sweeps of perennials. Some of them are large plants, or planted in such numbers as to have visual punch. The agastache Purple Haze is a haze of purple; the giant coneflower, with paddle-shaped blue-green leaves, is just revealing the petals of its golden daisies. The handsome and astonishingly underused native plant known as wild quinine is producing clusters of white flowers. The bees can’t wait.

The fresh and exuberant display of sophisticated perennials, grasses and shrubs would be reason enough to visit the garden. But what makes the Friendship Garden truly compelling is that it was created to show that the domestic landscape didn’t have to be one of passive lawns, foundation shrubs and bedding plants.

The original Friendship Garden and another, built in 1986 on the other side of the Arbor House, were designed by D.C. landscape architects Wolfgang Oehme and James van Sweden to demonstrate their naturalistic approach.

They had been recruited by the arboretum’s director, Marc Cathey, who also wanted to move the residential model beyond turf and shrubs.

Cathey said he believed that the shift to perennials and grasses could make the garden come alive while reducing the use of pesticides and fertilizers.

Oehme and van Sweden were convinced of it, and demonstrated it through their work (prominent clients included Oprah Winfrey) and a series of books. Cathey, a consummate showman, dubbed the initial garden “the New American Garden,” and the name stuck as shorthand for gardens that were inherently more herbaceous, dynamic, multi-seasonal and naturalistic, with romanticized echoes of the prairie.

The Friendship Garden was shadier but in its pioneering spirit was much the same.

Both gardens “were very important because other than the Federal Reserve [garden], this was one of the early outlets people could come and understand what the New American Garden was,” said Eric Groft, vice president of Oehme, van Sweden & Associates. He worked with arboretum horticulturists in restoring the Friendship Garden. (The firm’s founders have since died, as has Cathey.)

As the original garden waned, the innovative idea behind it survived and evolved. The new garden reflects the sunnier site conditions but also the subsequent shifts in horticultural emphasis. It is these differences that make the Friendship Garden particularly interesting.

The native perennial named rattlesnake master (Eryngium yuccifolium) readies to bloom. It is grown for its foliage effect, however. (Adrian Higgins/The Washington Post)

The garden uses current design techniques of interplanting perennials such as the coneflower Green Jewel with little bluestem grasses. (Adrian Higgins/The Washington Post)

In general, top gardeners today are more ecologically minded, use more native plants and have an aversion to invasive ones such as butterbur. There is a greater emphasis on harboring pollinators and other wildlife.

This environmental sensibility is also pushing designers to mix plants in a way not done in the 1980s and 1990s, when the preference was to assemble them in discrete drifts. The advantage is that the Friendship Garden has a more seamless display through the season — the skill is to mix plants that are compatible. In one bed, you find the coneflower Green Jewel mixed in with the grass little bluestem and Eryngium yuccifolium. In another, the wild quinine is integrated with switchgrass and giant coneflower.

What’s gone from Oehme’s bag of plant tricks? Invasive miscanthus grasses are out, replaced with native switchgrasses and purple moor grass. Sedum Autumn Joy has been replaced with Autumn Fire. Rudbeckia Goldsturm grew sickly (not to mention trite) and is now bettered by the more robust Rudbeckia fulgida var. deamii . Oehme loved a clumping bamboo named fargesia, but it was programmed to self-destruct. It is monocarpic: When it bloomed, it died, and no one bothered to replace it.

Scott Aker, the arboretum’s head of horticulture and education, said the new garden, like the old, is meant to inspire visitors to try new plants in their home gardens. Did the original vision hold water? “I think so,” he said. “It moved everybody beyond the landscape that was turf, bushes and bedding plants. What this garden and this movement did for perennials is truly amazing.”

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