If shooting flowers is your thing--or if you're looking for a beautiful floral backdrop for wedding or engagement photos--Washington has some excellent options. There's the blooming of the cherry blossoms around the Tidal Basin in early spring, followed by the flowering of the tulips and other blossoms all around town. The National Arboretum, the Botanical Gardens, River Farm, and the Smithsonian's Edith A. Haupf Garden all have a range of exotic flowers from around the world and are all expertly maintained. And there are a lot more wonderful gardens around the area as well as some good options for catching fall foliage.
And there's even an English-style rose garden hidden away in Arlington. It has a spectacular range of different kinds of roses, each variety carefully identified. They're all neatly arranged, interspersed with walking paths, wooden park benches, and small arched gazebos.
The roses are at their peak bloom around the end of May through June and start fading in early July. The rest of the time it's a pleasant garden, but if you want to see the flowers, head there in June. (If you're heading there earlier in the spring, walk a little way down the path towards the playground and check out the gardens on your right, past the Sunny Garden sign. That garden's different kinds of flowers mostly bloom earlier and also quite beautiful.)
There are no access restrictions day or night, and there's no problem using tripods or flash.
No advance permissions or permits are required for personal, wedding, engagement, or family photos. Shoots for commercial use require a permit.
The garden is at the corner of Wilson Blvd and N Lexington Street in Arlington. There's not really a close metro stop, although it is possible to walk from either East Falls Church or Ballston (both Orange line) but it's a very long walk from both.
It's also on a bike path that connects directly to both the Custis Trail, which runs alongside the 66 freeway towards DC, and Four Mile Run/W&OD Trail, which comes out down at the George Washington Parkway next to Reagan National Airport. If you don't have your own bike, you can rent one from Capital Bikeshare at a bunch of locations in DC and Arlington.
There's also plenty of parking right next to the garden. If you're driving down Wilson Blvd, keep a lookout for the tennis courts--the garden is right next to those.
There's also plenty of room for picnics, walking, and playing in the adjacent park areas.
Oh, and please don't pick the roses (as I saw someone do recently).
See also the Arlington Rose Foundation which created and provides support for the garden.