USDA plant hardiness zones are based on average low temperatures in an area. For example, Austin is in Zone 8b with lows averaging from 15 to 20 degrees Fahrenheit. Only 100 miles south, San Antonio – although inland -- is in coastal Zone 9a with lows of 20 to 25 degrees Fahrenheit, according to the 2012 Zones Map. But along the city’s famed River Walk, temperatures can be a zone warmer in the winter.
Although an area may be defined as being within a single Zone, it may actually contain microclimates depending on natural or constructed features that tend to warm up or further cool parts of the landscape. If your home has a south-facing wall that captures and radiates heat, you may be able to winter over perennials that are listed for a warmer Zone.