Botanic Gardens Conservation U.S.
From the review: The goal of Botanic Gardens Conservation International US is to collaborate with numerous botanic gardens in the country to "raise awareness and scientific understanding of threatened plants in North America." and "The site features two unique databases with global scope: GardenSearch!, a directory of 3,199 botanical gardens, and PlantSearch!, a database of plants, especially rare plants, maintained by botanical gardens (1,252,417 records at 1,000-plus institutions)