Hardiness Zone vs. Frost Date: Different Questions

The zone answers "will it survive winter"

The USDA Plant Hardiness Zone is built from the average coldest night of the year, averaged over 30 years. It tells you whether a perennial, a tree, or a shrub rated for that zone can survive the winter where you are. It says nothing about when spring actually starts.

The frost date answers "when can I plant"

Frost dates are about the specific day the temperature crosses 32°F in spring and fall, which is what actually kills tender annuals, seedlings, and warm-season vegetables. Two ZIP codes can share a hardiness zone and still have last-frost dates weeks apart, if one sits at a higher elevation or further from a coastline that moderates temperature swings.

Using both together

A perennial's tag ("hardy to zone 6") is a zone question. "Is it safe to move my tomato seedlings outside" is a frost-date question. This site reports both on every ZIP page, because a gardener usually needs the zone once per plant purchase and the frost date every single spring.

Where to go next

See the range this plays out across: Stanley, ID, Farber, MO, Indian Creek, FL.

What the Early, Typical and Late Frost Dates Actually Mean · Why Your Own Yard Might Not Match Your ZIP Code's Number