How Plan & Plant calculates planting dates
Maintained by Aleksandr Gulbickij · Last changed 8 October 2026 · Changelog coming soon
Short version: we take daily weather for the grid cell your town sits in, find the last spring frost and first autumn frost in each year of the sample, and use how those dates vary to set planting windows for each crop. Everything on a place page is calculated by a program from the sources below; people choose the rules, check the results and write the explanations.
Data period in use today
Published dates currently use 2019–2026 statistics from the self-hosted Open-Meteo archive. That is a short recent window, not the WMO 1991–2020 climate normal. A full thirty-year extract (with a minimum usable-winters gate) is planned; until then we do not describe these figures as a climate normal, and confidence claims that depend on a long winter sample are withheld.
1. Weather data
- Source: ERA5-Land and ERA5 reanalysis from the Copernicus Climate Change Service (C3S), read through a self-hosted copy of the open-source Open-Meteo server.
- Period today: 2019–2026 (see above). Future target: complete winters across 1991–2020.
- Which cell: the ERA5-Land cell containing the town. Towns that share a cell share one set of numbers; their pages say so when that applies.
- What reanalysis is, and isn't: a physically consistent reconstruction of past weather, blending observations with a weather model. It is not a thermometer in your garden.
2. Frost dates
- A frost day is a day whose minimum air temperature (2 m) is at or below 0 °C (32 °F).
- For each winter in the sample we find the last frost day of spring and the first frost day of autumn.
- Across those winters we report a typical last frost (about half of winters later). When the archive has enough winters (about 24+), we also show a safer planting date (fewer winters still frosted after that date). On the current short sample we hide the safer date rather than claim a 1-in-10 risk from too few winters.
- Dates are estimated from weather model data (ERA5-Land via Open-Meteo), not from a thermometer in your garden. We are not yet applying a station bias correction (Option A). A station comparison table will appear here once reference normals are committed under
docs/validation/. - Where frost is rare in the sample, pages say so rather than inventing a firm date.
3. Growing season and temperature windows
- Season length is the number of days between the frost dates we show.
- Daily temperatures are smoothed before deciding which weeks suit cool-season or warm-season crops, so single odd weeks don't flip the answer. Extreme possible highs can produce a heat note; they do not open a gap in the warm-season window by themselves.
- The place-page warm-season window uses the median of warm crops' tolerable maximum temperatures (not a single reference crop).
4. Crop rules
Each crop has a hardiness group and a few numbers. Shared sources are listed below; we plan to list sources on each grow guide:
| Group | Planting out | Examples |
|---|---|---|
| Hardy | From weeks before the safer last frost | Peas, onions, spinach |
| Semi-hardy | Closer to the safer last frost | Lettuce, carrots, beets |
| Tender | After the safer last frost, once nights are mild enough | Beans, sweet corn |
| Very tender | Later still | Tomatoes, peppers, basil, melons |
Direct sowing waits until soil temperature reaches the crop's minimum germination temperature where we have that record. Last sowing dates count back from the first autumn frost using days to maturity and harvest allowances. Rules follow extension guidance such as Purdue HO-186, RHS advice, and Virginia Tech SPES-801.
Garlic is planted in autumn and harvested the next summer. We do not judge it by frost-free season length. Timing anchors on modelled soil freeze (4–6 weeks before) or, in milder places, a cool spell; very mild climates get a pre-chill note. The mild-winter anchor is a Plan & Plant proposal, not a single extension rule. Garlic pages stay out of the sitemap until that rule passes review.
5. Fleece, cold frames and greenhouses
Cover options apply typical night-time and daytime temperature offsets to the outdoor series and then run the same rules. A single-skin unheated greenhouse gives little frost protection on clear nights but warms days; fleece or an inner layer is what moves frost dates. Offsets are estimates; night cloud weights are not yet validated against measurements.
6. Limitations
- Short sample. 2019–2026 is not enough years to match a climate normal. Treat dates as estimates for recent years, not a long-run expectation.
- Reanalysis can miss cold nights. On clear, still nights, gardens (and especially frost pockets) get colder than a grid cell's average. Prefer later plantings for tender crops until you know your own garden; the safer date returns when the 30-year archive is loaded.
- Coasts and mixed terrain: a grid cell can be partly sea or mixed elevation, which makes nights milder or colder than inland gardens nearby.
- Your garden is not average. Walls, slopes, raised beds and city heat all shift your dates by days to weeks.
7. What is automated, and what people do
- Automated: every date, window, verdict, chart and table on place and crop pages is computed by our program from the data above. Short sentences around them are templates filled with those numbers.
- People: chose the method, the crop rules and thresholds, and the sources; check results; write and edit the grow guides. How we use AI tools, if at all, is on the editorial policy.
8. Changes
When data or rules change, we recalculate, and pages whose numbers changed show a new "last recalculated" date. Material changes will be listed in a changelog when that page ships.
9. Data credits and citations
- Weather data by Open-Meteo.com, CC BY 4.0. Zippenfenig, P. (2023). Open-Meteo.com Weather API. Zenodo. doi:10.5281/ZENODO.7970649.
- Contains modified Copernicus Climate Change Service information 2026. Neither the European Commission nor ECMWF is responsible for any use that may be made of the Copernicus information or data it contains.
- Hersbach, H. et al. (2018): ERA5 hourly data on single levels from 1940 to present. C3S Climate Data Store (CDS). DOI: 10.24381/cds.adbb2d47.
- Muñoz Sabater, J. (2019): ERA5-Land hourly data from 1950 to present. C3S Climate Data Store (CDS). DOI: 10.24381/cds.e2161bac.
- Place names: GeoNames, CC BY 4.0.
- We computed frost dates, seasons and planting windows from these data; the original providers have not reviewed or endorsed them.
See also About and the editorial policy.