Dame's-violet, also known as Sweet Rocket (Hesperis matronalis) is one of my favourite plants in the garden. The pink flowered form is nice but the white flowered variety is to my eye even more beautiful. As twilight approaches the flower heads seem almost luminescent.
 Even more exquisite is when there happens to be a seedling where a pollinator has flitted between the pink and the white; the result is a form with white flowers that blush with subtlest hint of pale lilac.