[LorenPechtel]

The plant zones are not about average, but about worst case. Will your plant survive the coldest night? It is a good general guide for what you can grow locally. You check the zones the plant is rated for and if relevant chilling hours. It's not perfect but it's a very good starting point.

[nmbrskeptix]

Right, coldest expected. My figs have died three years in a row because we keep getting worst case 5b instead of 6b

[LorenPechtel]

I got the reverse. Some yams (she likes the leaves) that would be killed back by the first frost, return sometime in April. This winter there were two frosts that did damage but no full winter kill--the surviving stems produced new leaves.