Started in 1850, the United States Botanical Gardens in Washington DC, is one of the oldest botanical gardens in North America. It houses about 26,000 plants, including medicinal plants, orchids, and cacti. The Conservatory was added later in 1933, and contains a jungle, desert, and primeval paradise. Outdoors, the National Garden features a variety of mid-Atlantic plants.
The gardens are open year round from 10:00 AM to 5:00 PM. Admission is free.