Data & Methodology
How GardeningByZone calculates planting dates — and the open dataset behind it.
Every planting date on this site is computed, not hand-entered. This page documents the data sources, the calculation, what's covered, and the known limitations — so the numbers are reproducible and citable.
Data sources
- USDA Plant Hardiness Zones — the standard 13-zone (a/b half-zone) system based on average annual minimum winter temperature.
- Representative average frost dates per zone — a typical average last-spring and first-fall frost for each zone, listed in full below.
- Per-crop timing offsets — for each of 100+ crops, how many weeks before or after the last spring frost to start seeds indoors, transplant, and direct sow.
How a planting date is calculated
For a given crop and zone, each date is the zone's average last-spring-frost date shifted by the crop's offset:
date = zone average last-spring frost + (crop offset in weeks)
A negative offset means before the last frost (e.g. cool-season crops and indoor seed starting); a positive offset means after it (e.g. transplanting heat-loving crops once the soil has warmed). The offsets are zone-independent — the zone supplies the frost date, the crop supplies the shift — which is why the same crop's calendar slides earlier in warm zones and later in cold ones.
Average frost dates by zone
The foundational dataset. Frost-free days is the span between the two frost dates.
| Zone | Region (representative) | Avg. last spring frost | Avg. first fall frost | Frost-free days |
|---|---|---|---|---|
| Zone 2A | great plains | May 30 | Sep 1 | 94 |
| Zone 3B | great plains | May 15 | Sep 15 | 123 |
| Zone 4A | midwest | May 8 | Sep 25 | 140 |
| Zone 4B | midwest | May 1 | Oct 1 | 153 |
| Zone 5A | midwest | Apr 25 | Oct 8 | 166 |
| Zone 5B | midwest | Apr 18 | Oct 13 | 178 |
| Zone 6A | midwest | Apr 21 | Oct 15 | 177 |
| Zone 6B | midwest | Apr 10 | Oct 20 | 193 |
| Zone 7A | southeast | Apr 5 | Oct 28 | 206 |
| Zone 7B | southeast | Mar 28 | Nov 3 | 220 |
| Zone 8A | southeast | Mar 20 | Nov 10 | 235 |
| Zone 8B | southeast | Mar 12 | Nov 18 | 251 |
| Zone 9A | texas | Feb 28 | Nov 28 | 273 |
| Zone 9B | florida | Feb 15 | Dec 10 | 298 |
| Zone 10A | florida | Feb 1 | Dec 20 | 322 |
| Zone 10B | florida | Jan 15 | Dec 31 | 350 |
| Zone 11A | florida | Jan 1 | Dec 31 | 364 |
Coverage
100+ crops (vegetables, herbs, and flowers) × 17 half-zones × three timing windows (start indoors, transplant, direct sow) — roughly 5,000 computed planting dates. Browse the individual plant guides or planting schedules by zone.
Limitations
- Frost dates are zone-level averages, not your exact location. Local microclimate, elevation, and a given year's weather can shift them by a week or more — always check local conditions.
- Half-zone frost dates are representative values for the zone, not a per-zip-code lookup. For a zip-code-specific calendar, use the planting tool.
- Dates assume typical outdoor culture; season-extension (row cover, greenhouse) changes them.
Use & citation
The frost-date dataset is available as JSON:
/data/frost-dates.json. It is offered under
CC BY 4.0 — free to
reuse with attribution:
Frost-date data: GardeningByZone (gardeningbyzone.com/methodology)
Last updated June 9, 2026.