The United States Botanic Garden (USBG) is a botanical garden on the grounds of the US Capitol in Washington, D.C.
The Botanic Garden is supervised by the Congress through the Architect of the Capitol, who maintains the grounds of the Capitol. The USBG is open every day of the year, including federal holidays. It's the oldest continually-operating botanic garden in the USA.
Links: www.usbg.gov, en.wikipedia.org