Hardiness zone by ZIP code
Your USDA zone, worked out from ten years of actual recorded low temperatures at your coordinates — not interpolated from a distant weather station.
What the number is
A hardiness zone answers exactly one question: how cold does it get here in a typical winter? It is the average of each year's single coldest night, sorted into 10°F bands and split into half-zones.
That is genuinely useful and genuinely narrow. It tells you whether a plant will survive your winter. It tells you nothing about whether it will survive your summer, your rainfall or your humidity — which is why a plant can be perfectly hardy in your zone and still fail in your garden.
How this is calculated
| Source | Ten years of daily minimum temperatures at your ZIP's coordinates, from the Open-Meteo historical archive |
|---|---|
| Method | The coldest night of each winter is taken, then averaged — which is the USDA's own definition of the zone boundary |
| Bands | 10°F per zone, 5°F per half-zone. Zone 1a begins at −60°F |
| Frost dates | The average first and last day the temperature touched 32°F, from the same record |
Questions
What does a hardiness zone actually mean?
It is one number: the average coldest night of the year, in 10°F bands. Zone 9 averages 20–30°F at its coldest, zone 10 averages 30–40°F. Each band splits into "a" (the colder half) and "b". It says nothing about summer heat, rainfall or humidity — a plant can be hardy in your zone and still die of your August.
How is this different from the official USDA map?
The USDA map is the authority and is drawn from a large station network. This calculates the same quantity — the average annual extreme minimum — from ten years of observed daily minimums at your exact coordinates. It usually agrees; where it differs, your local reading may actually be closer to your garden than a map interpolated between distant stations. Treat it as an estimate, not a replacement.
Can I grow a plant rated one zone colder than mine?
Usually yes, with room to spare — a zone 8 plant in zone 10 is entirely safe from cold. The risk runs the other way: a zone 10 plant in zone 8 will be killed by a normal winter unless you bring it in.
Does a microclimate change my zone?
It can shift it by roughly half a zone within one garden. A south-facing wall stores heat and releases it overnight; a low spot collects cold air and frosts first. If a plant is borderline for your zone, where you put it genuinely decides whether it lives.
Why does my zone matter for houseplants?
Only when they go outside. Most houseplants are tropical — zone 10 or 11 — so almost anywhere in the US they are summer visitors that must come indoors before frost. Your zone tells you the deadline.