Links United States :

University of Chicago - Medical Herb Garden:

http://nnlm.gov/pnr/uwmhg/

University of California - Berkeley - Botanical garden :

http://botanicalgarden.berkeley.edu/collections/collections_intro.shtml

United States  Botanic Garden :

http://www.usbg.gov/

University Garden Arboreta - Links :

http://www.plantations.cornell.edu/visitor/links_university.cfm

United States Natinal Arboretum :

http://www.usna.usda.gov/

Links "Botanical garden" in U.S.:

http://en.wikipedia.org/wiki/List_of_botanical_gardens_in_the_United_States