U.S. Botanic Garden, Washington, D.C.
The conservatory at the U.S. Botanic Garden is a historic structure dating back to the 1930s. With nearly 29,000 feet of growing space, it has enough room to host a diverse collection of plant life. The conservatory's lineup includes a desert exhibit, an orchid garden, a tropical jungle and rooms with plants native to the Southwest and Mexico. Best of all it is completely free to visit the garden, like almost all of the federally owned attractions in Washington, D.C.
There is also a children's garden, a medicinal plant room, a collection of rare and endangered plants and a garden that showcases species that date back to prehistoric times. Both the outdoor gardens and the conservatory are open every day of the year, including holidays.