Dates from extension services
Every state has a land-grant university with an extension service that publishes gardening advice for that state: the University of Minnesota for Minnesota, NC State for North Carolina, UF/IFAS for Florida, and so on. Their planting dates are tested in local conditions, so they are our first source. For each topic we take dates from extension services in several regions, from the coldest zones to the warmest.
Zones from the USDA map
A hardiness zone is the average coldest winter night of a place, in 10 °F steps, from the 2023 USDA Plant Hardiness Zone Map. To label each region in our calendars with its zone, we use the map’s ZIP-code listing from the PRISM Group, Oregon State University (prism.oregonstate.edu, accessed October 11, 2026).
A zone describes winter cold, not the date of your first frost, and a state can span several zones. So each calendar row names the region its source covers, and we say when a date is tied to frost or soil temperature rather than to the calendar.
How to read our calendars
- Each row is one source, named under the bar, with its window in its own words.
- Solid bars are dates the source gives. Pale bars mark a season given without dates (“late fall”), drawn approximately. Striped bars are explained in each chart’s key.
- Sources disagree sometimes, on depth especially. When they do, we show each one rather than pick a winner, and say what the difference tracks, such as colder places planting deeper.
Our own arithmetic
Where we calculate, such as harvest dates from days to maturity, or how far a cubic yard of mulch spreads, we say so and show the math. The calculators show every step with your numbers.
Checking and updating
Each guide shows the date it was last updated. If your county extension office says something different, trust them for your garden, and let us know so we can check the page.