Our Data Sources
Last updated: March 29, 2026
We believe in showing our work. Here's exactly where our data comes from and how we use it.
Frost Date Data
Source: NOAA Climate Normals
Our frost dates come from the National Oceanic and Atmospheric Administration's Climate Normals database, which calculates 30-year averages from weather station data across the United States. These are the same averages used by agricultural professionals, university extension services, and the National Weather Service.
The frost dates we display represent the date by which there is approximately a 50% probability that the last spring frost (or first fall frost) has occurred. In any given year, the actual frost date could be earlier or later than this average.
USDA Hardiness Zone Data
Source: USDA Plant Hardiness Zone Map
Zone assignments come from the USDA Plant Hardiness Zone Map, maintained by the Agricultural Research Service. This map divides North America into zones based on average annual extreme minimum temperatures, using data from over 8,000 weather stations.
We map zip codes to zones based on the USDA's published data. In some cases, zip codes span multiple zones — we assign the predominant zone for the area.
USDA Plant Hardiness Zone Map →
Plant Growing Data
Sources: University Cooperative Extension Services, Seed Companies, Gardening References
Plant-specific information (days to germination, weeks before/after frost to plant, soil temperature requirements, spacing, companion plants) is compiled from multiple sources:
University cooperative extension publications — every state has an extension service that publishes research-based gardening guides. These are among the most reliable sources of growing information available.
Seed company planting guides — companies like Burpee, Johnny's Selected Seeds, and Baker Creek provide extensively tested planting guidance with their products.
Established gardening references — we cross-reference multiple sources to verify planting recommendations before including them in our database.
State-Specific Information
Soil descriptions, climate notes, and region-specific growing tips for each state are compiled from state extension service publications, USDA soil surveys, and published gardening guides specific to each region.
How We Calculate Planting Dates
When you enter your zip code, we look up your USDA zone and the corresponding average frost dates. When you select a plant, we apply that plant's timing requirements relative to your frost dates.
For example, if your average last frost date is April 15 and tomatoes need to be started indoors 6 weeks before last frost, we calculate an indoor seed starting date of approximately March 4.
This is straightforward math applied to reliable data. No AI-generated guesses, no crowdsourced averages — just established horticultural timing applied to NOAA frost data.
Data Accuracy & Updates
NOAA Climate Normals are updated every 10 years (most recently in 2021). USDA hardiness zones are updated periodically as climate patterns shift. We review and update our plant data regularly to ensure accuracy.
If you notice information that seems incorrect for your area, please contact us. Local gardeners often have better ground-truth knowledge than any database.