With nearly 1,800 trees across the Smithsonian campus, Smithsonian Gardens (SG)’s Tree Collection is one of the Smithsonian’s most visible, widespread, and accessible collections. Over 25 million visitors each year have an opportunity to engage with the Tree Collection, a significant visitation number which presents both a wealth of opportunities and some unique challenges.    

Tree root cells need oxygen, just like other living entities with respiring cells. A healthy soil for plant growth in the mid-Atlantic region is typically about half solid (soil particles) and half porosity (air-filled macropores and micropores). Porosity allows for both water and oxygen retention. However, soil structure is fragile and can be easily damaged as visitors stray off defined walks and onto the soil. With heavy pedestrian compaction, that porosity is crushed out of the soil, eliminating necessary reservoirs of oxygen that trees need for healthy root growth. Without oxygen in the soils, trees cannot thrive.   

This fall, Smithsonian Gardens completed a multi-year project to help preserve the Smithsonian Gardens Tree Collection along Constitution Avenue outside the National Museum of Natural History. This project, funded through the Smithsonian’s Collections Care Preservation Fund, addressed care for the historic Constitution Avenue elms through remediation of their damaged soils. 

In addition to being living collection items and critical green infrastructure, the Constitution Avenue elms are also iconic components of the historic and cultural landscape of Washington, DC’s Monumental Core. At the turn of the 20th century, when the National Mall and Monumental Core were being comprehensively designed, the Senate Park Commission and McMillan Plan specifically identified the American elm as the tree to underpin this effort: “The American elm was chosen not only because of the architectural character of its columnar trunk and the delicate traceries formed by its wide spreading branches, but also because in the District of Columbia this tree is at its best. . . “ (Senate Park Commission, 1902). 

This preservation project included three components to restore the trees’ soils and provide the highest quality housing for these living collections in both the short- and long-term: street tree fencing (to protect the soils from unintended pedestrian damage); soil remediation (to improve soil conditions for tree growth); and native groundcover plantings (to protect and enhance the soils with designed native plant communities). 

The project included the design of a campus-wide protocol for a signature street tree fence with an arched design that references the historic elm trees’ open, vase-like canopies. This new fence design was reviewed and approved by a number of external review agencies including the National Capital Planning Commission, Commission of Fine Arts, DC Historic Preservation, and Interagency Working Group.  

Soil remediation to aerate the compacted soil, reintroduce nutrients and organic matter, and increase water infiltration played a critical role in the project. Vertical mulching with a pneumatic air excavator was used to de-compact the soils without damaging tree roots. The soils were then backfilled with organic-rich compost that added welcome nutrients. 

Following soil remediation, Smithsonian Gardens installed 15,000 native plants in the street tree planting beds. These plants serve many purposes, including discouraging pedestrian traffic, improving soil structure, enhancing soil aeration and drainage, providing ecosystem resources to wildlife, and beautifying the Smithsonian’s urban streetscape. The use of interplanted native plants in this context is particularly compelling because it embodies an emerging trend in contemporary horticulture: designed native plant communities. 

This pilot project is the first of its kind in Washington DC’s Monumental Core. It will both prolong the health and life of these living collections and serve as a model for stewardship of healthy urban trees and urban forests throughout DC’s Monumental Core and beyond.