Odd thing is the best luck I've had with them was in heavy clay. It was well drained though, and water never puddled there. Perhaps the soil here is too moisture retentive? I could try amending with fine bark and sand; maybe that would be a porous enough media to overwinter them.
The big chunk of my current clump that died at the end of last winter, died during the melting of lots of snow. The soil was certainly cold and wet.
