Over the past year, the University of Washington Botanic Gardens (UWBG) prepared a plan assessing current and future challenges posed by climate change to the Arboretum plant collection, and also outlining some key collection-management measures for mitigating these challenges. The document was researched and written by UWBG associate director and curator Ray Larson and Miro Stuke, a PhD candidate at the UW School of Environmental and Forest Sciences specializing in the impacts of climate change on plants. Funding for the project came from a Stanley Smith Horticultural Trust grant (see below), secured by the Arboretum Foundation.
As UWBG director Christina Owen writes in the Foreword to the plan, Seattle’s temperate, maritime climate has long been the “envy of the horticultural world.” Our historically mild, wet winters and warm, dry summers have allowed us to cultivate a wide variety of plants from around the world. However, over the last few decades, climate change has started to rewrite this script. Increased heat, drought, and extreme weather events have begun to cause noticeable stress to both native and non-native trees and shrubs in the Arboretum and beyond. We’re already seeing lethal impacts on certain plant species and groups, including western red cedar, western hemlock, pines and birches.
The Arboretum’s plant collection contains more than 15,000 individual specimens, repre-senting nearly 5,000 unique species and varieties. Says Christina, “The work presented in this Climate Adaptation Plan is intended to help us better understand the impacts of climatic changes on these vast and diverse collections…Most critically, it identifies the investments and changes we can start making now in order to provide the appropriate horticultural interventions that will allow our plants to continue to thrive in spite of these challenges.”
Declining birch trees along Azalea Way in the Arboretum. A combination of drought stress and insect infestation is decimating European and Asian birches in our region.
Rating Plants for Our Future Climate
To examine the current and future effects of climate change on the plant collection, Ray and Miro used a variety of tools, including the Climate Assessment Tool (CAT), a predictive modelling software co-developed and hosted by Botanic Gardens Conservation International. The software helps botanic gardens figure out the suitability of tree and shrub species to predicted future climate scenarios in their region.
According to the UW’s Climate Impact Group, the mean annual temperature in the greater Seattle area is projected to increase from between 4.5 to 8.0 degrees Fahrenheit by the end of the century, depending on what measures, if any, are taken to reduce greenhouse gas emissions. Predictions also include warmer, wetter winters (with decreasing snowpack), drier summers, more extreme heat, and more extreme precipitation events. Using CAT, they calculated suitability ratings for close to a thousand species in the Arboretum collection based on three differ-ent greenhouse gas emissions scenarios: 1), current climate conditions, with no further increase in emissions, 2) intermediate scenario, with emissions curtailed but still increasing over a 30-year period, and 3) business as usual, with gases increasing at their current rate through 2090.
CAT uses a variety of bioclimatic metrics such as “mean annual temperature” and “precipitation of driest quarter” to rate how well a plant species will perform under different emissions scenarios. Looking specifically at the metrics for the hottest and driest quarters of the year (June to early October in our region), the projections for the Arboretum are not good. It turns out most of our collection plants are already on the edge of what they can tolerate in terms of summer heat and drought, and they are predicted to be outside of their suitability range by mid-century. These include many of the Arboretum’s nationally accredited core collections of maple, magnolia, oak and holly.
Regular mulching is recommended for protecting soil and making our plant collections more climate resilient.
Managing Existing Collections
The report recommends a variety of actions to mitigate against a warming climate and preserve core and other collections. These include expansion and upgrading of the Arboretum’s irrigation systems in order to be able to do targeted watering of vulnerable trees and shrubs. “Most of the Arboretum’s irrigation infrastructure is decades old and inefficient in terms of coverage and water use,” Ray notes. “These should be audited and upgraded to maximize efficiency.”
Caring for soil is a critical part of growing healthy plants, and the adaptation plan devotes a lot of attention to managing the Arboretum’s mostly heavy, clay soils, which create drainage problems in the wet months and turn dry and hard in summer. Ray recommends various solutions, especially for larger garden areas and new garden development. Some areas can be improved over time with mulching, composting, and aeration. In others, such as we see in the New Zealand Forest, planting on berms of amended, well-draining soil is the better option.
Since decreased snowpack in the winter may reduce the water available for irrigation, the plan recommends building in more green infrastructure, such as underground cisterns, that can capture rainwater during the wet months for use in the dry season. Also, creating a well-resourced propagation program will enable the Arboretum to have a reserve of plants to reinstall in the gardens if existing specimens fail due to climate stress.
A well-resourced propagation program will enable the Arboretum to replace plants lost to climate-change-related stress.
Climate Adaptive Collections
For developing the collections of the future, Ray recommends sourcing and trialing plants from regions around the globe with climates that are analogous to what’s predicted for Seattle. He discusses a variety of tools and resources for determining these analogous regions.
For instance, the CityApp tool, developed by the University of Maryland Center for Environmental Science, is a climate-planning model that allows users to forecast what their city climate will feel like in 60 years by matching it to other U.S. urban area that already have those conditions. (For Seattle, CityApp shows the closest analog to be Three Rivers, California, a town in the foothills of the Sierra Nevada at the eastern side of the Central Valley of California.)
Walter-Lieth Climate Diagrams show regional information from around the world and can be used for matching purposes. (The Arboretum used them to select climate-compatible ecosystems in China, Chile, Australia, and New Zealand for the Pacific Connections Garden.) Species from certain drier, Mediterranean-like areas are recommended for trials in our climate, says Ray.
“Regions that show similar patterns offer the promise for finding plant species that will be suitable for the Arboretum long term. Seed collections from species in those areas should be grown and trialed in the Arboretum to examine performance and for inclusion in future collections and garden spaces.”
The full climate plan document is posted on the “History & Planning” page of the UW Botanic Gardens website, along with an Appendix showing the climate vulnerability ratings of Arboretum plants (organized from “high” to “low”). The hope is that the plan will not only help guide plant curation and management at the Arboretum but also offer useful lessons for other gardens and landscapes in the region.
Niall Dunne is the editor of the “Bulletin” and the communications manager at the Arboretum Foundation.
Stewartia malacodendron, seen here along Arboretum Drive, received a “low” climate vulnerability rating in the plan. The species is native to the U.S. Southeast.
Stanley Smith Horticultural Trust
The development of the Arboretum’s “Climate Adaptation Plan” was made possible in part by the generous support of the Stanley Smith Horticultural Trust. Created in 1970 by May Smith, in honor of her late husband Stanley Smith, the Trust supports education and research in ornamental horticulture and the development and maintenance of public gardens and arboreta. We are deeply grateful for the Trust’s support and investment in climate-smart horticulture, and plan to amplify this investment by sharing our findings with regional gardens, stakeholders, and community partners. Learn more about the Trust—and May and Stanley Smith—at https://smithht.org.
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This article is reprinted from the winter 2026 issue of the Arboretum Bulletin, our quarterly botanical and horticultural magazine. Published since 1937, the Bulletin is a benefit of Arboretum Foundation membership and features in-depth articles on the Arboretum collections and history, as well as general information of interest to plant lovers in the Pacific Northwest and beyond. Join/subscribe today.
