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Methodology & sources

WhenPlant does not guess planting dates — every date is computed from three public, authoritative datasets. Here's exactly how, and where every number comes from.

1. Your hardiness zone

Your ZIP code is mapped to a USDA hardiness zone using data from the 2023 USDA Plant Hardiness Zone Map, produced by the PRISM Climate Group at Oregon State University from 1991–2020 temperature data. We query it via phzmapi.org, a public ZIP-keyed build of that dataset. (It is a derived build, not the official USDA lookup.)

2. Your frost dates

The two dates the whole calendar counts from — your average last spring frost and first fall frost — come from the NOAA NCEI 1991–2020 U.S. Climate Normals. We use the 32°F (0°C) frost-freeze probabilities at the 50% level from each station (variables ANN-TMIN-PRBLST-T32FP50 and ANN-TMIN-PRBFST-T32FP50). We match your location to its nearest NOAA weather station (of 1,093 with complete frost normals) and show you which station and how far away it is.

3. The planting rules

Each crop's planting window is transcribed verbatim from a cited source:

• Timing relative to frost — University of New Hampshire Cooperative Extension — “Growing Vegetables: When to Plant Your Vegetable Garden”
• Weeks to start seeds indoors — Johnny's Selected Seeds — Seed-Starting Date Calculator chart
• Days to maturity — University of Georgia Extension — Vegetable Planting Chart (Circular C963)
• Fall planting — the UGA count-back method: fall sowing date = first frost − (days to maturity + a 14-day fall factor + a 14-day frost-tender factor for tender crops).

What we don't do

We never invent a date. Where a source gives no value for a field, it's left blank. Frost tolerance labels are derived from each source's own timing (planted before the last frost = frost-hardy; after = frost-tender) and are labelled as derived. Local microclimate, elevation, and the specific year vary — treat WhenPlant as a reliable, sourced starting point, not a guarantee.

Last verified 2026-07-16.