The Chicago Botanic Garden has a bunch in their English Wall Garden that are, as far as I know, perennial. They also have more or less perfect soil, and being against a wall gives them a zone. If you call their plant information desk, they may be able to give you more info. I'm also in a zone 6 pocket and have the well-drained soil they supposedly like. The one I planted two falls ago bloomed fantastically the first spring; this spring all I got was foliage. And a couple of late-season bargain shorties I planted last fall didn't come up at all. I may try feeding the one that had leaves and see if that helps for next year.

I planted 10 two falls ago. Six of them came up the first spring. This year all six of them came up, but only 4 had blossoms. One of them had multiplied.