I have grown Prairie Blue Eyes in both a zone 5/6 and this zone 4 garden for over twenty years and it was/is hardy in both locations. I divided my clump a few years ago and the plants have been left undisturbed since. Interestingly, unlike the other reports my PBE has not flowered in several years (it did flower for several years after being divided). I imagine that is because it is growing under almost wild/natural conditions. It has been growing with no added water (just rain), no fertilizing and occasional weeding.
Many years ago I used it in a test and I can report that although it is both pod and pollen fertile with other cultivars it seems completely self-incompatible (all self-pollinations set pods which were then aborted). If I ever get around to studying self-incompatibility I will be using it in crosses with Stella de Oro which is self-compatible to check on the inheritance of compatibility/incompatibility. Of course, to do that I will need to fertilize it heavily to start it blooming again. It will take probably at least a full year of heavy fertilizing before it flowers again so a two year wait. I imagine the seedlings from that cross will have horribly coloured flowers.
