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New Trail For Washington Park Arboretum To Mitigate SR-520 Construction

After multiple full-weekend closures for final work rerouting and restriping, the new 520 bridge across Lake Washington should improve traffic flows.

Another side effect of the new construction will be better access to parts of one of Seattle’s most loved parks. The Washington Park Arboretum that borders on the Seattle side of the bridge is losing nearly five acres of land to the wider bridge and staging work as it has been built. But it's also getting millions of dollars in mitigation funding. 

In fact, the $7.8 million dollars in mitigation money from the Washington State Department of Transportation is the largest single amount the arboretum has ever received.

Curator Ray Larson, who was hired using some of that money, recently provided a tour via electric cart along the arboretum's iconic Azalea Way. The 80-year-old path through the park is part of the original design by the legendary Olsen brothers. It's currently lined with blooms in a rainbow of colors that frame vistas in multiple directions. He paused to point out the ash collection on the west side of the park.

“But you can see that it's really wet right now, so people can’t get over there very easily,” Larson said.  

For most of the year, even when the sun is shining, the wetlands that dominate most of the park make it hard to see much without losing your shoes in places where there isn’t pavement or gravel, Larson says.

So they’re adding 1.2 miles of trail and several footbridges to form a loop. This is part of a longstanding master plan for the arboretum. Larsen expects it will make the park feel a lot bigger.

Credit The Berger Partnership
The Berger Partnership
The Arboretum Loop Trail is outlined by the thick white and black dotted lines. The white portion is the new section of trail, while the black portion is the existing Arboretum Drive, which will connect to form a 2.5-mile loop.

“I think people ... when they walk through, they’ll realize how much more space there is that maybe they haven’t explored," he said, adding that the timing is great because Seattle is currently growing so rapidly. 

"It’s really going to open up a lot of areas for people. Even though we’re not adding land, we’re going make it a lot more accessible for everybody,” Larson said.  

They’re also daylighting Arboretum Creek, which runs through the heart of the park, but is currently mostly invisible in underground pipes and culverts. 

Credit Ryan Garrison photo / University of Washington Botanical Garden
University of Washington Botanical Garden
Grand Fir- Abies grandes - Along Arboretum Creek

“People look at creeks now differently than they used to. And so we’re adding woody debris and pools and things to slow the water down and store it. And then, quite a diversity of plantings. About 20,000 plants will be added as part of the mitigation effort,” Larson said.  

Trees are being replaced at ratio of 6 to 1, under the plan. And older trees that have been hard to reach should be easier to visit, such as a 130-foot noble fir that's a staff favorite. It predates the Arboretum and can be hard to get to now, Larson says. 

And the partially built freeway “ramps to nowhere” that were installed in the 1960s and never used for traffic are being taken out.


Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to