USDA Plant Hardiness Zone by ZIP Code

What your USDA zone actually means

A hardiness zone is one statistic: the average annual extreme minimum winter temperature for an area. USDA describes the map as “based on the average annual extreme minimum winter temperature, displayed as 10-degree F zones ranging from zone 1 (coldest) to zone 13 (warmest)”.

Read that phrase carefully, because it is narrower than it looks. It is not the average winter temperature. It is not the coldest temperature ever recorded. It is the coldest night you would expect in a typical winter, averaged over the 2023 map’s reference period. A plant rated to your zone should survive an ordinary winter where you are; an unusual winter is exactly the case the average does not describe.

That is why the number is used for perennials — trees, shrubs, and plants expected to live through winter — and why it says nothing useful about annuals, which are replanted each year and never face the cold.

Why zones have an “a” and a “b”

Each whole zone covers a 10°F range, which is wide enough to matter. USDA therefore splits every zone into half zones “designated as ‘a’ and ‘b’” in 5°F increments, where a is the colder half and b the warmer.

So zone 7a and zone 7b are not interchangeable: 7a bottoms out around 0°F to 5°F, 7b around 5°F to 10°F. For a plant rated “hardy to zone 7” the distinction is usually harmless; for one rated “hardy to 7b” it is the difference between surviving and not. If you are choosing something near the edge of its range, the half zone is the part of the answer that counts.

The limits of a ZIP-code lookup

A ZIP code is a mail-delivery area, not a climate area. USDA’s underlying grid resolves to about half a mile per cell; a ZIP code can span many miles and, in mountainous country, thousands of feet of elevation. Assigning one zone to the whole ZIP code is a simplification USDA makes for convenience, and it is the same simplification this page inherits.

What is and is not covered. The 2023 map publishes ZIP-code listings for Conterminous U.S., Alaska, Hawaii and Puerto Rico — 40,502 ZIP codes in total, and that is the whole of the official listing. It publishes none for U.S. Virgin Islands, Guam, American Samoa, Northern Mariana Islands, so this tool returns nothing for postal codes there rather than borrowing a zone from somewhere else. PRISM does publish climate normals for other places; a normal is a different quantity from a zone assignment and is not a substitute for one. A ZIP code missing from the data does not mean the location has no hardiness zone — only that USDA publishes no zone against that ZIP code.

The practical consequence: the zone you get here is right for your area and approximately right for your garden. USDA is direct about the gap. Microclimates — “small heat islands” or “cool spots (frost pockets)” — can shift conditions within a single property, and “no hardiness zone map can take the place of the detailed knowledge that gardeners learn about their own gardens through hands-on experience”. A south-facing wall, a hollow that collects cold air, or shelter from wind can each move a spot a half zone or more from its ZIP code’s label.

Why the next ZIP code over can differ

Adjacent ZIP codes often land in different half zones, and the reason is usually elevation or water. Cold air pools in valleys, so a valley floor can be a half zone colder than the ridge above it. Large lakes and coastlines moderate winter minimums, so a shoreline ZIP code can be warmer than one a few miles inland.

This is also why this tool refuses to substitute a nearby ZIP code when yours is missing. The substitution would be invisible in the result and wrong often enough to kill a plant.

What hardiness zones do not tell you

Frost dates

A hardiness zone describes how cold it gets, not when. Two places in the same zone can have first and last frost dates weeks apart, which is what actually governs when you plant. USDA’s map carries no frost dates and neither does this page.

Heat and the length of the growing season

Nothing in a hardiness zone describes summer. A plant can be perfectly winter-hardy in your zone and still fail in your summer heat or humidity, or run out of growing season before it ripens. Heat tolerance is a separate rating on a separate scale.

Everything else that kills plants

USDA lists the rest plainly: “Wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants.” Snow cover in particular insulates roots, so a reliably snowy zone 5 can be gentler than a bare, windy zone 6.

How to use the result

Treat the zone as a filter, not a plan. A plant tag saying “zones 5–9” means the plant is expected to survive winter anywhere in that band; if your zone is outside it, no amount of care in summer changes the answer. USDA’s own framing is that these maps “should serve as general guides for growing perennial plants”.

Three things worth doing with the number. Check the half zone, not just the whole one, if a plant is near the edge of its range. If your garden has an obvious microclimate — a frost hollow, a sheltered south-facing wall — plan a half zone either side of your label. And where you need timing rather than survival, look up local frost dates instead, because this is the wrong number for that question.

Who publishes this data

The map designation is USDA’s. The USDA Plant Hardiness Zone Map is published by the Agricultural Research Service, and the zone assigned to a ZIP code is USDA’s assignment, not this site’s.

The files themselves come from Oregon State University. USDA states that GIS data “can now be downloaded directly from the PRISM Climate Group at Oregon State University” in “grid and shapefile formats, as well as by ZIP code”, under a Specific Cooperative Agreement between USDA and OSU. So the four 2023 USDA Plant Hardiness Zone GIS Datasets this tool reads are hosted by PRISM, not by USDA — which is why the source links at the foot of this page point where they do. PRISM developed the mapping technology and produced the underlying grids; USDA remains the source of the zone designation.

Nothing in the zone assignments is created by DotGovSource. What this site adds is the ZIP lookup, the temperature interval computed from USDA’s published definition, and the Celsius conversion.

Other property and data tools

If you are assessing a property rather than a planting plan, the solar ROI calculator works out the payback period on a solar installation, and FHA loan limits by county gives the HUD mortgage cap wherever you are looking. Both are built the same way as this page: official federal data where it exists, clearly labelled arithmetic where it does not.