Thāne is in hardiness zone 12a

What the zone tells you

A hardiness zone answers exactly one question: how cold does it get here in an average winter, at worst. It is calculated from the mean annual extreme minimum — the coldest night of each year, averaged over the record. For Thāne that figure is 12.2°C (54°F), which places it in zone 12a.

That makes it a good tool for one job. When a nursery says a plant is hardy to zone 12a, a perennial, shrub or tree rated to this zone or a lower number should survive a normal winter here without protection. Anything rated to a higher number is a gamble, a container plant, or a write-off in a cold year.

What it does not tell you

It says nothing about when to plant. Two places can share a zone and have growing seasons that differ by six weeks, because a zone is a single winter night and a season is the gap between the last frost of spring and the first of autumn. A maritime climate and a continental one can sit in the same zone and behave nothing alike.

It also says nothing about heat, drought, humidity or the length of your summer, all of which can matter more than winter cold. A zone number is a floor, not a description of a climate.