Sources and method | Planting Weeks

Planting advice on this site comes from state university Extension services, the public outreach arm of each state’s land-grant university. The hardiness zone lookup uses the USDA map data as published. Nothing here comes from seed sellers, forums, or our own garden.

Extension services we use

Clemson Cooperative Extension
Colorado State University Extension
Illinois Extension
Oregon State University Extension

We add a service only after reading its website terms. Some services do not allow their pages to be stored or read by scripts, and we leave those out even when their advice is good.

How a statement gets on a page

We read the Extension page, write the fact in our own words, and put the service’s name in front of it: “Oregon State University Extension says”. The line links to the page it came from and shows the date we last checked that page.

When only one service gives a number, that number is shown as theirs alone. When services disagree, each one gets its own line and we do not pick a winner. We do not average numbers or fill a gap with a guess.

State planting dates belong to the state or region they were written for, and are labelled that way.

Hardiness zones

The ZIP lookup uses the ZIP code listing for the 2023 USDA Plant Hardiness Zone Map, published by the PRISM Group at Oregon State University. Zones and temperature ranges are shown exactly as published. We do not draw a map.

USDA Plant Hardiness Zone Map, 2023: map creation and use (USDA Agricultural Research Service), checked 2026-09-27
2023 USDA hardiness zones by ZIP code (lower 48 states) (PRISM Group, Oregon State University), checked 2026-09-27

USDA Plant Hardiness Zone Map, 2023. Agricultural Research Service, U.S. Department of Agriculture. Accessed from https://planthardiness.ars.usda.gov.

What we do not cover

The date of your last frost. We have no source for them that we can use, so every crop page counts in weeks from your last frost and leaves the date to you. The NOAA 1991–2020 climate normals and your state’s Extension service can give you one.

Our own growing trials. We have not grown these crops side by side, and no page claims otherwise.

Keeping it current

Extension pages get revised and moved. We recheck the sources behind each page and update the check date when we do. If a link is dead or a line no longer matches its source, please write to hello@plantingweeks.com.