Low temperature ratings can be deceptive. In the dry west microphylla/greggii can take colder temperatures than they can in the east. I have found that many of these cultivars are hardy in my garden in a park where the soil is basically sand and not so hardy in my home garden where the soil is an improved mix of sand and clay. My home garden is also about a half zone colder. Most of these, other than those yellow types, will survive short periods to 0F and a little below but be killed to the ground.
