A Brief Introduction to the Oak Collection of Seattle’s Washington Park Arboretum

[Adapted with permission from the Washington Park Arboretum Foundation from a version that appeared in the Foundation’s Summer 2021 Bulletin.]

The Washington Park Arboretum is an unusual partnership of public institutions, located on the shoreline of Lake Washington in the City of Seattle, State of Washington, USA. The land is owned by the City, and all the trees and the plant collection are owned by the University of Washington, a public university with its main campus a mile or two to the northwest. The City maintains the park functions of the space, and UW Botanic Gardens (a unit of the University’s School of Environmental and Forest Sciences) takes care of the gardens, trees, and plant collection. The Arboretum was founded in 1934 and laid out on a Master Plan prepared by the Olmsted Brothers design firm in 1936 in park land that the Olmsted firm previously included in their design for a “Comprehensive System of Parks and Parkways” for Seattle of 1903.¹

When I was a child growing up near Seattle in the 1960s, my family made a ritual annual stroll with my grandmother down “Azalea Way” in the Arboretum near the peak bloom of spring. The abundance of flowers was impressive, but what caught my eye—even as a kid—was the resplendent golden “English” oak, Quercus robur ‘Concordia’, now part of the Golden-Leaf Group of cultivars, on the uphill bank of the pedestrian way. What exactly was my emotional response to this standout tree, I can’t recall. It is a vivid memory, though, of a vivid tree.

The Arboretum would look very different without this and the scores of other oaks in the collection. When, much later in life and inspired by the late Art Kruckeberg, I wanted to figure something out about oaks, the Arboretum was one of my best teachers. I picked up the now out-of-print booklet, “The Woody Plant Collection in the Washington Park Arboretum” (1994), and scouted out all 74 taxa listed there. Just one of them, Q. garryana, is native to this territory.

The years march on. I’ve been wondering, how are all these exotic species faring in Seattle’s cool, sub-Mediterranean climate? How is the Arboretum using oaks in its research and conservation roles? What’s new or up-and-coming in terms of collection additions or modifications?

Ray Larson, Interim Director and Curator of Living Collections at UW Botanic Gardens, graciously met me over Zoom to talk about some of these questions.

Where did the original collection come from?

In terms of specimen numbers, the familiar, large deciduous oaks of Eastern North America (ENA)—“old war-horses” of urban planting such as Q. rubra, Q. palustris, and Q. coccinea— predominate the oak collection at the Arboretum. (You can see the full list by searching for “Quercus” in the online Collections Database at depts.washington.edu/uwbg/gardens/bgbase.php) Two historical factors are responsible.

First, when the Massachusetts-based Olmsted firm was laying out street plantings for the 1903 plan, they emphasized big trees from ENA. Many of the oldest of the oak specimens are strung along Lake Washington Boulevard, which runs north-south through the Park. For these accessions, original data is quite thin.

Most accessions from that period do not have complete accession number dates (they are instead designated with an “X,” indicating uncertainty). Of the oaks marked with an “X,” most are trees of ENA origin. In addition to the three species mentioned above, there are a good number of Q. alba, Q. macrocarpa, and Q. montana (Q. prinus in the database). Another abundant tree in the early planting is Q. robur, popular of course in parks and public gardens worldwide. (It has proved weedy in Seattle. In my little corner of a neighboring suburb, Q. robur and its hybrid derivatives are by far the most common volunteers.)

The second reason for the predominance of eastern oaks can be traced to early partnerships with other arboreta. When the idea of a collection arboretum was implemented in 1936–38, a call for seed contributions went out in substantial measure to large, institutional sources east of the Rocky Mountains, such as Harvard’s Arnold Arboretum and the Morris Arboretum in Philadelphia. Washington Park Arboretum’s founders laid out a taxonomic arrangement for the new collections, and the oaks were sited in the north-central section of the Park in accordance with the Olmsted Brothers’ Master Plan.

The accession numbers from the late 1930s tell how the range of species from the East Coast, Southeast, and Mississippi Valley filled out the main collection site. Among taxa with the most individual specimens are Q. bicolor, Q. falcata, Q. imbricaria, Q. muehlenbergii, and many specimens of Q. nigra.

Putting these species together, you might think you were visiting the humid forests of Kentucky, Tennessee, or southeast Missouri, where just about all of them converge. However, accessions from this period also included lots of Q. dentata—and a few other selections from East Asia, again emphasizing origins with humid summers!

How have the big trees from continental climates fared?

The big oaks from ENA have grown big—and are still growing! From Ray Larson’s perspective, the original choice to plant, say, five specimens of each taxon rather than maybe two has had some negative consequences. The oak canopy, together with the native canopy of conifers and Acer macrophyllum, is really as much forest now as an arboretum in some places. Even some more recent accessions, like a Q. hypoleucoides from the southwestern U.S. and Mexico, dating to 1968 and added in the main Oak Collection area, has grown tall and narrow.

To extend the variety of the collection, Ray can’t help but contemplate some editing.

This may occur naturally. So far, the translocation of these exotic species to Seattle’s climate has not yielded any widespread early senescence or losses. However, fast growth in Seattle conditions may ultimately be threatening to the humid-summer or cold-winter species. Plus climate change—bringing longer, hotter, drier summers to the region—may take a toll.

How have plants from more intense, Mediterranean-like climates performed?

One of the challenges in the Arboretum is wet soil, and one of the victims, in Ray’s view, was a very old Q. kelloggii—a rare montane California component of the original Olmsted plantings that appears to have succumbed from poor drainage. He suspects some other species with provenance in hotter dry climates, like Q. wislizeni from California’s interior foothills, have—over past decades—lacked the summer heat they need to sustain good health. However, summers are getting hotter, and Ray is interested in building the collection of Mediterranean-climate oaks from diverse provenances to test how they will fare here. Two specimens of Q. suber are doing nicely in the “Mediterranean” section of the Arboretum where soils are faster draining.

What paths forward?

Despite the emphasis here on the early-planted ENA oaks, there is enough diversity of Quercus in the Arboretum to merit recognition as a Nationally-Accredited Plant Collection through the Plant Collections Network of the American Public Gardens Association. Ray Larson is excited about the possibilities of collaboration with other network members in ex-situ conservation of threatened species. This effort is also linked to the Global Conservation Consortium for Oak sponsored by Botanic Gardens Conservation International.

One example is the exceedingly rare Q. acerifolia, found only in isolated locations in Arkansas. The Arboretum received acorns of this taxon two years ago. Another is Q. boytonii, from very limited sites in Alabama, which the Arboretum received in 1999. I stumbled upon the Arboretum’s five specimens of Q. boyntonii by chance in some rough brush about 10 years ago—one wonders whether the wildish, discreet setting was designed to hide this rarity from plant thieves.

Ray has other partners in propagation. For example, he has received a number of seedlings and acorn selections from seed exchanges at IOS Conferences in 2015 and 2018 (through this author). And it is a two-way street: Ray participated in a seed-collection expedition to the Siskiyou Mountains of southern Oregon two years ago with staff from the Kruckeberg Botanic Garden and Heronswood Garden (two other Pacific Northwest icons) and is sharing seeds of the Northwest’s charming shrubby oaks—Q. sadleriana, Q. vacciniifolia, and Q. garryana var. breweri—with public gardens around the country. (It was these species, purchased from the late Mareen Kruckeberg after reading about them in Art’s “Gardening with Native Plants of the Pacific Northwest,” that got me going in the oak world and revived my childhood wonder.)

Ray is currently assessing areas in the oak collection, and other areas with enough sun, for new planting sites. He hopes to plant out many of the new accessions in the coming year.

¹ Seattle is named for a leader of the Indigenous Coast Salish peoples. Soon after European-American settlers first took up residence in the future city’s territory in 1851, the man Seattle and his people were forced to relocate to several other places across Puget Sound or further inland.

_{All photos © Niall Dunne}