No, it takes more than that to kill a greggii. I've seen them keep their green leaves when temperatures fall into the single numbers. I guess fresh green growth could be damaged by a hard freeze, but I've never had a greggii killed by cold at this time of year. Darcyi might be another story, and could be killed back to the ground. Mine has not started to grow yet so I have no clue whether it is even alive.

My greggii also have lost their leaves over the winter and even with the recent run of warm temps I have seen no sign of them coming to life. They look dead, but time will tell Ive heard that it takes a good warmup for them, maybe that means the soil has to warmup, first.