Hi, A fresh look at ebay, revealed that temperature loggers are now available at low cost. They're a small device which will run for a year off an internal battery recording the temperature every so often and they can be plugged into a USB port to read off the data. So discovering how low the temperatures underground go is now practical. Oddly I recall an undergraduate problem I was given which gave a mathematical model of temperature variation in the ground - a lot must already be known, but of course not under anyone's precise circumstances. I doubt it would be of much interest if I carried out this experiment, I have a poor man's version, I leave dahlia tubers just under the surface, and these usually survive. In the two "worst winter for 30 years", 2009-10 and 2010-11, I lost a lot of bulbs on shelves in the greenhouse, but the bulbs I would expect to survive, narcissus, galanthus, tulips, crocus in pots all survived. I still think they are hardier than others. However in those desperate winters, the minimum temperature was around minus 5C, that's with living close to the sea in North West England. -- David Pilling email: david@pilling.demon.co.uk web: http://www.davidpilling.net/