A planting on the south side of a house and sheltered by evergreens may never get as cold as the plot the exposed north side. It may make a difference that one garden is on a hill, the other in a valley. This is especially true if you are near the southern edge of a zone, or in an urban area. Heat is important. We tend to think of plant survival in terms of cold survivability.
Heat can matter, too.
Some plants can't take the heat. This may not be as obvious as is the factor of cold survivability. Most plants can tough out high temperatures. They may not flourish, but they survive. A few may perish from the heat, but most will live through it, though they may be weakened.