If you ever visit Washington DC, you should stop by and visit the US Botanical Gardens. It's a short walk from the White House.
Started in 1850, the United States Botanical Gardens is one of the oldest botanic gardens in North America. It houses about 26, 000 plants, including medicinal plants, orchids, and cacti.
The Conservatory, built in 1933, houses a jungle, desert, and primeval paradise.
Outdoors, the National Garden features a variety of mid-Atlantic plants.
The gardens are open 10-5 daily, including weekends and holidays. Admission is free.