ZIP Code Garden Zone & Frost Date Lookup

What this lookup actually returns

Four numbers come back from a ZIP code, and each one answers a different question. The USDA hardiness zone (from the 2023 map) is the average annual extreme minimum winter temperature for the location, split into 10°F zones and 5°F half zones — it tells you what a perennial can be expected to survive, and nothing else. The average last spring frost and first fall frost are 1991–2020 climate normals, and they set the window you can plant tender crops in. The frost-free growing days figure is simply the interval between those two dates, and it is the single most practical number for choosing varieties: a tomato that needs ninety days will not finish in a seventy-day window no matter how early you start it.

The underlying dataset covers 35,248 U.S. ZIP codes, combining USDA 2023 hardiness zones with NOAA 1991–2020 frost normals. It is published as open data at /data/zip-zone-frost.json under a Creative Commons Attribution licence, so you can check or reuse the figures behind any answer you get here. Nothing about the lookup requires an account and the query runs in your browser.

Why “average” is the word to watch

Every one of these four figures is a long-term average, and averages hide the years that cause trouble. An average last frost date is a midpoint: in roughly half of years the last frost arrives later, sometimes considerably later. That is why a planting date chosen from the average is a reasonable bet rather than a safe one, and why the practical approach is to plant the hardy crops on schedule, hold the tender ones until the forecast supports them, and keep a way to cover plants for a late cold night. The same caution applies to the zone figure — a zone describes a typical winter minimum, and an exceptional cold snap can still kill a plant rated for that zone.

The second limitation is spatial. A ZIP code is a delivery area, not a climate zone, and the dataset resolves it to a representative point. That is far more precise than a state or metro average, but it still cannot see your yard. A slope facing south, a wall that radiates heat at night, a low spot where cold air pools, a raised bed, and a downtown courtyard can each behave like a different half zone from the number this lookup returns. Treat the lookup as the correct starting point, then adjust from what your own site does over a season or two.

Turning the four numbers into a plan

Start with the frost-free window and pick crops and varieties that fit inside it, allowing a margin for a slow spring. Use the zone to decide what can overwinter and what must be treated as an annual, and to sanity-check the perennials you are considering. Then take the dates to the planting calendar to turn them into indoor-start, sow, and transplant dates for each crop, and to the monthly zone checklists for what the current month asks of you. Print the report — the tool produces a sheet you can take to a garden centre, which is more reliable than remembering the numbers while standing in front of a seed rack.

If you want context rather than your own numbers, the city guides explain how a metro’s microclimate differs from the surrounding state, and the state guides cover the regional picture including grass types and common pests. The three views are designed to be used together: ZIP for your exact dates, city for local context, state for the wider climate and pest pressure.