The other Saturday garden highlight in DC were the gardens around the Smithsonian Castle.
The Haupt Garden is behind the Castle, and actually on top of the Sackler and African History museums, which are built underground. It's split into three sections, including a Moongate Garden, based on a garden in Beijing, and a Fountain Garden, based on a garden in the Alhambra. (It was raining so hard when we were there that they'd blocked off the Fountain Garden due to slight flooding.) The main part of the Haupt is a formal green area, and they've surrounded it with planters filled with exotic plants. It's a really heat space to wonder through.
The Folger Rose Garden is on the Mall side of the buildings, between the Castle and the Arts and Industries building. It's still a bit early in the season, but there were roses out to see.