About Safe to Plant

What this is

Almost every frost table on the internet covers the United States, and a good many of the rest are the same American table with the units changed. If you garden in Kenya, Chile, Poland or New Zealand, the honest answer you usually get is a shrug and a hardiness zone borrowed from somebody else's continent.

This site exists because that gap is closeable. The daily temperature record needed to compute a real last-frost date for anywhere on land is public, free and forty years long. Nobody had turned it into pages a gardener could read, so this site does: last spring frost, first autumn frost, the length of the frost-free season, a hardiness-zone equivalent, and a sowing window for 307 crops, all derived for your town rather than interpolated from a neighbouring one.

Three answers are published for every date, not one, because a gardener with a cold frame and a gardener with nothing but hope need different numbers from the same data. The method page sets out the arithmetic in full.

Who makes it

Safe to Plant is an independent project, self-funded, run by one gardener who also writes software. It is not affiliated with NASA, GeoNames, the USDA, any national meteorological service, or any seed company or nursery. No supplier pays for placement here, because there is no placement to buy: the site sells nothing and links to no shop.

That independence is the point. A planting date that quietly nudges you towards a particular catalogue is worth nothing, and a frost date is too easy to fudge for anyone to be able to check it after the fact. So the source is named, the method is published, and the whole calculation is reproducible by anyone who wants to check it.

Where the data comes from

Daily minimum and maximum temperatures come from NASA POWER, a public-domain global record that reanalyses satellite observations and model output into a continuous daily series on a worldwide grid. This site uses the 30 years from 1995 to 2024 — long enough for percentiles to mean something, recent enough that a warming trend has not been averaged out by decades of colder records.

Place names, coordinates, elevations and populations come from GeoNames, used under CC BY 4.0.

Crop timings — weeks before or after the last frost, days to maturity, minimum soil temperature, how much cold each plant will take — are compiled from published extension-service and seed-industry guidance rather than derived from the climate record, and are stated in the same relative form a seed packet uses so that they travel between hemispheres without breaking.

Nothing is fetched while you are reading. Every figure is precomputed into a database by scheduled jobs, so a page view is a handful of database reads and some arithmetic. That is why the site is fast, and it is also why no upstream service sees your visit.

How it is kept honest

Four rules govern what gets published here.

• Nothing is invented. A location with no usable record gets no page. Where a value cannot be computed, the page says so instead of printing a plausible-looking number.
• Uncertainty is shown, not hidden. Frost dates are published as percentiles with the odds stated in plain words, because a single average date is wrong roughly half the time and saying so is more useful than pretending otherwise.
• The limits are stated on the page. The grid resolves a district, not a garden; cold air pools in hollows; a walled town plot can run a fortnight ahead of open fields a mile off. Every page that gives you a date also tells you what it cannot know.
• Corrections are welcome. If a figure here contradicts a long local record you keep, that is worth hearing about. The grid is good; it is not infallible.

How it is paid for

Safe to Plant carries advertising. That is the whole business model: no subscription, no paywall, no newsletter list to sell, no affiliate links, no sponsored crop pages. Ads are served by third parties and are marked as advertising; they are not editorial, and nothing an advertiser does changes a date on this site.

What advertising means for your data — cookies, personalisation, and how to turn it off — is set out in the privacy policy. The short version: this site sets no cookie of its own, keeps no account, and asks you for nothing.

Using the data

The frost percentiles, hardiness zones and planting windows derived here are published under CC BY 4.0, the same licence the place data arrives under. Use them in a blog post, a school project, a garden-club newsletter or an app, and credit Safe to Plant with a link. The full terms are on the terms of use page.

Missing town? Crop that should be here and is not? A date you think is wrong? Get in touch — those three messages are the ones that improve the site fastest.