Hi, I'm a little surprised to hear that neither coum nor hederifolium survived last winter. You are colder than us here in SE Pennsylvania, but not so much more that I'd expect that to happen. Cold hardiness is very much influenced by drainage - was the ground these tubers were in at all soggy? Freezing and thawing in less than perfectly well-drained ground could cause you to lose plants that would be hardy with perfect drainage in a garden a zone colder. On our S facing perfectly drained hillside I have a number of hederifolium that are 25-50% exposed to the air - they look like flying saucers that have crashed into the slope and stuck half-way in. They are up to 8" diameter - but they come through the winters just fine. I'd try to optimize the situation and drainage - and then you can certainly plant deeper as well if you want, as long as whatever is on top drains perfectly then the flowers and leaves will find a way through. You could put up to 6" of pure gravel on top of them without issue - but don't create a sump for water to gather on top of the tubers. Good luck! J. John T Lonsdale PhD 407 Edgewood Drive, Exton, Pennsylvania 19341, USA Home: 610 594 9232 Cell: 484 678 9856 Fax: 801 327 1266 Visit "Edgewood" - The Lonsdale Garden at http://www.edgewoodgardens.net/ USDA Zone 6b