Cinta, That's very odd as I grew them well in MA (Zone 4) and saw 32 below F more than once. The only cultivar I lost was 'Commander Hay', which I lost 3 times before giving up. Never lost another.
If the semps are in too heavy soil or don't get enough drainage they are much more likely to rot. Be sure you have those sort of conditions for the plants. Only the toughest tectorums will take wet feet.
Kevin
