The team building the trail took painstaking care to protect the Washington Park Arboretum’s 230-acre collection of 20,000 trees and other plants from around the world.

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Try to imagine building an asphalt trail through the exhibition halls of the Seattle Art Museum — wending your way around paintings and sculptures, or through the Burke Museum — dodging dinosaur bones, carvings and masks.

That’s what it was like to design and construct Washington Park Arboretum’s loop trail, which quietly opened to walkers, runners and cyclists in November. In laying 1.2 miles of pavement, the team building the trail took painstaking care to protect the arboretum’s 230-acre collection of 20,000 trees and other plants from around the world.

The meandering route, lined with 18 benches cast in a classic style from the 1939 New York World’s Fair, runs all the way from East Madison Street to the Graham Visitors Center between Lake Washington Boulevard East and the arboretum’s iconic Azalea Way.

“You can think about the arboretum as a living museum — that’s how we treat it,” said Fred Hoyt, interim director of the University of Washington Botanic Gardens, which manages the tree sanctuary in cooperation with Seattle Parks and Recreation.

The trail descends below Pacific Rim foliage, crosses over Arboretum Creek, stretches through a restored wetland and skirts past sturdy walnuts and oaks.

Ushering visitors into the park from East Madison Street and connecting with the existing Arboretum Drive to create an approximately 2-mile recreational loop, the $7.8 million trail presented parks project manager Garrett Farrell with a number of unique challenges.

Farrell had to avoid tree cutting wherever possible, work with botanical experts, contend with rainy weather and boggy terrain and rebid the project in order to make the budget work during a construction boom, scrapping concrete walls in favor of rockery.

Before breaking ground in 2016, the project’s team designed the trail six times on paper, plotted the route with stakes and poles and blew away a layer of soil to determine where there were important tree roots in harm’s way. Local company Ohno Construction was the contractor.

The team altered the route to work around some trees, moved others and turned those they had to cut into landscape elements and snags where wildlife can thrive.

Including trees planted in the arboretum and around Washington Park Playfield, the complex has gained seven new trees for every tree cut down, Farrell said.

“If we had been building a road, we would have been done a long time ago,” he said. “What we did was bob and weave through the arboretum’s collection.”

That precision was costly, eating up more than $1,000 per foot of pavement in a city where better schools and homeless shelters are needed. But the money in this case came from a mitigation fund for the Highway 520 bridge-replacement project.

“The new bridge is taking another bite out of the arboretum,” Farrell said.

Seward Park and Taylor Creek water projects in South Seattle are among other recipients of Highway 520 mitigation money, according to the state transportation department.

Hoyt says advocates reacted cautiously when the trail was proposed years ago, concerned about damage to trees and other environmental impacts.

The project included construction of a larger parking lot downhill from Azaela Way, adding asphalt in a part of the arboretum where drainage already was a problem.

But it also included the clearing away of blackberry bushes, the installation of bioswales and the daylighting and restoration of a section of Arboretum Creek.

Mallard ducks paddled there on a recent afternoon, soaking up rays of sunlight that reach the new stretch of trail with more intensity than along shady Arboretum Drive.

Three bridges with iron railings cross the creek, which had been buried underground, and the restoration work has opened up new east-west vistas across the arboretum. Even in the winter, the trail is getting a lot of use, Farrell said.

“This used to be an impenetrable blackberry thicket,” Farrell said. “Unless you were super able-bodied, this area was off-limits to you.”

Access is something Jane Stonecipher, Arboretum Foundation interim director, appreciates about the loop. The trail is giving new life to a neglected area and allowing older visitors, who can no longer use the arboretum’s dirt paths, to ride a tram through.

There were concerns that a bikeable trail parallel to Lake Washington Boulevard East would become a thruway for speedsters, so the route is purposefully sinuous.

Signs remind cyclists the speed limit is 10 mph, and Stonecipher believes they can coexist alongside the stroller-pushers, botanists and tree-huggers. A grand opening is planned for April 8.

“I like how we’re bringing different users to the arboretum,” she said. “People can come for the recreation and come back when they realize this is such a unique place.”