Margo, I don't think it is the cold getting your semps. It sounds more like the soil/soil mix you are using.
Semps are hard to keep happy in a forest product/peat type mix. It might work if you dig ups the area of clay soil and add lots of crushed rock, the sharp edged type, not rounded like pea gravel. This will greatly improve your clay soil and make it drain faster. To this process you can add some of the organic material mixing everything really well for about 6" to 12" deep. That is for growing in the ground.
For container growing semps I would use a good potting mix and add lots of chicken grit and some sharp sand, not fine sand. Or if you can get sandy loam delivered mix it about 1/3 and 1/3 chicken grit and 1/3 with the high quality potting mix, not potting soil.
I'm not sure if you are wanting information for in ground or in pots information?
