How we check facts

Every number on GardenFacts comes from a public data set. Each claim on a page links to the fact it rests on, and each fact links to its source.

Where the data comes from

• Hardiness zones: the 2023 USDA Plant Hardiness Zone Map, made by USDA's Agricultural Research Service and the PRISM Group at Oregon State University. We read the map's 800 meter grid of the average annual extreme minimum temperature (1991 to 2020) at each city's center point and assign the zone with USDA's 5°F half-zone bands. We also show the official zone of every ZIP code that overlaps the city, because a city can span more than one zone. PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed October 30, 2026. Our pages are not the official USDA map.
• Frost dates and growing season: NOAA NCEI U.S. Climate Normals 1991-2020, from a nearby weather station (within 25 miles; we prefer stations with at least 25 years of records). NOAA gives the date after which the last spring frost (32°F) comes in 9, 5 and 1 of 10 years, and the same for the first fall frost. They are odds from past weather, not a forecast. We do not show dates where frost happens in fewer than 9 of 10 years, because NOAA's dates then describe only the years with frost.
• Planting rules: Colorado State University Extension, GardenNotes #720 "Vegetable Planting Guide" (reviewed September 2024). It sorts vegetables into hardy, semi-hardy, tender and very tender groups and ties each group to the average last spring frost. The guide was written for Colorado. Your local Cooperative Extension office may publish dates tuned to your area.
• Cities: U.S. Census Bureau population estimates (Vintage 2024) and Gazetteer files.

Our calculations

Planting dates on city pages apply CSU's rule to NOAA's median (50%) last frost date at the station. First harvest dates add CSU's typical days to harvest to the earliest planting date. These are simple arithmetic, marked "GardenFacts calculation" on each page, not advice from the source. Weather in any one year can be earlier or later.

How a page is made

1. We download the data sets and match each city to its zone and weather station.
2. Code builds every table, chart and date straight from the data.
3. A writing step explains what the facts mean for a gardener in plain English. Every number carries a reference to its fact.
4. A script checks every number and flags words that go beyond the facts. Then a separate review step checks each claim against the facts before the page is published.

What we do not cover

We do not recommend fertilizer or pesticide products or doses, and we do not say whether a plant is toxic, invasive or banned unless an official source (USDA, a state agency, ASPCA or a university extension service) says so on the page where we cite it.