#3 - Perennial geraniums have long been an English garden staple, but they haven't really established themselves in North American gardens until recently. 'Johnson's Blue', Rozanne' and Geranium sanguinium have been grabbing all the attention with their flashy colors. For a white garden, Clark's Geranium or Geranium clarkei 'Kashmir White' is a perfect choice. Again, you may get a few less blooms in partial shade, but you'll notice them even more. And hardy geraniums require little attention, as they spread a bit further every year. They are also extremely easy to divide and transplant, to fill in any bare spots as your garden matures.
My favorite choice is Geranium clarkei 'Kashmir White': Zones 4 - 8 (20" x 15"), Bloom Period: May - July
But a lovely alternative with a similar season of bloom would be Meadow Anemone (Anemone canadensis): Zones 2 - 6 (18" x 12"), Bloom Period: May - July