On my way to see "Perspectives: Chiharu Shiota" at the Sackler, I saw a sign in the Haupt Garden directing me to see "Paradise in a Pot," a collection of tropical plants. Following the sign's suggestion, I walked around and found an array of tropical plants, many of them unfamiliar to me, at the back of the garden. They will be moved to the Smithsonian's greenhouse in Suitland in the near future; I'm guessing as soon as the weather turns colder. "Limited time only" was the warning on the sign. I didn't realize the Smithsonian had a greenhouse, but they do. It serves as a production facility for plants that are displayed in the museums, and as the home of the Smithsonian Gardens Orchid Collection. I was reminded once again that the museum exhibits that you see on the Mall are only a fraction of what the Smithsonian really owns.
Verdict: I can't tell you when the plants will retreat to their winter quarters, but if you happen to be in the Haupt Garden before it's really fall, have a look.