USDA Plant Hardiness Zone Map
2023 USDA Plant Hardiness Zone Map, published by USDA Agricultural Research Service. We use the 2023 map for official zone definitions and half-zone temperature bands. Accessed 2026-07-16.
How to Use the Maps, also published by USDA Agricultural Research Service, supports our explanation of how the map should and should not be used. Accessed 2026-07-16.
The current Zone Finder sends ZIP searches to USDA. We have not found a reviewed, stable public ZIP-to-zone API or CSV contract that we can responsibly reproduce.
University Extension crop guidance
Starting seeds indoors, published by University of Minnesota Extension. The seed calculator uses the crop-specific indoor lead times in its reviewed table. Accessed 2026-07-17.
Dedicated crop guides use the more specific source when one is available: tomatoes, peppers, eggplant, broccoli, cabbage, cauliflower, leeks, cucumbers, lettuce, onions and sweet corn from University of Minnesota Extension, plus the peer-reviewed carrot fact sheet from Utah State University Extension. Each crop page states the rule it uses and the source access date.
The calculator uses crop-specific indoor timing when it conflicts with the general table: eggplant 8 weeks, broccoli 5 weeks, cabbage 4-6 weeks, cauliflower 5 weeks, onions 10-12 weeks and leeks 10-15 weeks. Celery retains the general 12-13 week value because the reviewed crop-specific URL was unavailable. Conflicts are not averaged.
NOAA freeze probability normals
1991-2020 U.S. Climate Normals, published by NOAA National Centers for Environmental Information. The frost tool uses the v1.0.1 agricultural normals for 32°F first- and last-freeze probability dates. Accessed 2026-07-17.
We include only stations whose six 10%, 50% and 90% fields meet NOAA S completeness and use at least 24 years of observations.
Census ZCTA locations
2025 Gazetteer ZCTA National File, published by U.S. Census Bureau. Internal points provide an approximate location for nearest-station matching. ZCTAs are not USPS ZIP Code delivery boundaries. Accessed 2026-07-17.
Source standards
- Primary government and university Extension sources are preferred.
- A source must support the specific statement or calculation where it appears.
- Access and review dates are recorded so guidance can be checked again.
- Conflicting regional guidance is described, not averaged into a false national rule.