River Farm was George Washington's northernmost farm. It now serves as the Headquarters of the American Horticultural Society. The gardens are open to the public from April through September, Mondays-Fridays, 9 a.m.-5 p.m., Saturdays from 9 a.m.-1 p.m. They are closed on national holidays. Gates that stood at the White House for nearly 100 years mark the entrance to the main house.