A Missing Walnut, a Mortarless Wall and a Closed Walkway: The Arboretum's Year of Repair

This summer, horticulture staff at Washington Park Arboretum saw that the upper crown of a black walnut in the bee meadow, just south of the Graham Visitors Center, was dying back fast. Plant Health Specialist Dr. Ryan Garrison suspected a specific cause. Arborist Lincoln Erbeck climbed up and took live tissue samples from the top of the tree, and Garrison drove them to the Washington State University Plant & Insect Diagnostic Laboratory in Puyallup. The tests confirmed his guess.

That tree is gone now, and the stump has been ground out. On a walk past the Visitors Center you'll find open ground where it stood. A lot of the Arboretum's 2026 looks like that. A tree came out, a stretch of shoreline went behind a fence, and part of a trail closed. In each case, staff did the work before the damage spread.

The bee meadow walnut appears to be a first for Western Washington

The diagnosis was Thousand Cankers Disease. The Arboretum Foundation's October 15 report quotes the Arboretum's Manager of Horticulture, Joanna Long, on what came next:

"The tests confirmed the diagnosis for us. This appears to be the first documented case of Thousand Cankers Disease in Western Washington. We alerted the US Department of Agriculture, which is also testing a sample now. We also removed the tree and ground up the stump."

The disease comes from two organisms working together. The fungus Geosmithia morbida is carried by the walnut twig beetle, a small red bark beetle about the size of a sesame seed that originally came from the U.S. Southwest and northern Mexico. Adult beetles fly in spring and summer and bore into walnut bark to lay eggs, bringing fungal spores in with them. Cankers then form in the inner bark and disrupt the tree's flow of nutrients. According to the Foundation's report, a tree can die in as few as two years after the first symptoms appear.

The disease isn't new to the state. WSU reported it near Prosser in the summer of 2008, and the Pacific Northwest plant disease handbook lists it as detected in eastern Washington. What changed this fall is that a case turned up on the west side of the Cascades, a few minutes' walk from Madison Park.

Black walnut takes the worst of it. The Foundation notes that the species is native to eastern and central North America and was once a popular ornamental, grown for its foliage and its edible nuts. It isn't planted much around here anymore, but large old specimens still stand in yards and parks. Long put it plainly: "There are many black walnuts growing in the Seattle area, and I'm afraid we may start seeing more cases of TCD soon." She added that there are no known treatments.

Garrison has checked every susceptible tree in the Arboretum's small walnut collection and cleared most of them. The exceptions are several walnuts out on the water along the WSDOT Peninsula. They show symptoms, and staff say they don't have enough access to remove them. In the same report, Long lists the signs a homeowner can watch for on a black walnut:

• Brown, wilted foliage at the top of the tree, followed by branches dying
• Small cankers just under the outer bark of affected branches
• Tiny beetle entry and exit holes on dead or dying branches

If you think your tree has it, her advice is to have a certified arborist look at it. The Pacific Northwest handbook also advises against moving firewood or logs with the bark still on from infected trees into areas that don't have the disease.

At the Japanese Garden, the fence comes down to a few final days

The bee meadow walnut was taken out to slow a threat. Across Lake Washington Boulevard, the Seattle Japanese Garden spent the year rebuilding something before it could fail. Its $2.8 million project replaces the garden's deteriorating north stone wall and adds an accessible looped walkway at the north end of the central pond.

The ishigaki, or stone wall, was finished on September 10. It's a traditional Japanese dry-stone wall, built without mortar. Walls like it were made for stability and earthquake resilience under castles and temples. This one was built by Suminori Awata, a 15th-generation master stonemason, working with a crew from Green Man Builders. The garden said goodbye to Awata and most of that crew on September 14. Project lead Kyle Schlagenhauf has stayed on with the rest of the crew to build the new pathway. According to the garden, it will let people using wheelchairs or pushing strollers reach the shoreline area for the first time.

The garden closed completely for five weeks this summer and reopened July 28. It has been about 80% open since then, with the construction zone at the north end, near the Yamasaki Pine. The garden suggests the southern sections for anyone who wants a quieter visit while work continues.

The 2026 season ends Sunday, November 29, and the garden stays closed through the winter. From this Thursday, that leaves a little over a week of open days.

If you go before November 29, you'll see the finished stonework while the crew is still building the new pathway at the north end of the pond. The garden has also posted a photo gallery of the wall going up, from bare stone to finished courses.

The same institution, under a name that's ten months old

The walnut removal, the wall and the plant sales now come from one organization. On January 9, 2026, the Arboretum Foundation and UW Botanic Gardens announced they would combine as Seattle Botanic Gardens. The merger brings the Foundation's fundraising, advocacy and volunteers together with UW Botanic Gardens' horticulture, programs and conservation research. The September 19 Fall Plant Sale at the Graham Visitors Center was the first sale held under the new name, and it raised more than $4,300.

The combined staff is also behind the third gap on the map. Phase 1 of the Waterfront Trail restoration is rebuilding the Foster Island floating walkway, and part of that trail is closed during construction. When construction started, Seattle Parks expected Phase 1 to open in early 2027. Phase 2 design work is focused on Marsh Island. For now, the Foster Island loop that many walkers count on has a section they can't use.

Madison Park's own shoreline is in the same kind of cycle. Friends of Madison Park reports that its October 24 Party for the Park raised more than $117,000. The money goes to free community events, neighborhood beautification and the next phase of the Bathhouse and shoreline restoration.

Where the Arboretum and the park open back up in December

Once the Japanese Garden closes, the year's repair work gives way to a winter calendar, and several of the dates fall on the same days in the Arboretum and down on Madison Street. Many of the Arboretum events are fundraisers for the same organization doing the repairs.

1. Monday, November 23 to Monday, December 7. Friends of Madison Park is promoting the Wellspring Family Services toy drive, with donation bins at Red Wagon Toys and Parkshore Senior Living.
2. Sunday, November 29. It's the Japanese Garden's last day of the season. That evening, Bamboo Thai has live music from 5 to 8 p.m.
3. Tuesday, December 1. Seattle Botanic Gardens hosts a free virtual Plant ID Basics class for Giving Tuesday, from 5:30 to 6:30 p.m.
4. Saturday and Sunday, December 5 and 6. Gifts & Greens Galore at the Arboretum sells botanical decorations, greens and locally made gifts. On Saturday the 5th, you can also catch Winter Wonderland crafts at the Madison Park Bathhouse from 3 to 4:30 p.m. and then the 5 p.m. tree lighting by the playground, where McGilvra Elementary students lead the singalong.
5. Saturday, December 12. An outdoor holiday artisan market fills the Bank of America lot at Madison and 41st Avenue from 10 a.m. to 3 p.m.
6. Friday and Saturday, December 18 and 19. The Winter Solstice Luminaria Walk starts from the Graham Visitors Center, with entry at 6 or 7:30 p.m. It follows a one-mile lantern-lit route with a light installation, music, hot drinks and cookies. Tickets are $30 for general admission, $25 for members and $10 for kids under 12. Earlier on Saturday the 19th, Santa photos run at the Bathhouse from 12:30 to 2:30 p.m., and the Christmas Ships and a beach bonfire follow from 2:35 to 2:55 p.m.

The full neighborhood schedule is on the Friends of Madison Park holiday page. The lantern route starts from the same Visitors Center as the bee meadow, so walkers on the solstice nights will pass close to where the walnut used to stand.

At Michele Schuler, we follow how the Arboretum and the Madison Park shoreline are being cared for because those places shape everyday life in the neighborhoods around them. If you're weighing what nearby changes mean for your own property, or what the mature trees on your lot add to it, we'd be glad to prepare your Private Home Valuation.