How We Write Our Guides

Where the zones come from

Hardiness zones are the USDA's 2023 Plant Hardiness Zone Map, as published for each ZIP code by PRISM at Oregon State University, which produces the map for the USDA. All 40,502 ZIP codes in that table have a zone. When a guide says a source was written for "zones 8b–9a", those zones are read from this table for every ZIP code in that source's state or county.

Where the frost dates come from

Freeze dates are NOAA's 1991–2020 US Climate Normals. 7,125 weather stations publish usable freeze probabilities, and each ZIP code is matched to its nearest station within 100 km. The tools name that station, its identifier and its distance.

• What "freeze" means here. NOAA publishes freeze dates at several temperatures; this site reads the 32 °F one — the first night in fall, or the last night in spring, that falls to 32 °F or below.
• Median and cold-year dates. The headline date is NOAA's 50% date, a median: in half of years the freeze comes earlier. Where a guide gives a cold-year date, it is NOAA's 10% date for the first fall freeze: about one year in ten, the freeze comes before it.
• Zone tables. A guide's zone-by-zone dates are the median of every station that the ZIP codes in that zone use. Where a table shows it, each row gives how many stations and ZIP codes it is built from, and the spread across the middle half of them.
• Light and hard frost. A hard or killing frost comes on or after the first 32 °F night, and a light frost can form above 32 °F. Guides that time a job to a frost say which side of that line the 32 °F date falls on.

7,108 ZIP codes have no station close enough. Pages tell those readers why — a PO box with no published location, or a remote place with the real distance — and do not borrow a station from somewhere else.

How sources are chosen

Planting rules, depths, timings and rates are quoted from university cooperative extension publications — the land-grant universities' public advice for gardeners in their own state or county. Only text that has been read from the publication itself is quoted, and each quote is credited on the page to the source it came from.

• Every source shows who it was written for. Each entry in a guide's source list says the state or county it covers and that area's USDA zones, so a Minnesota rule is never read as advice for Georgia.
• Disagreements are shown, not settled. Where two publications give different figures — for example the storage temperature for dahlia and canna roots — the page prints each one with its source.
• Gaps are stated. Where no source we could use covers a region, the page says so — for example, garlic timing for Texas and Louisiana — and points readers to their own county extension office.
• Judgement is labelled. The zone ranges in the plant tables are conventional guidance, not measurements, and the pages label them as a judgement next to the measured freeze data.

What the checks cover

Every build of the site runs automated checks, and a build that fails one is not published. Among them, the checks:

• re-derive every zone row's freeze date on the fall and winter guides from the NOAA station data, without using the code that drew the table;
• confirm each zone row is the zone it is labelled with, so a ZIP lookup reads the right row;
• confirm every source a page cites is linked, and every question in a page's FAQ markup is shown on the page;
• re-check every Arizona planting date against the university calendar it comes from;
• fail a plant page that prints a temperature the station data does not produce, and a drawing that shows a figure its page does not state.

The checks are tested by deliberately breaking a page and confirming the build fails.

How pages are kept current

• Each guide and plant page shows the date it was published and, when its wording changes, the date it was updated. Rebuilding the site does not move those dates.
• The site is rebuilt every week, so the guides the homepage and the state pages suggest for the month follow the calendar.
• The sitemap lists every page on the site, and is regenerated with every build.

Corrections

If a figure disagrees with your own records, a station looks wrong, or a quote does not match its source, email [email protected]. Station names and identifiers, and every source's title and link, are printed so a claim can be checked. When a correction changes what a page says, its updated date changes with it.

Institutions cited

The 52 guides and 44 plant pages cite 563 publications from these institutions:

• ASPCA Animal Poison Control Center — 1 publication
• Clemson Cooperative Extension — 61 publications
• Colorado State University Extension — 3 publications
• Cornell Cooperative Extension — 3 publications
• Iowa State University Extension and Outreach — 28 publications
• LSU AgCenter — 1 publication
• Mississippi State University Extension — 13 publications
• NC State Extension — 88 publications
• New Mexico State University Extension — 3 publications
• North Dakota State University Extension — 2 publications
• Ohio State University Extension — 2 publications
• Oregon State University Extension Service — 20 publications
• Penn State Extension — 43 publications
• Pet Poison Helpline — 1 publication
• Purdue Extension — 2 publications
• Rutgers Cooperative Extension — 1 publication
• Texas A&M AgriLife and Texas A&M Forest Service — 20 publications
• University of Arkansas Cooperative Extension Service — 6 publications
• University of California Cooperative Extension and UC Master Gardeners — 22 publications
• University of Delaware Cooperative Extension — 1 publication
• University of Florida IFAS — 72 publications
• University of Georgia Extension — 14 publications
• University of Illinois Extension — 66 publications
• University of Kentucky Cooperative Extension Service — 1 publication
• University of Minnesota Extension — 53 publications
• University of Minnesota Turfgrass Science — 1 publication
• University of Missouri Extension — 6 publications
• University of New Hampshire Extension — 2 publications
• University of Vermont Extension — 1 publication
• University of Wisconsin–Madison Extension — 13 publications
• Utah State University Extension — 13 publications

The Arizona planting calendars cite University of Arizona Cooperative Extension publications and print their planting dates as those publications give them.

USGrowersGuide is an independent project and is not affiliated with any of these institutions, NOAA or the USDA. For decisions that matter, check with your state's cooperative extension service. See also About this site.