I don't get it.
I bought two four-packs of Salvias, potted them, and put them out during the day and bring them in at night (north WI).
The other night I forgot to bring them in and there was a frost. Six of the plants died but two survived - they're as healthy as the day I brought them home. How is this possible?
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The only thing I could figure is maybe the survivors were seeds from very strong parents. I guess they have a nice mix of seed sources.
Two plants are like upright soldiers and the other six are a sorry sight. I got them because Hummingbirds are supposed to like Salvias. Will go shopping tomorrow and get more but this time I'll remember to bring them in after dinner. 
The log-sided house provides just enough thermal protection to see them through the night. You'd think the plants closest to the house would make it, but they died along with the plants farthest away from the house, and the ones snuggled in the center. The survivors were on the two sides parallell to the house.