How the figures are sourced

The rule

Every figure on this site comes from a source we saved and read, and each page cites it. We keep a copy of every source with the site's code. Automated tests check that each figure on a page matches the saved copy, that every source a page cites is registered, and that no page text contains a hand-typed number: the words pull each figure from typed data.

A page that we cannot source that way is not published. When a source cannot be saved, the page that depends on it is taken down until one can be, as the change log records.

Frost dates

Frost dates come from NOAA's most recent U.S. Climate Normals, a thirty-year record published by its National Centers for Environmental Information. The frost dates page names the exact period. We downloaded NOAA's archive of annual and seasonal normals for every station, kept a record of the file's size and checksum, and extracted the frost and freeze columns for each station without changing a value. A test compares the extract with one station's original file, value for value.

NOAA does not publish a single frost date. For each station it simulated thousands of years from the station's real nightly lows and reports dates at levels of risk: for spring, the chance that the last night at or below freezing falls on or after a date, and for fall, the chance that the first one falls on or before it. We show the even-odds date as typical and the one-year-in-ten date as late in spring and early in fall, because a single date hides the risk that kills transplants. Where frost is too rare for NOAA to date, or possible in any month, the page says so instead of showing a date.

You can pick a station yourself, share your location, or type a ZIP code. A ZIP code is placed at the point the Census Bureau gives for its ZIP Code Tabulation Area in the latest gazetteer file, and the page lists the closest stations by straight-line distance. Each result names the station, its elevation and its distance from you or from that point, and warns when the nearest station is far enough away for its dates to differ from yours. Your location and the ZIP code you type never leave your browser: the page downloads a small file of ZIP codes and does the lookup itself.

City frost date pages

The pages for individual cities are generated from data, and the rules that decide which cities get one are written down so nobody has to take them on trust. We ranked the incorporated places of the fifty states and the District of Columbia by the Census Bureau's latest population estimates and walked down the list. A city gets a page only when its nearest NOAA station with a complete record is within twenty-five miles of the point the Census Bureau gives for it, when that station has frost dates and a typical year has a frost, and when no larger city already uses the station. Cities that fail are left out and listed, with the reason, on the city frost dates index; we do not publish a short or padded page to fill the gap.

Every figure on a city page comes from that city's own station record, the Census files or the count rules of the extension publications named above, so no two pages can differ only by a swapped name. The wording is written from the numbers: a sentence about a long season, a wide spread between the early and late dates or a hard freeze that follows the first frost closely appears only where that city's figures say so. The Census files have no elevation for a city, so each page gives the station's elevation and says it cannot compare it with your garden's. Tests run the same content checks as the hand-written pages on every generated page: the word floor, a worked example, sources, working links, no hand-typed number in the text, and a limit on how many sentences two pages may share.

Hardiness zones

The hardiness zone lookup reads the ZIP code listings that the PRISM Group at Oregon State University published with the latest USDA Plant Hardiness Zone Map. We saved PRISM's four files, for the conterminous states, Alaska, Hawaii and Puerto Rico, recorded their checksums, and copy each ZIP code's half zone and temperature band without changing them. A test rebuilds the lookup from the saved files and compares every ZIP code. We draw no map of our own, so the page is a lookup of PRISM's listing, not a version of the official map.

Extension sources

The charts rest on publications from land-grant university Extension services, written for home gardeners and reviewed by specialists: NC State Extension for plant spacing, days to harvest and the central North Carolina planting calendar; the University of Georgia for the Georgia calendar and its fall count-back rule; the University of Minnesota for seed starting, leggy seedlings and hardening off; Oklahoma State and the University of Florida for soil pH; Clemson, Illinois, New Mexico State and Tennessee for mowing height; Georgia, South Dakota State and Texas A&M for tomato leaf curl, plus Maine, West Virginia and Maryland for seedlings; Minnesota, Georgia, Penn State, Utah State, Ohio State, Iowa State, UMass and Colorado State for garlic planting; Maryland, Iowa State, South Dakota State and Utah State for squash rot; UC IPM and Utah State for tomato powdery mildew; and Minnesota, Illinois, UMass and South Dakota State for winter squash harvest. We saved each page, or the PDF and a text extract of it, and quote it line by line in our notes, and a test checks every quote against the saved copy. We do not use seed-company pages, competing garden sites or forum answers as the basis for a figure.

State planting pages

The state planting pages copy one state's own Extension calendar for one crop, a row per region, and set it beside NOAA's frost dates for that state's cities. Virginia, Illinois, Kentucky, South Carolina, Missouri, California, Alabama and Texas come from newly saved copies of their calendars, and Georgia and central North Carolina from the calendars above. We save the page or PDF, record its checksum, quote it line by line in a notes file, and a test checks every quoted line, every table row and every printed window against the saved copy. The seed-starting weeks on those pages come from other states' Extension services and are labelled as general, never as the state's own advice.

We left a state out when its table could not be read with confidence. Florida's PDF text came apart into wrapped fragments, Mississippi's table gives two unlabeled sets of dates for each crop, Arkansas's dates depend on a zone map we did not save, and Oklahoma's pages refused our download. Texas A&M's spring guidance counts from freeze dates rather than giving dates, so Texas has fall pages only. Where a state's text does not say which region a city belongs to, the page does not guess.

Age, region and doubt

Some sources are older than we would like, and some were written for one state. The NC State table is the oldest we use, and every page built on it gives its publication date. The calendars from North Carolina and Georgia are labelled for the regions they were written for, and the pages explain how to adjust or when not to copy them. Where one source contradicts itself, as the Georgia calendar does on its last snap bean date, the page points it out. Where sources disagree with each other, we do not average them: the soil pH table comes from one source only, and the mowing chart keeps each university's rows separate and labelled, because they were written for different climates, and the garlic lookup shows each state's statement on its own and does not carry one state's dates to another.

How the pages use the data

A chart page is a table built from the typed data. The frost lookup and the fall planner read NOAA's dates for the station you choose; the planner subtracts a crop's published days to harvest and the University of Georgia's harvest allowance from your first frost, and shows that it did. A troubleshooter is a decision tree whose every path ends at a likely cause and what to do next. A checklist names the source behind each step.

Dates and rechecks

Each page shows when its figures were last checked against their sources and when the next check is due. A daily job on the site's server reports pages past their recheck date so they are not left to go stale.

What we deliberately leave out

We do not draw our own hardiness zone map or guess a zone between ZIP codes. We do not give pesticide doses or mixing rates. And we do not use affiliate links today.

Our own wording

Page text is ours. We quote short phrases with attribution. We reproduce tables only from public university Extension publications and NOAA, credit and link each one on the page that uses it, and never copy tables from commercial garden sites. We do not reproduce a diagnostic key or photograph from a source. If you think we have been too close to a source, tell us on the contact page.