Methodology

Zones

ZIP-to-zone assignments are the official USDA Agricultural Research Service 2023 Plant Hardiness Zone Map data (hosted by PRISM Climate Group, Oregon State University).

Freeze dates

We match every NOAA weather station inside each zone's boundaries (point-in-polygon) and pull its 1991–2020 freeze/frost normals: dates of first and last freeze at 36°F, 32°F and 28°F, at 10%, 50% and 90% probability. Zone pages show the median across member stations plus the 10th–90th percentile spread — frost varies within a zone; your ZIP's weather may sit at either end.

Planting windows

Crop windows are week-offsets relative to last/first freeze, compiled from University of Maryland Extension's central-Maryland calendar (stated frost basis April 15 / October 25) and Cornell Cooperative Extension's timing buckets, then anchored to each zone's median 32°F/50% dates. Your local extension office remains the authority for varieties and microclimates.

Limits

Offsets derive from mid-Atlantic and Midwest extension climates; cold-zone and no-freeze-zone extrapolation is unvalidated until reviewed. Probabilities are climatological, not forecasts.