How sowing dates work
Bia moves each seed-packet sowing date to match the climate of your town, using decades of climate records, your frost dates and your height above sea level.
From a climate grid to your date
Five steps
Start with climate records
Bia uses ERA5-Land, a reconstruction of the weather from 1995 to 2024, on a grid of points about 55 km apart (half a degree of latitude and longitude). It covers every town in the UK, the US, Canada and New Zealand.
Find the town on the grid
Each town is blended from the four grid points around it, with closer points counting for more.
Correct for height
Air cools as you go up, roughly 0.65°C for every 100 m. A town on a hill gets a cooler year than the valley below, and later dates to match.
Work out the frost dates and the warmth
From decades of records, Bia finds the usual last frost in spring and first frost in autumn, and how warm each week of the year tends to be. Where frost is rare, dates go by warmth alone.
Move the seed-packet date
Seed suppliers give sowing windows for a typical year. Bia moves each window to match when spring arrives in your town, and drops it when the season is too short for the crop to grow.
One town, four grid points
Leeds sits between four grid points. Each one carries its own record of the weather, and Bia blends them for the town, then corrects for its height.
Where it can be wrong
Frost pockets and hollows
Cold air sinks, so a plot at the bottom of a slope can frost when the town doesn’t.
Cities and coasts
Big cities and seafronts are often a little milder than the grid suggests.
An unusual year
Dates are for a typical year. A late cold snap can still catch tender plants, so check the forecast before planting out.
Your own plot
A sunny wall, a windy corner or raised beds can all shift things by a week or two. Bia’s frost alerts use the forecast for your area, not just averages.
Sources
- ERA5-Land time series, 1995 to 2024, from the Copernicus Climate Change Service (C3S) at ECMWF. Contains modified Copernicus Climate Change Service information; neither the European Commission nor ECMWF is responsible for any use of it.
- Town positions and heights from GeoNames (CC BY 4.0), and ERA5-Land’s own terrain height for each grid point.
- Sowing windows and days to harvest from UK seed-packet dates, which suit the English Midlands.
See it for your town
Get these dates as reminders
Bia works out what to sow, plant out and harvest, and when, for your own plot, and reminds you on the day. Free for 2 months.
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