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The Arnold Arboretum of Harvard University.  
Boston, MA.
http://arboretum.harvard.edu

The Morris Arboretum of the University of Pennsylvania.
Philadelphia, PA.
Morrisarboretum.org

US National Arboretum.  Home of the National Bonsai and Penjing Museum.
Washington, DC.
http://usna.usda.gov

Louisiana State Arboretum.
http://www.crt.state.la.us/parks/iarbor.aspx

Cylburn Arboretum.
Baltimore, MD.
Cylburnassociation.org

Adkins Arboretum.  
Ridgely, MD.
http://adkinsarboretum.org

Tyler Arboretum.
Media, PA.
Tylerarboretum.org