Understanding the current state of tree diversity within the United States is imperative to protect those species, their habitats, and the countless communities they support, as well as the ecosystem services they provide. Despite the U.S. having ample resources and a relatively well-studied flora, before 2022 there was not a universally recognized, up-to-date, comprehensive checklist of U.S. tree species based on the standardized definition of a tree for the Global Tree Assessment initiative, nor were U.S. trees well represented on the International Union for Conservation of Nature (IUCN) Red List of Threatened species. Through a collaborative, cross-sector partnership including botanical gardens, NGOs and government agencies, we created an updated checklist of all tree species native to the contiguous 48 United States, created over 700 new or updated IUCN Red List assessments and NatureServe Global Ranks and developed a replicable assessment data-sharing process between the IUCN Red List and NatureServe. These results lay the groundwork for tree conservation efforts in the U.S. that contribute to achieving critical national and global conservation targets, including the Global Tree Assessment initiative. Fortunately, there are actions we can take in our own gardens that have a positive impact on trees and forests across the country, and can contribute to safeguarding the 11-16% of U.S. tree species that are threatened with extinction.