My wife and I were in Washington DC a few weeks back for a wedding. I’ve been to DC a handful of times and it is one of my all-time favorite places to visit. I love history so the memorials, monuments and Smithsonian museums are all high on my list. But DC has a lot to offer plant geeks as well. In honor of Memorial Day and the unofficial start of summer vacation season, here are my top three DC Horticultural Highlights.
National Botanic Garden. Located nearly adjacent to the US Capitol, the Botanic Garden is easy to miss if you’re not looking for it. The garden is comparatively small but offers a nice respite from the hustle and bustle (and interminable school groups) of the rest of the National Mall. It’s also a cooler oasis to beat the heat if you visit the Mall in the summer and offers some unique views of the Capitol. The conservatory has great on-going and rotating exhibits. A current one was on medicinal plants. 
The National Arboretum. The Arboretum is a little more of challenge to get to and probably best if you have a car. Some highlights include the Gotelli collection of dwarf conifers, the grove of state trees, and The National Bonsai & Penjing Museum. Be sure to check on hours of operation before you go. The first time I visited the Arboretum I arrived at 4:30 thinking I would have a several hours of prime late afternoon and evening light for pictures, only to find out they closed at 5:00.
Arlington Cemetery. Arlington is a special place is so many ways. If you’re an American, these are images steeped in our collective consciences; the tomb of the Unknowns, the Kennedy gravesites, the Challenger memorial, and tens of thousands of headstones marking those that gave the last full measure of devotion. The grounds are wonderfully tended and, like the Botanic Garden, provide a break from the din on the nearby National Mall. The Cemetery grounds include dozens of memorial trees and several state Champion trees. Motorized trams are available but if you can walk a couple miles it’s a fascinating and moving place for a stroll. Arlington is easily accessible by DC Metro or walk across the Memorial Bridge at the Lincoln Memorial end of the Mall.
The botanic garden is a great place but why do they have so many castor beans around? It’s toxic, for goodness sake.
Also, datura…
Dunbarton Oaks is another great garden to visit in D.C., although it is a ways away from the mall. The Ripley garden is another great one and that is right on the Mall by the Arts and Industry building.
I’d also recommend the Old Congressional Cemetery. It’s a beautiful place to walk and you can call a number on your cell phone to get quick bios on the more notable occupants, like John Phillips Sousa, J Edgar Hoover, and John Quincy Adams, among others. It’s also an off-leash dog park.
Dumbarton Oaks is great, but if you’re willing to drive there, you might as well go down to Mt. Vernon and River Farm, the AHS hq.