About half of the country moved into a slightly warmer zone in the Agriculture Department’s new “plant hardiness” map, an important guide for gardeners. Climate change may be a factor.
"The temperature of each zone is determined by the average minimum temperature during winter months. The hardiness zones are separated by 10°F. This means that Zone 9 is 10°F colder than Zone 10 and Zone 8 is 10°F colder than Zone 9.
Each zone has two subsets. The Zone 9 subsets are Zone 9a and Zone 9b. Each is separated by 5°F. That means the temperature ranges for Zone 9 are:
Zone 9: The minimum average temperature range is 20°F to 30°F.
Zone 9a: The minimum average temperature range is 20°F to 25° F.
Zone 9b: The minimum average temperature range is 25°F to 30°F."
Zone map here, just punch in your zip code:
https://planthardiness.ars.usda.gov/
Mine came back as “9a” so here’s what that means:
https://www.lovetoknow.com/home/garden/gardening-zone-9
"The temperature of each zone is determined by the average minimum temperature during winter months. The hardiness zones are separated by 10°F. This means that Zone 9 is 10°F colder than Zone 10 and Zone 8 is 10°F colder than Zone 9.
Each zone has two subsets. The Zone 9 subsets are Zone 9a and Zone 9b. Each is separated by 5°F. That means the temperature ranges for Zone 9 are:
Zone 9: The minimum average temperature range is 20°F to 30°F.
Zone 9a: The minimum average temperature range is 20°F to 25° F.
Zone 9b: The minimum average temperature range is 25°F to 30°F."
The second thing you need is a plant book which describes hardiness of each species by what zones it can live in. That lets you plan a garden.