Montana -- zone 3, drought tolerant, and so dainty - a long blooming perennial, with deadheading. This flower prefers full sun but since I never water it; it only survives in the shady spots.
Plus, the Rockets, just like the Montana, get a bad rap because they are problematic in well-watered gardens. Reseed like crazy and kill off other wildflowers. In this spot, beneath this native tree, where they never receive a drop of water... they are very well-behaved.
* Dame's Rocket has been deemed a noxious weed in the rich, moist, acidic soils of the southeast.