Planting data sources

PlantWhenNow generates planting calendars using publicly available government data. Here is what we use and how.

USDA Plant Hardiness Zone Map

Source: USDA Agricultural Research Service

What it provides: The hardiness zone map divides the U.S. into zones based on average annual minimum winter temperature. Each zone spans a 10-degree Fahrenheit range. We use the 2023 revision, which reflects data from 1991 to 2020.

How we use it: We map your 3-digit ZIP code prefix to the corresponding hardiness zone. This determines your approximate frost dates and growing season length.

NOAA Climate Normals

Source: NOAA National Centers for Environmental Information

What it provides: 30-year climate averages (1991-2020) including freeze/frost probability dates for weather stations across the U.S.

How we use it: We derive average last spring frost and first fall frost dates for each hardiness zone from these records. These frost dates anchor every planting window calculation.

Planting schedules

Sources: University cooperative extension programs including Clemson, Cornell, University of Minnesota, UC Davis, and Texas A&M.

What they provide: Crop-specific planting guidance expressed as weeks before or after the last frost date. For example, "start tomato seeds indoors 6 to 8 weeks before last frost."

How we use it: We compiled recommendations from multiple extension programs into a database of 31 common vegetables and herbs. Each crop has timing offsets for seed starting, direct sowing, transplanting, and harvest relative to the last frost date.

Limitations

ZIP-to-zone mapping uses 3-digit prefixes, so it is an approximation. Two addresses with the same 3-digit prefix but different elevations might be in different zones. Frost dates are averages from 1991-2020 and do not account for year-to-year variation or recent climate shifts. Always check your local forecast before planting frost-sensitive crops.

Data updates

We update the zone mapping when the USDA releases new hardiness zone data. The most recent update uses the 2023 map revision.